Chapter 8: Study of Compounds A. Hydrogen Chloride
In the early nineteenth century, the alkali works of northern England and Scotland began making washing soda on an enormous scale. The first step of the process was simple enough: common salt was heated with sulphuric acid. Sodium sulphate stayed behind in the pan, and a gas came off. That gas was hydrogen chloride, and for years the works had no use for it, so they sent it up the chimney.
The countryside downwind soon told them what the gas was doing. Hedges browned, leaves fell out of season, and metal fittings on nearby buildings corroded. The gas itself was invisible, yet the air near the works looked hazy, because every trace of moisture it met turned into a fine mist of acid.
Complaints turned into lawsuits, and lawsuits turned into law. Manufacturers began running the escaping gas up tall towers packed with brushwood while water trickled down from the top, and the gas vanished into the water almost as fast as it arrived. In 1863 the British Parliament passed the Alkali Act, which required the works to condense the great bulk of the gas instead of releasing it.
What had been a nuisance became a product. The liquid draining from the bottom of those towers was hydrochloric acid — and today it is one of the most widely used acids in industry.
Diagram
Fig. 8.0-1 — Absorbing Hydrogen Chloride In A Packed Tower Tall vertical tower shown in cut-away, standing beside a factory building with a chimney; label the inlet near the base where hydrogen chloride gas enters, the loose packing filling the tower, the water inlet at the top with arrows showing water trickling downward, the upward arrows showing gas rising against the falling water, and the outlet pipe at the base carrying hydrochloric acid away to a storage tank.
8.1 The Hydrogen Chloride Molecule
Before we prepare a gas, it helps to know what one particle of it looks like. Hydrogen chloride is a gas whose molecule is built from one atom of hydrogen joined to one atom of chlorine. Almost everything the gas does later in this chapter — its choking smell, its density, the way it vanishes into water — follows from that single small molecule.
8.1.1 Formula, Molecular Mass And Bonding
A hydrogen atom has one electron in its only shell, and needs one more to reach a stable arrangement. A chlorine atom has the arrangement 2, 8, 7, and needs one more electron to complete its outermost shell. Neither atom can simply take an electron from the other, because both of them need to gain one.
So the two atoms share instead. Each contributes one electron to a shared pair, and that pair is counted by both atoms. Hydrogen then has two electrons around it, chlorine has eight in its outer shell, and the shared pair holds the two atoms together. The molecule is drawn both ways in Fig. 8.1-1.
Diagram
Fig. 8.1-1 — The Hydrogen Chloride Molecule Two drawings side by side. Left: electron-dot (Lewis) structure of HCl — H and Cl with one shared pair of dots between them and three lone pairs around Cl; below it the same molecule written as H–Cl; label the shared pair and one lone pair. Right: orbital (shell) diagram — a small circle labelled H carrying one electron and a larger circle labelled Cl with three shells holding 2, 8 and 7 electrons, the two circles overlapping so the shared pair lies in the overlap; label the shared pair, the hydrogen nucleus and the chlorine nucleus.
Because the atoms are held by a shared pair and not by transferred electrons, the bond in hydrogen chloride is covalent, and the compound is a covalent compound. Its formula and molecular mass follow directly from the two atoms it contains.
Molecular formula: \(\mathrm{HCl}\) Bond: covalent Molecular mass: \(1 + 35.5 = 36.5\)
The two atoms do not, however, hold the shared pair equally. Chlorine attracts the shared electrons more strongly than hydrogen does, so the pair sits nearer the chlorine atom. That leaves the chlorine end of the molecule slightly negative and the hydrogen end slightly positive. A covalent bond with its shared pair pulled towards one atom in this way is called a polar covalent bond. Keep this unevenness in mind — it is why the gas dissolves so readily in water, and why its solution behaves as an acid at all.
8.1.2 Occurrence
Hydrogen chloride was being made in the laboratory long before anyone understood what it contained. In 1648 Johann Rudolf Glauber obtained the acid by heating common salt, \(\mathrm{NaCl}\), with concentrated sulphuric acid — the same method a school laboratory uses today. Lavoisier named it muriatic acid. In 1810 Davy showed what the acid was actually made of, and named it hydrochloric acid, the name it still carries.
The compound is not confined to the laboratory. It is found in two very different places in nature.
- Hydrogen chloride gas occurs in the free state in volcanic emissions, escaping from vents in the earth along with steam and other gases.
- Hydrochloric acid, at a concentration of about \(0.2 - 0.4\) percent, is present in the gastric juice of mammals, where it helps in digestion.
The gastric juice in your own stomach is therefore a dilute solution of the very acid this chapter prepares — dilute enough to leave the stomach lining unharmed, yet strong enough to do its work on food.
8.2 General Methods Of Preparing Hydrogen Chloride Gas
There are two general ways of getting hydrogen chloride. One builds the molecule straight from its own two elements. The other takes the chlorine that is already locked up in a metal chloride and sets it free as the gas. The second is the one a laboratory actually uses, and Section 8.3 works through it in detail.
8.2.1 By Direct Combination Of The Elements
Hydrogen chloride can be made by direct combination — the union of two elements to form a single compound, with nothing else produced. Here the two elements are hydrogen and chlorine, and the compound is hydrogen chloride.
If moist hydrogen gas is mixed with chlorine and the mixture is left in diffused sunlight, the two gases combine steadily:
hydrogen + chlorine → hydrogen chloride
\[\mathrm{H_2}\text{(g)} + \mathrm{Cl_2}\text{(g)} \xrightarrow{\ \text{diffused sunlight}\ } 2\,\mathrm{HCl}\text{(g)}\]
The light is doing real work here, and how much light there is decides everything. In direct sunlight the reaction is explosive. In the dark it is negligible — the mixture can stand almost unchanged. Diffused sunlight is the middle setting, bright enough to start the change but not to run it away.
The reaction can also be made to go in the dark, by adding a catalyst such as activated carbon. Activated carbon absorbs hydrogen on its surface, and hydrogen held in this way is more reactive, so the two gases combine without any light at all.
There is a second version of the same combination. A jet of hydrogen already burning in air will go on burning when it is lowered into a jar of chlorine, and hydrogen chloride is formed. The hydrogen is not burning in oxygen now — it is burning in chlorine, which shows that burning does not always mean combining with oxygen.
8.2.2 By Heating A Metallic Chloride With Concentrated Sulphuric Acid
The second general method starts from a chloride that already exists as a solid. When a metallic chloride is heated with concentrated sulphuric acid, the acid takes the metal and the chloride gives up its chlorine as hydrogen chloride gas:
metallic chloride + concentrated sulphuric acid → metallic sulphate + hydrogen chloride
The reason this works is worth stating plainly, because it is the reason the whole method exists. Sulphuric acid boils at a high temperature and is very hard to vaporise, while hydrogen chloride is a gas at ordinary temperature. When the two are heated together, the acid that is difficult to vaporise stays in the flask and drives out the one that is easy to vaporise. A less volatile acid displaces a more volatile acid from its salt.
With sodium chloride, the product depends on the temperature. Below \(200^\circ\text{C}\) only one of the two hydrogen atoms of the sulphuric acid is used, and sodium hydrogen sulphate is left behind:
\[\mathrm{NaCl} + \mathrm{H_2SO_4} \xrightarrow{\ <200^\circ\text{C}\ } \mathrm{NaHSO_4} + \mathrm{HCl}\]
Above \(200^\circ\text{C}\) the second hydrogen atom is used as well, a second molecule of sodium chloride is consumed, and the residue is sodium sulphate:
\[2\,\mathrm{NaCl} + \mathrm{H_2SO_4} \xrightarrow{\ \text{above } 200^\circ\text{C}\ } \mathrm{Na_2SO_4} + 2\,\mathrm{HCl}\]
Sodium chloride is not the only chloride that behaves this way. Copper(II) chloride heated with concentrated sulphuric acid gives copper(II) sulphate and the same gas:
\[\mathrm{CuCl_2} + \mathrm{H_2SO_4} \xrightarrow{\ \Delta\ } \mathrm{CuSO_4} + 2\,\mathrm{HCl}\]
8.3 Laboratory Preparation Of Hydrogen Chloride Gas
In the laboratory, hydrogen chloride gas is prepared by the action of concentrated sulphuric acid on sodium chloride. The whole operation has four stages — generate the gas, dry it, collect it, and then test that the jar is full. FC 8.1 sets the four out in order, and the rest of this section takes them one at a time.
Flowchart
FC 8.1 — Preparing And Collecting Dry Hydrogen Chloride Gas Common salt and conc. sulphuric acid in flask → gentle heating below 200 °C → moist HCl gas leaves through delivery tube → bubbled through washer bottle of conc. sulphuric acid (drying) → dry HCl gas → gas jar filled by upward displacement of air → jar tested with rod dipped in ammonium hydroxide → dense white fumes show the jar is full
8.3.1 Reactants And Choice Of Acid
The reactants are sodium chloride and concentrated sulphuric acid.
Any metallic chloride would give the gas, as you saw in Section 8.2.2, so why sodium chloride? Because it is common salt — cheap, pure and available anywhere. That single practical reason is why it is preferred over other metal chlorides.
The choice of acid needs more care. Concentrated nitric acid is not used, because nitric acid is itself volatile — it vaporises easily — and its vapour would come over along with the hydrogen chloride and contaminate the gas. Sulphuric acid stays in the flask, and that is exactly what is wanted.
8.3.2 Apparatus And Procedure
The apparatus is set up as shown in Fig. 8.3-1.
Diagram
Fig. 8.3-1 — Laboratory Preparation Of Hydrogen Chloride Gas From Common Salt Round-bottomed flask clamped on a stand over a burner, containing sodium chloride; a thistle funnel passes through the stopper with its lower end dipping below the surface of the liquid, labelled conc. sulphuric acid at the top; a bent delivery tube leads from the flask into a washer bottle containing conc. sulphuric acid, with the inlet tube dipping into the acid and a short outlet tube above it; from the washer bottle a tube leads to the mouth of an upright gas jar standing in a trough, with an arrow marked air leaving the jar and the label dry hydrogen chloride inside it. Label sodium chloride, conc. sulphuric acid, thistle funnel, delivery tube, washer bottle, heat, gas jar.
Place some common salt in a flask, and pour concentrated sulphuric acid on to it through the thistle funnel. Either a flat-bottomed or a round-bottomed flask may be used, though a round-bottomed flask is normally chosen where heating is to be strong, since it stands heat better.
In the cold, the reaction proceeds slowly. On heating the mixture gently there is effervescence, and hydrogen chloride gas is evolved in a controlled manner. Gentle heating is the point — the gas is wanted at a steady rate that the rest of the apparatus can handle, not in a rush.
Concentrated sulphuric acid is corrosive and attacks skin and clothing, so it is poured carefully down the thistle funnel and never the other way about — acid is always added to water, never water to acid.
8.3.3 The Reaction And Its Temperature Conditions
Below \(200^\circ\text{C}\) the reaction is:
sodium chloride + concentrated sulphuric acid → sodium hydrogen sulphate + hydrogen chloride
\[\mathrm{NaCl} + \mathrm{H_2SO_4} \xrightarrow{\ <200^\circ\text{C}\ } \mathrm{NaHSO_4} + \mathrm{HCl}\uparrow\]
This is a reversible reaction — the products could in principle react back. Yet it goes to completion, because hydrogen chloride is a gas and escapes from the flask continuously. What has left the flask cannot come back, so the reverse change never gets its chance and the forward reaction runs on until the salt is used up.
If the mixture is heated above \(200^\circ\text{C}\), the sodium hydrogen sulphate formed in the first step attacks a further portion of salt, giving sodium sulphate and a second lot of gas:
\[\mathrm{NaHSO_4} + \mathrm{NaCl} \xrightarrow{\ \text{above } 200^\circ\text{C}\ } \mathrm{Na_2SO_4} + \mathrm{HCl}\uparrow\]
This second equation is worth a moment’s thought, because sodium hydrogen sulphate is here behaving as an acid — it still holds a replaceable hydrogen atom, and it uses it. Adding the two steps together gives the overall change above \(200^\circ\text{C}\):
\[2\,\mathrm{NaCl} + \mathrm{H_2SO_4}\text{(conc.)} \xrightarrow{\ \text{above } 200^\circ\text{C}\ } \mathrm{Na_2SO_4} + 2\,\mathrm{HCl}\]
Why The Higher Temperature Is Avoided
The higher temperature gives twice the gas from the same acid, so it looks like the better route. In practice it is not, for three reasons:
- The apparatus is made of glass, and at that temperature it may crack.
- Fuel is wasted in maintaining the stronger heating.
- The sodium sulphate formed sets as a hard crust that sticks to the glass and is very difficult to remove afterwards.
8.3.4 Drying The Gas
The gas leaving the flask carries water vapour with it. To obtain the dry gas it is passed through a washer bottle containing concentrated sulphuric acid, which absorbs that moisture. A substance used in this way, to take water out of a gas without reacting with it, is a drying agent.
Two other common drying agents cannot be used here. Phosphorus pentoxide and quicklime are both good at removing water, but both react with hydrogen chloride itself, so they would destroy the very gas being dried:
\[2\,\mathrm{P_2O_5} + 3\,\mathrm{HCl} \rightarrow \mathrm{POCl_3} + 3\,\mathrm{HPO_3}\]
\[\mathrm{CaO} + 2\,\mathrm{HCl} \rightarrow \mathrm{CaCl_2} + \mathrm{H_2O}\]
The first gives phosphorus oxychloride. The second is simply an acid reacting with a basic oxide, which is what quicklime is — and a gas that is acidic can never be dried over an alkaline solid.
8.3.5 Collection Of The Gas
Hydrogen chloride is collected by upward displacement of air, in an upright gas jar. It can be collected this way because it is 1.28 times as heavy as air: entering at the top of the jar, it sinks to the bottom and pushes the lighter air up and out.
It is not collected over water, for the reason Section 8.4.3 explores in detail — the gas is extremely soluble in water and would simply dissolve away.
8.3.6 Identification Of The Gas
Two observations tell you the gas is hydrogen chloride, and that the jar is full.
The first needs nothing at all. When the jar is completely filled, fumes appear above its mouth. Hydrogen chloride exposed to air gives white fumes, because it dissolves in the water vapour of the atmosphere and forms tiny droplets of hydrochloric acid.
The second is a definite chemical test. Bring a glass rod dipped in ammonium hydroxide near the mouth of the jar. Dense white fumes appear:
\[\mathrm{HCl} + \mathrm{NH_4OH} \rightarrow \mathrm{NH_4Cl} + \mathrm{H_2O}\]
The white fumes are solid ammonium chloride, and their appearance proves the jar is full of hydrogen chloride gas.
8.3.7 Precautions
Three precautions govern the preparation:
- The lower end of the thistle funnel must dip below the surface of the concentrated sulphuric acid in the flask. If it does not, the gas will escape up the funnel instead of going down the delivery tube.
- The inlet tube of the washer bottle must dip into the drying agent, the concentrated sulphuric acid, so that the gas bubbles through it and is dried rather than passing over the top of it.
- The temperature is maintained at nearly \(200^\circ\text{C}\) and is not allowed to rise above it, for the three reasons given in Section 8.3.3.
8.4 Physical Properties Of Hydrogen Chloride Gas
Two of the physical properties of hydrogen chloride matter more than all the rest — it is heavier than air, and it is extraordinarily soluble in water. Those two decide how the gas is collected, how it is stored, and how the acid is made from it. Table 8.1 sets out the full list, and the two experiments that follow demonstrate the important pair.
8.4.1 The Table Of Physical Properties
Table 8.1 Physical properties of hydrogen chloride gas
| S. No. | Property | Description |
|---|---|---|
| 1 | Colour | Colourless gas |
| 2 | Smell | Pungent, choking smell |
| 3 | Taste | Sour (acidic) |
| 4 | Physiological nature | Corrosive; it irritates the nose, throat and lungs |
| 5 | Density | About one and a quarter times heavier than air (vapour density of \(\mathrm{HCl}\) is 18.25, that of air is 14.4) |
| 6 | Boiling point | \(-85^\circ\text{C}\) |
| 7 | Melting point | \(-114^\circ\text{C}\) |
| 8 | Liquefaction and solidification | Under a high pressure of 40 atm at a low temperature of \(10^\circ\text{C}\) it is liquefied to a colourless liquid |
| 9 | Solubility | Highly soluble in water — 1 volume of water dissolves 452 volumes of the gas at room temperature. Being a polar covalent compound it also dissolves in organic solvents such as acetone and toluene |
The sour taste is listed because it is a real property of the substance, not because anyone tests it that way. No chemical in a laboratory is ever tasted, and a corrosive gas that irritates the lungs is not smelled directly either — a little is wafted towards the nose with the hand, and never inhaled from the mouth of the jar.
The density entry deserves unpacking, since it is stated as a comparison. The vapour density of a gas tells you how heavy it is compared with hydrogen, and dividing one vapour density by another compares the two gases with each other. Here \(18.25 \div 14.4 = 1.27\), which is where ‘about one and a quarter times heavier than air’ comes from, and why the gas is collected by upward displacement of air.
8.4.2 Experiment To Show That The Gas Is Heavier Than Air
That the gas is heavier than air can be shown directly, as in Fig. 8.4-1. A jar of hydrogen chloride is tilted over a second jar containing a burning candle, and the gas is poured out of it exactly as a liquid would be poured.
Diagram
Fig. 8.4-1 — Showing That Hydrogen Chloride Gas Is Heavier Than Air A gas jar held tilted at the top right, labelled HCl gas, with its mouth over a wide jar standing below; a short arrow marked air points out of the lower jar and a curved arrow shows the gas flowing downward from the tilted jar into it; inside the lower jar a lighted candle stands on a small block, drawn with the flame going out. Label HCl gas, air, burning candle.
The candle goes out. Being heavier than air, the gas occupies the lower portion of the jar and forces the air out of it, so the candle is left surrounded by hydrogen chloride instead of air.
Notice how much this one observation establishes. The gas can be poured downward, so it is denser than air. The flame dies in it, so the gas does not burn itself and does not support the burning of anything else — it is neither combustible nor a supporter of combustion.
8.4.3 Solubility And The Fountain Experiment
One volume of water dissolves 452 volumes of hydrogen chloride at room temperature. That figure is hard to believe until it is seen, and the fountain experiment of Fig. 8.4-2 is the classic way of seeing it.
Diagram
Fig. 8.4-2 — The Fountain Experiment Round-bottomed flask held inverted in a clamp on a stand, labelled HCl gas, closed by a rubber stopper with two holes; through one hole a long jet tube runs from inside the flask down into a beaker below, through the other a rubber-teat dropper containing a few drops of water; inside the flask a spray of fine jets is drawn rising from the top of the jet tube and labelled red fountain; the beaker below stands on the bench and is labelled blue litmus solution, with the lower end of the jet tube dipping into it. Label HCl gas, jet tube, dropper, blue litmus solution, red fountain.
A dry round-bottomed flask is filled with dry hydrogen chloride gas and closed with a rubber stopper carrying two holes. A long jet tube passes through one hole and reaches down into a beaker of blue litmus solution. A dropper holding a few drops of water passes through the other.
Now press the dropper. The blue litmus solution rushes up the jet tube with great force and sprays out at the top as a red fountain inside the flask.
The reason is a pressure difference, and it is worth following in two steps. The few drops of water from the dropper meet the gas, and because the gas is so soluble, a large volume of it dissolves in that tiny quantity of water almost at once. Gas that has dissolved no longer presses on the walls, so the pressure inside the flask falls sharply.
The atmospheric pressure outside is now the greater of the two. It presses on the surface of the liquid in the beaker and pushes it up the jet tube into the flask, where it emerges as a fountain. The fountain is red, and not blue, because the gas dissolving in it forms an acid — blue litmus turns red in the presence of an acid.
The same solubility explains something you can see whenever a bottle of the gas is opened. Hydrogen chloride fumes in moist air, dissolving in the water vapour present there to form tiny droplets of hydrochloric acid, and it is those suspended droplets that make the white fumes visible.
8.5 Chemical Properties Of Hydrogen Chloride Gas
Everything in this section concerns the dry gas, not its solution in water. That distinction matters, because the dry gas and the acid behave very differently — the dry gas is made of covalent molecules, and molecules do not do the things that ions do. Section 8.6 takes up what happens once water is added.
8.5.1 Combustibility
The gas is neither combustible nor a supporter of combustion. It does not burn itself, and it puts out a burning splint lowered into it, exactly as it extinguished the candle in Fig. 8.4-1.
8.5.2 Thermal Dissociation
Heat the gas strongly, above \(500^\circ\text{C}\), and it comes apart into the two elements it was made from:
hydrogen chloride → hydrogen + chlorine
\[2\,\mathrm{HCl} \xrightarrow{\ >500^\circ\text{C}\ } \mathrm{H_2} + \mathrm{Cl_2}\]
This is thermal dissociation — the breaking up of a compound by heat into simpler substances that join together again when the compound is cooled. It is the reverse of the direct combination you met in Section 8.2.1, which is why the arrow could as easily have been drawn both ways.
8.5.3 Action On Metals
Metals that come before hydrogen in the electrochemical series react with hydrogen chloride gas when heated, forming their chlorides and setting hydrogen free. The reactions are listed in Table 8.2.
The position in the series is the reason for the whole pattern, so it is worth saying plainly what that position means. A metal placed above hydrogen loses electrons more readily than hydrogen does. It can therefore take the place of hydrogen in the compound, keeping the chlorine for itself and pushing the hydrogen out as a gas. A metal below hydrogen — copper, silver, gold — cannot do this, and is left untouched.
Table 8.2 Action of hydrogen chloride gas on metals above hydrogen
| Metal | + hydrogen chloride (g) | → | Metal chloride | + hydrogen |
|---|---|---|---|---|
| \(2\,\mathrm{Na}\) | \(2\,\mathrm{HCl}\) | → | \(2\,\mathrm{NaCl}\) | \(\mathrm{H_2}\) |
| \(\mathrm{Ca}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{CaCl_2}\) | \(\mathrm{H_2}\) |
| \(\mathrm{Mg}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{MgCl_2}\) | \(\mathrm{H_2}\) |
| \(\mathrm{Zn}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{ZnCl_2}\) | \(\mathrm{H_2}\) |
| \(\mathrm{Fe}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{FeCl_2}\) | \(\mathrm{H_2}\) |
Read down the middle column and you will see why the equations balance as they do. A metal with a valency of two, such as calcium, magnesium, zinc or iron in this reaction, needs two chlorine atoms, so it uses two molecules of the gas. Sodium has a valency of one, so two atoms of sodium are needed to use up the two molecules and release one molecule of hydrogen.
8.5.4 Reaction With Ammonia
Hydrogen chloride combines with ammonia to form dense white fumes of ammonium chloride:
hydrogen chloride + ammonia → ammonium chloride
\[\mathrm{NH_3}\text{(g)} + \mathrm{HCl}\text{(g)} \rightarrow \mathrm{NH_4Cl}\text{(s)}\]
Look carefully at the state symbols. Two gases have combined to give a solid, with no liquid and no other product at all. The white fumes you see are not a gas but an enormous number of extremely fine solid particles suspended in the air, and they settle in time as a white powder. This is the reaction behind the identification test in Section 8.3.6, where the ammonia came from a rod dipped in ammonium hydroxide.
Diagram
Fig. 8.5-1 — Two Gases Forming A Solid Two gas jars held mouth to mouth in a horizontal line, the left one labelled ammonia and the right one labelled hydrogen chloride, each with its cover glass drawn just removed; dense white fumes shown filling the space where the two jars meet, labelled white fumes of ammonium chloride, with a thin layer of white powder settling on the lower inside surface of both jars.
8.6 Hydrochloric Acid And Its Preparation
When hydrogen chloride gas is dissolved in water, hydrochloric acid is formed. That one sentence hides the most important change in the chapter, because the gas that goes into the water is not acidic and the solution that comes out of it is strongly acidic. Something happens to the molecule when it meets water, and this section begins with what that something is.
8.6.1 Ionisation In Water And The Covalent Nature Of The Gas
You know from Section 8.1.1 that the bond in hydrogen chloride is polar — the shared pair sits nearer the chlorine, leaving the hydrogen end slightly positive. Water molecules are polar too, and they pull that bond apart. The hydrogen leaves without its electron, as a proton, and attaches itself to a water molecule; the chlorine keeps both electrons and leaves as a chloride ion:
\[\mathrm{HCl} + \mathrm{H_2O} \rightarrow \mathrm{H_3O^{+}} + \mathrm{Cl^{-}}\]
The splitting of a compound into ions in this way is ionisation, and the ion \(\mathrm{H_3O^{+}}\) is called the hydronium ion. It is the hydronium ion, and nothing else, that makes a solution acidic. Check the charges before moving on: the left side is neutral overall, and on the right one positive and one negative charge cancel, so the equation balances in charge as well as in atoms.
Now the consequences follow at once, and they are the ones students are most often asked about.
Dry hydrogen chloride gas and liquefied hydrogen chloride contain no water, so no ionisation takes place and no \(\mathrm{H^{+}}\) is produced. They therefore do not turn blue litmus paper red. The dry gas is not acidic.
Dry hydrogen chloride gas and the liquefied gas also do not conduct electricity, because conduction needs free ions to carry the charge and there are none — only whole molecules. This is direct evidence that hydrogen chloride is a covalent compound.
The Evidence From Toluene
There is a neat experiment that settles the matter. Hydrogen chloride dissolves in toluene, an organic solvent, just as it dissolves in water. But the toluene solution neither turns blue litmus red nor conducts electricity.
Dissolving, then, is not the cause of acidity. Toluene molecules are not polar enough to pull the bond apart, so the hydrogen chloride stays in toluene as whole molecules, no hydronium ions are formed, and the solution shows no acidic property at all. The same substance is acidic in one solvent and not in another — which shows that hydrogen chloride is itself a covalent compound, and that water is what converts it into an acid.
8.6.2 Laboratory Method Of Preparing Hydrochloric Acid
The aqueous solution of hydrogen chloride gas is what we call hydrochloric acid, and it is prepared simply by dissolving the gas in water. The gas is passed into the water until no more is absorbed. The product is concentrated acid, containing about 36 percent of hydrogen chloride by mass.
Passing the gas in is not as simple as it sounds, and the arrangement used is shown in Fig. 8.6-1. An inverted funnel is connected to the hydrogen chloride gas supply and held so that its rim just touches the surface of the water in the trough — dipping into the surface, not plunged below it.
Diagram
Fig. 8.6-1 — The Inverted Funnel Arrangement For Absorbing Hydrogen Chloride Two side-by-side panels of the same beaker of water in a trough, each with a wide inverted funnel held mouth-downward with its rim at the water surface, connected by a tube at the top to a supply labelled HCl gas. Left panel: water drawn risen up inside the funnel above the level outside, with an upward arrow labelled water rises as gas dissolves. Right panel: the water inside fallen back, with a gap drawn between the funnel rim and the water surface, labelled air gap, and downward arrows showing the level falling. Label HCl gas, inverted funnel, water, air gap.
Follow what happens in a cycle. The gas is so soluble that it is absorbed the instant it meets the water, and the pressure under the funnel drops. Water is pushed up into the funnel by the atmosphere outside — this is back-suction beginning. As the water rises inside, the level outside falls, and an air gap opens between the rim of the funnel and the water surface.
That gap is the safety valve. Air enters through it, the pressure inside and outside become equal, and the water that had risen in the funnel falls back down again. The process repeats over and over, and it continues until the water in the trough is saturated with hydrogen chloride — which is to say, until it has become hydrochloric acid.
8.6.3 Back Suction And How It Is Avoided
Back suction is worth understanding properly, because it is the one real hazard in this preparation.
Hydrogen chloride is absorbed by water more quickly than it is being generated in the flask. The gas therefore disappears from the delivery tube faster than it arrives, the pressure inside the tube and the flask falls below atmospheric pressure, and the atmosphere pushes the water backwards up the delivery tube. That is back suction.
Now suppose the gas were led into the water through an ordinary narrow glass tube instead of a funnel. The narrow tube would fill with water in a moment, and the water would pass over into the generating flask. Two things would follow. The preparation would stop, and — far more seriously — water would meet hot concentrated sulphuric acid, which releases a great deal of heat, and the flask could burst.
The funnel arrangement prevents this in two ways:
- Its wide mouth lets an air gap form at the rim as soon as the level drops, which breaks the suction before water can travel far. It therefore prevents or minimises back suction.
- Its wide mouth also presents a large surface area of water to the gas, so absorption is rapid and efficient.
As a further precaution, an empty flask is placed between the generating flask and the water trough, as in Fig. 8.6-2. It acts as an anti-suction device: if back suction does occur, the water collects in this empty flask and never reaches the hot reaction mixture.
Diagram
Fig. 8.6-2 — The Empty Flask As An Anti-Suction Device Left to right on one bench: a round-bottomed flask on a stand over a burner labelled reactants, with a thistle funnel; a washer bottle of conc. sulphuric acid; an empty round-bottomed flask standing alone with the tube entering it from the top and leaving from the top, labelled empty flask; and finally an inverted funnel dipping into a trough of water. Draw a dashed arrow running backwards from the trough to the empty flask, labelled back suction stops here. Label reactants, conc. sulphuric acid, empty flask, inverted funnel, water.
8.6.4 Concentration By Distillation And The Constant Boiling Mixture
A dilute solution of hydrochloric acid can be made stronger by distillation. Water boils off first, the solution left behind grows more concentrated, and this continues until the acid reaches 22.2 percent hydrogen chloride by weight and 77.8 percent water by weight. At that composition the liquid boils at \(110^\circ\text{C}\).
From there, boiling achieves nothing more. Beyond this point the hydrogen chloride begins to leave along with the water vapour, so the liquid boiling away has the same composition as the liquid left behind, and the concentration cannot rise any further.
A mixture that behaves in this way is called a constant boiling mixture, or an azeotrope.
8.7 Properties Of Hydrochloric Acid
Hydrochloric acid in water is strongly acidic, and it shows all the properties of an acid. What follows is that list, worked through one class of reaction at a time — indicators, metals, oxides and hydroxides, salts of weaker acids, and precipitation — followed by two things this acid does that most acids do not.
8.7.1 Physical Properties
Table 8.3 Physical properties of hydrochloric acid
| S. No. | Property | Description |
|---|---|---|
| 1 | Colour | Colourless |
| 2 | Smell | Pungent, choking smell |
| 3 | Taste | Sour (acidic) |
| 4 | Effect on skin | The concentrated acid is corrosive and causes blisters on the skin |
| 5 | Solubility | Readily soluble in water in all proportions |
| 6 | Boiling point | \(110^\circ\text{C}\) (a volatile acid) |
Because the concentrated acid blisters skin, it is handled with the bottle held away from the body, and any splash is washed off at once with plenty of running water. When the acid has to be diluted, the acid is poured slowly into the water with stirring, never water into the acid.
8.7.2 Action On Indicators
An indicator is a substance that shows one colour in acid and another in alkali, and the aqueous acid changes them as Table 8.4 shows.
Table 8.4 Action of hydrochloric acid on indicators
| Indicator | Original colour | Changes to |
|---|---|---|
| Moist litmus | Blue | Red |
| Methyl orange | Orange | Pink |
| Phenolphthalein | Colourless | Colourless |
The last row is not a misprint, and it is worth pausing on. Phenolphthalein is colourless in acid and pink in alkali, so adding an acid to it produces no change at all — the absence of a colour is the observation.
8.7.3 Action On Metals
Metals placed above hydrogen in the activity series react with the acid, forming the metallic chloride and evolving hydrogen. In general:
metal + hydrochloric acid → metallic chloride + hydrogen
Table 8.5 Action of hydrochloric acid on metals above hydrogen
| Metal | + hydrochloric acid | → | Metallic chloride | + hydrogen |
|---|---|---|---|---|
| \(\mathrm{Ca}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{CaCl_2}\) | \(\mathrm{H_2}\uparrow\) |
| \(\mathrm{Mg}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{MgCl_2}\) | \(\mathrm{H_2}\uparrow\) |
| \(\mathrm{Zn}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{ZnCl_2}\) | \(\mathrm{H_2}\uparrow\) |
What you see is effervescence — a steady stream of bubbles at the surface of the metal — and the metal slowly disappearing. The gas collected burns with a pop when a lighted splint is brought to it, which identifies it as hydrogen.
8.7.4 Action On Oxides And Hydroxides
Metallic oxides and hydroxides are bases, and an acid reacts with a base to give a salt and water. Nothing else is formed — no gas, so no effervescence:
metallic oxide or hydroxide + hydrochloric acid → metallic chloride + water
Table 8.6 Action of hydrochloric acid on metallic oxides and hydroxides
| Oxide or hydroxide | + hydrochloric acid | → | Metallic chloride | + water |
|---|---|---|---|---|
| \(\mathrm{MgO}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{MgCl_2}\) | \(\mathrm{H_2O}\) |
| \(\mathrm{ZnO}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{ZnCl_2}\) | \(\mathrm{H_2O}\) |
| \(\mathrm{FeO}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{FeCl_2}\) | \(\mathrm{H_2O}\) |
| \(\mathrm{Fe_2O_3}\) | \(6\,\mathrm{HCl}\) | → | \(2\,\mathrm{FeCl_3}\) | \(3\,\mathrm{H_2O}\) |
| \(\mathrm{CuO}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{CuCl_2}\) | \(\mathrm{H_2O}\) |
| \(\mathrm{NaOH}\) | \(\mathrm{HCl}\) | → | \(\mathrm{NaCl}\) | \(\mathrm{H_2O}\) |
| \(\mathrm{Ca(OH)_2}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{CaCl_2}\) | \(2\,\mathrm{H_2O}\) |
| \(\mathrm{Zn(OH)_2}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{ZnCl_2}\) | \(2\,\mathrm{H_2O}\) |
| \(\mathrm{Cu(OH)_2}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{CuCl_2}\) | \(2\,\mathrm{H_2O}\) |
Compare the two iron rows. In \(\mathrm{FeO}\) the iron is in the \(+2\) state and gives \(\mathrm{FeCl_2}\); in \(\mathrm{Fe_2O_3}\) it is in the \(+3\) state and gives \(\mathrm{FeCl_3}\), which needs three chlorides for every iron and therefore six molecules of acid for every two irons. The acid does not change the oxidation state of the metal — it simply exchanges oxygen for chlorine.
Several of these reactions are visible ones. Black copper(II) oxide dissolves in the warm acid to give a blue-green solution of copper(II) chloride, and reddish-brown iron(III) oxide gives a yellow solution of iron(III) chloride.
8.7.5 Action On Salts Of Weaker Acids
Hydrochloric acid is a strong acid, and a strong acid decomposes the salt of a weaker acid, driving the weaker acid out. Since carbonic, sulphurous and hydrosulphuric acids are all unstable, what is actually driven out is a gas — and the gas identifies the salt.
Carbonates And Hydrogen Carbonates
carbonate or hydrogen carbonate + hydrochloric acid → metallic chloride + water + carbon dioxide
Table 8.7 Action of hydrochloric acid on carbonates and hydrogen carbonates
| Salt | + hydrochloric acid | → | Metallic chloride | + water | + carbon dioxide |
|---|---|---|---|---|---|
| \(\mathrm{Na_2CO_3}\) | \(2\,\mathrm{HCl}\) | → | \(2\,\mathrm{NaCl}\) | \(\mathrm{H_2O}\) | \(\mathrm{CO_2}\uparrow\) |
| \(\mathrm{CaCO_3}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{CaCl_2}\) | \(\mathrm{H_2O}\) | \(\mathrm{CO_2}\uparrow\) |
| \(\mathrm{NaHCO_3}\) | \(\mathrm{HCl}\) | → | \(\mathrm{NaCl}\) | \(\mathrm{H_2O}\) | \(\mathrm{CO_2}\uparrow\) |
| \(\mathrm{Ca(HCO_3)_2}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{CaCl_2}\) | \(2\,\mathrm{H_2O}\) | \(2\,\mathrm{CO_2}\uparrow\) |
The gas is colourless and odourless, and it turns lime water milky — the standard test for carbon dioxide.
Sulphites And Hydrogen Sulphites
sulphite or hydrogen sulphite + dilute hydrochloric acid → metallic chloride + water + sulphur dioxide
Table 8.8 Action of dilute hydrochloric acid on sulphites and hydrogen sulphites
| Salt | + hydrochloric acid | → | Metallic chloride | + water | + sulphur dioxide |
|---|---|---|---|---|---|
| \(\mathrm{Na_2SO_3}\) | \(2\,\mathrm{HCl}\) | → | \(2\,\mathrm{NaCl}\) | \(\mathrm{H_2O}\) | \(\mathrm{SO_2}\uparrow\) |
| \(\mathrm{K_2SO_3}\) | \(2\,\mathrm{HCl}\) | → | \(2\,\mathrm{KCl}\) | \(\mathrm{H_2O}\) | \(\mathrm{SO_2}\uparrow\) |
| \(\mathrm{NaHSO_3}\) | \(\mathrm{HCl}\) | → | \(\mathrm{NaCl}\) | \(\mathrm{H_2O}\) | \(\mathrm{SO_2}\uparrow\) |
| \(\mathrm{MgSO_3}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{MgCl_2}\) | \(\mathrm{H_2O}\) | \(\mathrm{SO_2}\) |
Sulphur dioxide is colourless with the choking smell of burning sulphur, and it turns paper moistened with acidified potassium dichromate solution from orange to green.
Sulphides
metallic sulphide + dilute hydrochloric acid → metallic chloride + hydrogen sulphide
Table 8.9 Action of dilute hydrochloric acid on metallic sulphides
| Sulphide | + hydrochloric acid | → | Metallic chloride | + hydrogen sulphide |
|---|---|---|---|---|
| \(\mathrm{Na_2S}\) | \(2\,\mathrm{HCl}\) | → | \(2\,\mathrm{NaCl}\) | \(\mathrm{H_2S}\uparrow\) |
| \(\mathrm{FeS}\) | \(2\,\mathrm{HCl}\) | → | \(\mathrm{FeCl_2}\) | \(\mathrm{H_2S}\uparrow\) |
Hydrogen sulphide is colourless and smells strongly of rotten eggs, and it turns paper moistened with lead acetate solution silvery black. It is also poisonous, so it is generated only in small test-tube quantities and in a fume cupboard or a well ventilated room.
Thiosulphates
Dilute hydrochloric acid reacts with a thiosulphate to give sulphur dioxide, and here something extra happens — solid sulphur is thrown out as a pale yellow precipitate that clouds the liquid:
\[\mathrm{Na_2S_2O_3} + 2\,\mathrm{HCl} \rightarrow 2\,\mathrm{NaCl} + \mathrm{H_2O} + \mathrm{SO_2} + \mathrm{S}\downarrow\]
This gives a way of telling two similar salts apart. Both a thiosulphate and a sulphite give sulphur dioxide with dilute hydrochloric acid, but only the thiosulphate leaves a precipitate of sulphur.
8.7.6 Precipitation Reactions
Dilute hydrochloric acid does not normally react with nitrates. Three nitrates are exceptions, and they are exceptions for one reason — their chlorides are insoluble, so a solid is thrown down and the reaction has somewhere to go.
Lead nitrate and mercury(I) nitrate each give a white precipitate:
\[\mathrm{Pb(NO_3)_2} + 2\,\mathrm{HCl} \rightarrow \mathrm{PbCl_2}\downarrow + 2\,\mathrm{HNO_3}\]
\[\mathrm{Hg_2(NO_3)_2} + 2\,\mathrm{HCl} \rightarrow \mathrm{Hg_2Cl_2}\downarrow + 2\,\mathrm{HNO_3}\]
The precipitate of lead chloride behaves in a way worth remembering: it dissolves on heating, and comes back as white needles when the solution cools.
Silver nitrate solution gives a thick curdy white precipitate of silver chloride:
\[\mathrm{AgNO_3} + \mathrm{HCl} \rightarrow \mathrm{AgCl}\downarrow + \mathrm{HNO_3}\]
This precipitate is insoluble in nitric acid but soluble in ammonium hydroxide solution, in which it forms a complex salt called diammine silver(I) chloride:
\[\mathrm{AgCl} + 2\,\mathrm{NH_4OH} \rightarrow \mathrm{[Ag(NH_3)_2]^{+}Cl^{-}} + 2\,\mathrm{H_2O}\]
Add dilute nitric acid to that clear solution and the white precipitate of silver chloride returns. The complete sequence — precipitate, dissolve, precipitate again — is set out in FC 8.2, and it is one of the most reliable identifications in the whole of school analysis.
Flowchart
FC 8.2 — The Silver Chloride Sequence Silver nitrate solution + dilute hydrochloric acid → thick curdy white precipitate of AgCl → add dilute nitric acid: precipitate does not dissolve → add ammonium hydroxide: precipitate dissolves to a clear solution of diammine silver(I) chloride → add dilute nitric acid to the clear solution: white precipitate of AgCl reappears → leave the precipitate in sunlight: it turns black as silver is set free
Silver chloride left exposed to light blackens, because it decomposes into chlorine and metallic silver, the silver being liberated as a fine black powder:
\[2\,\mathrm{AgCl} \rightarrow 2\,\mathrm{Ag} + \mathrm{Cl_2}\]
8.7.7 Oxidation Of Concentrated Hydrochloric Acid To Chlorine
Concentrated hydrochloric acid can readily be oxidised to chlorine by a strong oxidising agent — a substance that supplies oxygen to another substance, or takes hydrogen from it. Manganese dioxide, lead dioxide and red lead all do this on heating, as does bleaching powder with the dilute acid.
Table 8.10 Oxidation of hydrochloric acid to chlorine
| Oxidising agent | + hydrochloric acid | Condition | → | Metallic chloride | + water | + chlorine |
|---|---|---|---|---|---|---|
| \(\mathrm{MnO_2}\) | \(4\,\mathrm{HCl}\) (conc.) | \(\Delta\) | → | \(\mathrm{MnCl_2}\) | \(2\,\mathrm{H_2O}\) | \(\mathrm{Cl_2}\uparrow\) |
| \(\mathrm{PbO_2}\) | \(4\,\mathrm{HCl}\) (conc.) | \(\Delta\) | → | \(\mathrm{PbCl_2}\) | \(2\,\mathrm{H_2O}\) | \(\mathrm{Cl_2}\uparrow\) |
| \(\mathrm{Pb_3O_4}\) | \(8\,\mathrm{HCl}\) (conc.) | \(\Delta\) | → | \(3\,\mathrm{PbCl_2}\) | \(4\,\mathrm{H_2O}\) | \(\mathrm{Cl_2}\uparrow\) |
| \(\mathrm{CaOCl_2}\) (bleaching powder) | \(2\,\mathrm{HCl}\) (dil.) | \(\Delta\) | → | \(\mathrm{CaCl_2}\) | \(\mathrm{H_2O}\) | \(\mathrm{Cl_2}\uparrow\) |
Follow the hydrogen and you can see both halves of the change at once. The oxide gives up its oxygen, which joins the hydrogen of the acid to make water; the chlorine of the acid, having lost its hydrogen, is set free as greenish-yellow chlorine gas. Since the acid has lost hydrogen to the oxide, the acid has been oxidised — and, said the other way round, hydrochloric acid has reduced these compounds.
Chlorine is toxic, so any reaction of this kind is carried out in a fume cupboard.
8.7.8 Aqua Regia — Composition And Uses
A mixture of three parts of concentrated hydrochloric acid and one part of concentrated nitric acid is called aqua regia. Neither acid alone attacks gold. The mixture does, and the reason lies in what the two acids make between them:
\[3\,\mathrm{HCl} + \mathrm{HNO_3} \rightarrow \mathrm{NOCl} + 2\,\mathrm{H_2O} + 2[\mathrm{Cl}]\]
The products are nitrosyl chloride, \(\mathrm{NOCl}\), and nascent chlorine, written \([\mathrm{Cl}]\). Nascent chlorine means chlorine at the moment of its formation, as single atoms rather than \(\mathrm{Cl_2}\) molecules, and in that state it is far more reactive. It is this nascent chlorine that attacks the noble metals, converting them into soluble chlorides:
\[\mathrm{Au} + 3[\mathrm{Cl}] \rightarrow \mathrm{AuCl_3}\]
\[\mathrm{Pt} + 4[\mathrm{Cl}] \rightarrow \mathrm{PtCl_4}\]
The products are gold(III) chloride and platinum(IV) chloride. Notice that the nitric acid here is not acting as an acid at all — it is the oxidising agent that liberates the chlorine from the hydrochloric acid.
Aqua regia has three uses at this level. It dissolves noble metals such as gold and platinum. It is used in making chloroauric acid, the electrolyte used in refining gold. And it is used in analytical work to dissolve certain ores that ordinary acids will not touch.
8.8 General Uses Of Hydrochloric Acid
Almost every use of hydrochloric acid follows from one of the properties in Section 8.7 — it dissolves oxides, it supplies chloride, or it is simply a cheap strong acid.
In The Laboratory And In Manufacture
- In the laboratory it is used as a reagent, and for the preparation of aqua regia.
- In the manufacture of chlorine and of chlorides — for example ammonium chloride, which is used in dry cells.
- In the manufacture of dyes, drugs, paints and photographic chemicals, silver chloride among them.
- In the manufacture of glucose from starch.
Cleaning And Treating Metal Surfaces
Iron and steel exposed to air carry a layer of oxide, and no paint, plating or coating will stick to it. Hydrochloric acid dissolves that oxide and leaves clean metal behind, which is why so many of its uses are surface treatments.
- In industry, to pickle steel. Pickling is the removal of the oxide coating from the surface of a metal before it is painted, electroplated or galvanised. Steel that is to be plated with tin or chromium is first cleaned by pickling in hydrochloric acid together with some inhibitors.
- To remove rust from iron sheets.
- For cleaning metal surfaces before painting, electroplating, galvanising and soldering.
FC 8.3 shows where the pickling bath sits in the sequence.
Flowchart
FC 8.3 — Pickling Steel Before Coating Steel sheet carrying a layer of oxide scale → immersed in a bath of hydrochloric acid containing an inhibitor → oxides dissolve, leaving bare clean metal → washed and rinsed in water → dried → passed on for painting, tin plating, chromium plating or galvanising
Other Industrial Uses
- For purifying bone black, since the acid dissolves the calcium phosphate present in bones.
- In the extraction of glue from bones.
- In the tanning and calico printing industries.
In Medicine
- Dilute hydrochloric acid is prescribed to patients whose gastric juices have become less active than they should be.
8.9 Tests For Hydrogen Chloride Gas And Hydrochloric Acid
Everything needed to identify hydrogen chloride has already appeared in this chapter, scattered across the preparation and the reactions. This section gathers it into one place, as a chemist would use it at the bench. Table 8.11 sets out the four tests, what you do, what you see, and what it proves.
The first test is only a pointer, never a proof, since several gases irritate the nose. The second and third are the definite ones. Note that the ammonia test and the silver nitrate test between them cover both forms of the substance — the ammonia test works on the gas or on the vapour above the acid, while the silver nitrate test works on the solution.
The equations for the second, third and fourth tests were given in Sections 8.3.6, 8.7.6 and 8.7.7 respectively, and are collected again in the Summary.
The last line of the table brings in one new piece of laboratory practice. Starch iodide paper is filter paper soaked in potassium iodide solution and starch. Chlorine sets iodine free from the iodide, and free iodine gives an intense blue-black colour with starch, so the colour change is really two reactions happening one after the other on the same piece of paper.
8.10 Summary
- Hydrogen chloride has the molecular formula \(\mathrm{HCl}\) and a molecular mass of 36.5. Its single bond is formed by one shared pair of electrons, so the molecule is covalent.
- The shared pair lies nearer the chlorine atom, making the bond polar. Hydrogen chloride is therefore a polar covalent compound.
- The gas occurs free in volcanic emissions. Its solution, hydrochloric acid, is present to the extent of 0.2 to 0.4 percent in the gastric juice of mammals, where it helps in digestion.
- Glauber prepared the acid in 1648 from common salt and concentrated sulphuric acid. Lavoisier named it muriatic acid; Davy renamed it hydrochloric acid in 1810.
- By direct combination, moist hydrogen and chlorine unite in diffused sunlight: \[\mathrm{H_2}\text{(g)} + \mathrm{Cl_2}\text{(g)} \xrightarrow{\ \text{diffused sunlight}\ } 2\,\mathrm{HCl}\text{(g)}\] The reaction is explosive in direct sunlight and negligible in the dark, but proceeds in the dark with activated carbon as catalyst.
- A burning jet of hydrogen continues to burn in chlorine, forming hydrogen chloride.
- Any metallic chloride heated with concentrated sulphuric acid gives the gas, because the less volatile acid displaces the more volatile one from its salt.
- In the laboratory the gas is prepared from sodium chloride and concentrated sulphuric acid. Sodium chloride is chosen because it is cheap; concentrated nitric acid is not used because it is volatile and would come over with the gas.
- Below \(200^\circ\text{C}\): \[\mathrm{NaCl} + \mathrm{H_2SO_4} \xrightarrow{\ <200^\circ\text{C}\ } \mathrm{NaHSO_4} + \mathrm{HCl}\uparrow\] Above \(200^\circ\text{C}\): \[2\,\mathrm{NaCl} + \mathrm{H_2SO_4} \xrightarrow{\ \text{above } 200^\circ\text{C}\ } \mathrm{Na_2SO_4} + 2\,\mathrm{HCl}\]
- The reaction is reversible but goes to completion because the gas escapes continuously.
- Temperatures above \(200^\circ\text{C}\) are avoided: the glass may crack, fuel is wasted, and sodium sulphate forms a hard crust on the glass.
- The gas is dried over concentrated sulphuric acid. Phosphorus pentoxide and quicklime cannot be used, as both react with hydrogen chloride.
- It is collected by upward displacement of air, being 1.28 times heavier than air, and never over water, in which it is highly soluble.
- A full jar shows fumes at its mouth, and a rod dipped in ammonium hydroxide held near it gives dense white fumes: \[\mathrm{HCl} + \mathrm{NH_4OH} \rightarrow \mathrm{NH_4Cl} + \mathrm{H_2O}\]
- The gas is colourless, with a pungent choking smell, corrosive, boiling at \(-85^\circ\text{C}\) and melting at \(-114^\circ\text{C}\), and liquefied at 40 atm and \(10^\circ\text{C}\).
- Its vapour density is 18.25 against 14.4 for air, so it is about one and a quarter times heavier than air. Poured into a jar containing a burning candle, it extinguishes the flame — it is neither combustible nor a supporter of combustion.
- One volume of water dissolves 452 volumes of the gas at room temperature. The fountain experiment demonstrates this: the gas dissolving in a few drops of water lowers the pressure inside the flask, and atmospheric pressure drives blue litmus solution up the jet tube as a red fountain.
- Because of this solubility the gas fumes in moist air, forming droplets of hydrochloric acid.
- Above \(500^\circ\text{C}\) the gas dissociates: \[2\,\mathrm{HCl} \xrightarrow{\ >500^\circ\text{C}\ } \mathrm{H_2} + \mathrm{Cl_2}\]
- Metals above hydrogen in the electrochemical series react with the heated gas to give their chlorides and hydrogen.
- With ammonia the gas forms a solid directly: \[\mathrm{NH_3}\text{(g)} + \mathrm{HCl}\text{(g)} \rightarrow \mathrm{NH_4Cl}\text{(s)}\]
- In water the polar molecule ionises: \[\mathrm{HCl} + \mathrm{H_2O} \rightarrow \mathrm{H_3O^{+}} + \mathrm{Cl^{-}}\] The dry gas and the liquefied gas contain no ions, so they neither turn blue litmus red nor conduct electricity. In toluene the gas dissolves without ionising, and that solution is likewise non-acidic and non-conducting.
- Hydrochloric acid is prepared by dissolving the gas in water through an inverted funnel whose rim just touches the surface. The concentrated product contains about 36 percent hydrogen chloride by mass.
- Back suction occurs because the gas is absorbed faster than it is generated, lowering the pressure inside. The funnel breaks the suction by allowing an air gap to form, and also offers a large surface for absorption; an empty flask placed between the generator and the trough acts as an anti-suction device.
- On distillation the acid concentrates only to 22.2 percent hydrogen chloride and 77.8 percent water, boiling at \(110^\circ\text{C}\). This is a constant boiling mixture, or azeotrope.
- The acid turns blue litmus red and methyl orange pink, and leaves phenolphthalein colourless.
- With metals above hydrogen it gives the chloride and hydrogen; with metallic oxides and hydroxides it gives the chloride and water only.
- It decomposes salts of weaker acids — carbonates and hydrogen carbonates give carbon dioxide, sulphites and hydrogen sulphites give sulphur dioxide, and sulphides give hydrogen sulphide.
- With a thiosulphate it gives sulphur dioxide and also a yellow precipitate of sulphur, which distinguishes a thiosulphate from a sulphite: \[\mathrm{Na_2S_2O_3} + 2\,\mathrm{HCl} \rightarrow 2\,\mathrm{NaCl} + \mathrm{H_2O} + \mathrm{SO_2} + \mathrm{S}\downarrow\]
- Lead nitrate and mercury(I) nitrate give white precipitates of the chlorides; the lead chloride dissolves on heating. Silver nitrate gives a curdy white precipitate of silver chloride: \[\mathrm{AgNO_3} + \mathrm{HCl} \rightarrow \mathrm{AgCl}\downarrow + \mathrm{HNO_3}\]
- Silver chloride is insoluble in nitric acid but dissolves in ammonium hydroxide as diammine silver(I) chloride, from which dilute nitric acid precipitates it again. In light it blackens: \[2\,\mathrm{AgCl} \rightarrow 2\,\mathrm{Ag} + \mathrm{Cl_2}\]
- Strong oxidising agents — manganese dioxide, lead dioxide and red lead with the concentrated acid, bleaching powder with the dilute acid — oxidise the acid to chlorine: \[\mathrm{MnO_2} + 4\,\mathrm{HCl}\text{(conc.)} \xrightarrow{\ \Delta\ } \mathrm{MnCl_2} + 2\,\mathrm{H_2O} + \mathrm{Cl_2}\uparrow\]
- Aqua regia is three parts concentrated hydrochloric acid to one part concentrated nitric acid: \[3\,\mathrm{HCl} + \mathrm{HNO_3} \rightarrow \mathrm{NOCl} + 2\,\mathrm{H_2O} + 2[\mathrm{Cl}]\] The nascent chlorine dissolves gold and platinum as \(\mathrm{AuCl_3}\) and \(\mathrm{PtCl_4}\). It is used to dissolve noble metals, to make chloroauric acid for refining gold, and to dissolve certain ores in analysis.
- The acid is used as a laboratory reagent; in making chlorine and chlorides, dyes, drugs, paints, photographic chemicals and glucose from starch; in pickling steel and cleaning metal surfaces; in removing rust; in purifying bone black and extracting glue from bones; in tanning and calico printing; and in medicine.
- Hydrogen chloride is identified by its irritating smell, by white fumes with ammonia, by a curdy white precipitate with silver nitrate that is insoluble in nitric acid but soluble in ammonium hydroxide, and by the chlorine liberated on heating the concentrated acid with an oxidising agent, which turns moist starch iodide paper blue-black.
Flowchart
FC 8.4 — From Common Salt To Hydrochloric Acid Common salt + conc. sulphuric acid, heated below 200 °C → moist hydrogen chloride gas → dried over conc. sulphuric acid → dry HCl gas collected by upward displacement of air → same gas led through an inverted funnel on to water, with an empty flask guarding against back suction → water saturated with the gas → hydrochloric acid, about 36 percent by mass → distilled → constant boiling mixture, 22.2 percent HCl, boiling at 110 °C
8.11 Key Terms
- Aqua regia — A mixture of three parts concentrated hydrochloric acid and one part concentrated nitric acid, able to dissolve gold and platinum.
- Back suction — The drawing of water backwards into the apparatus, caused when a gas is absorbed faster than it is produced and the pressure inside falls below atmospheric pressure.
- Constant boiling mixture — A solution that boils at a fixed temperature without any change in its composition; hydrochloric acid forms one at \(110^\circ\text{C}\) containing 22.2 percent hydrogen chloride.
- Covalent bond — A bond formed when two atoms share a pair of electrons, the pair counting towards the outer shell of both.
- Direct combination — The union of two elements to form a single compound, with no other product.
- Drying agent — A substance used to remove water vapour from a gas without reacting with the gas itself.
- Fountain experiment — The experiment in which water admitted to a flask of hydrogen chloride dissolves the gas, lowers the pressure inside, and lets atmospheric pressure drive litmus solution in as a fountain, demonstrating the gas’s high solubility.
- Hydrochloric acid — The solution formed when hydrogen chloride gas is dissolved in water.
- Hydrogen chloride — The colourless, pungent covalent gas of formula \(\mathrm{HCl}\), molecular mass 36.5.
- Ionisation — The splitting of a compound into ions, as when hydrogen chloride in water gives hydronium and chloride ions.
- Oxidising agent — A substance that supplies oxygen to another substance, or removes hydrogen from it.
- Pickling — The removal of the oxide coating from a metal surface with acid, before painting, electroplating or galvanising.
- Polar covalent bond — A covalent bond in which the shared pair lies nearer one atom, leaving that atom slightly negative and the other slightly positive.
- Thermal dissociation — The reversible breaking up of a compound by heat, the products recombining on cooling.
- Volatile — Easily converted into vapour; a volatile acid vaporises readily on heating.
8.12 Points To Ponder
Hydrogen and chlorine combine explosively in direct sunlight, steadily in diffused sunlight and hardly at all in the dark. Does the light change how much hydrogen chloride is finally formed, or only how quickly it forms? What difference does that distinction make to a chemist?
The same gas is strongly acidic in water and shows no acidic property at all in toluene. Is acidity, then, a property of a substance — or is it a property of a substance together with the solvent it is placed in?
The preparation of hydrogen chloride is reversible, yet it goes to completion because the product escapes as a gas. Could a chemist use the same trick on a reversible reaction in which every product stays in the flask? What would have to be removed instead, and how?
Boiling cannot concentrate hydrochloric acid beyond 22.2 percent, because the vapour has the same composition as the liquid. What kind of method might get past a barrier of that sort, and what would it have to do that distillation cannot?
Neither concentrated hydrochloric acid nor concentrated nitric acid will dissolve gold, but a mixture of the two will. What does this suggest about the reactive species in a mixture, and how safe is it to predict the behaviour of a mixture from the behaviour of its parts?
Silver chloride darkens where light falls on it and stays white where light does not. What use could that behaviour be put to, and what would have to be controlled for it to be useful rather than a nuisance?
8.13 Objective Questions
- The number of electron pairs in a molecule of hydrogen chloride that take no part in the bonding is: [1]
- one
- two
- three
- four
- Hydrogen chloride gas is not collected over water because: [1]
- it is lighter than air
- it is highly soluble in water
- it reacts with water to give chlorine
- it is combustible
- One reason for keeping the temperature below \(200^\circ\text{C}\) in the laboratory preparation of hydrogen chloride is that above this temperature: [1]
- no gas at all is evolved
- the glass apparatus may crack
- the reaction stops being reversible
- the gas becomes insoluble in water
- Dilute hydrochloric acid is added separately to sodium sulphite and to sodium thiosulphate. The observation seen only with the thiosulphate is: [1]
- a colourless gas is evolved
- the liquid turns milky with a yellow precipitate
- a white precipitate forms that dissolves on heating
- a gas smelling of rotten eggs is evolved
- Aqua regia contains concentrated hydrochloric acid and concentrated nitric acid in the proportion: [1]
- 1 : 1
- 1 : 3
- 3 : 1
- 2 : 1
The following questions are assertion–reason based. Choose the answer from the codes given below. (1) Both A and R are true and R is the correct explanation of A. (2) Both A and R are true but R is not the correct explanation of A. (3) A is true but R is false. (4) A is false but R is true.
- Assertion (A): Silver chloride turns black when left in sunlight. Reason (R): Silver chloride decomposes into metallic silver and chlorine. [1]
- Assertion (A): Concentrated nitric acid is not used in place of concentrated sulphuric acid for preparing hydrogen chloride. Reason (R): Nitric acid is volatile. [1]
- Assertion (A): A solution of hydrogen chloride in toluene does not conduct electricity. Reason (R): Hydrogen chloride is insoluble in toluene. [1]
- Assertion (A): An inverted funnel is used when hydrogen chloride gas is dissolved in water. Reason (R): The funnel offers a large surface for absorption and lets an air gap form that breaks the suction. [1]
- Match each reaction in Column A with the observation in Column B. [4]
| Column A | Column B |
|---|---|
| a. Dilute hydrochloric acid is added to sodium hydrogen carbonate | i. A yellow precipitate appears in the liquid |
| b. Dilute hydrochloric acid is added to iron(II) sulphide | ii. A greenish-yellow gas is evolved |
| c. Dilute hydrochloric acid is added to sodium thiosulphate | iii. A gas is evolved that turns lime water milky |
| d. Concentrated hydrochloric acid is heated with red lead | iv. A gas smelling of rotten eggs is evolved |
- Fill in the blanks, choosing from the words in the brackets. [3]
- Hydrogen chloride gas is collected by ________ displacement of air. [upward / downward]
- The constant boiling mixture of hydrochloric acid contains ________ percent hydrogen chloride by weight. [22.2 / 36 / 77.8]
- Hydrogen chloride in water gives the ________ ion, which makes the solution acidic. [hydroxide / hydronium / chloride]
- State whether each statement is true or false. [3]
- Dry hydrogen chloride gas turns dry blue litmus paper red.
- The white precipitate formed when dilute hydrochloric acid is added to lead nitrate solution dissolves on heating.
- Hydrogen chloride gas supports the combustion of a burning splint.
- Name the following. [4]
- The gas evolved when dilute hydrochloric acid acts on magnesium sulphite.
- The substance used to dry hydrogen chloride gas in the laboratory.
- The complex salt formed when silver chloride dissolves in ammonium hydroxide.
- The acid mixture in which platinum can be dissolved.
- Give one word or phrase for each of the following. [3]
- A solution that boils at a fixed temperature without any change in its composition.
- The breaking up of a compound by heat, the products recombining on cooling.
- The removal of the oxide coating from a metal surface by means of an acid.
- Pick the odd one out in each set and give a reason for your choice. [2]
- \(\mathrm{MnO_2}\), \(\mathrm{PbO_2}\), \(\mathrm{Pb_3O_4}\), \(\mathrm{CuO}\)
- \(\mathrm{Na_2CO_3}\), \(\mathrm{NaHCO_3}\), \(\mathrm{CaCO_3}\), \(\mathrm{Na_2S}\)
8.14 Check Your Concepts
Define thermal dissociation, and give one example of it from the chemistry of hydrogen chloride. [2]
Give a reason why quicklime cannot be used to dry hydrogen chloride gas, and support your answer with a balanced equation. [2]
Differentiate between hydrogen chloride and hydrochloric acid, stating clearly the basis of the difference. [2]
Give a reason for each of the following. [3]
- A bottle of concentrated hydrochloric acid gives fumes when it is opened.
- Copper does not liberate hydrogen from hydrochloric acid, although magnesium does.
- Sodium chloride is preferred to other metallic chlorides for preparing hydrogen chloride.
Why is hydrogen chloride gas collected by upward displacement of air? Support your answer using the vapour densities of hydrogen chloride and of air. [3]
With reference to the fountain experiment, state what is observed when the dropper is pressed, and explain both why the liquid rises and why the fountain is red. [4]
Explain, in terms of ions, why a solution of hydrogen chloride in water is acidic and conducts electricity, while a solution of the same gas in toluene does neither. State what you would observe with blue litmus in each case. [4]
What would happen if hydrogen chloride gas were led into water through a plain narrow delivery tube instead of an inverted funnel? Describe the effect, explain its cause, and state the precaution that guards against it. [4]
Explain why a dilute solution of hydrochloric acid cannot be concentrated beyond 22.2 percent by boiling, and state the boiling point of the mixture obtained. [3]
Describe the laboratory preparation of dry hydrogen chloride gas under the following heads: reactants, condition, drying, collection, and identification of a full jar. [5]
8.15 Equations, Conversions & Identification
Write balanced equations, with conditions, for the action of concentrated sulphuric acid on sodium chloride: [2]
- below \(200^\circ\text{C}\)
- above \(200^\circ\text{C}\)
Complete and balance the following equations, stating in each case whether the dilute or the concentrated acid is used. [4]
- \(\mathrm{Fe_2O_3} + \mathrm{HCl} \rightarrow\)
- \(\mathrm{Ca(HCO_3)_2} + \mathrm{HCl} \rightarrow\)
- \(\mathrm{NaHSO_3} + \mathrm{HCl} \rightarrow\)
- \(\mathrm{Pb_3O_4} + \mathrm{HCl} \rightarrow\)
Name the gas evolved in each case below, and give one test by which you would identify it. [3]
- Dilute hydrochloric acid is added to magnesium sulphite.
- Dilute hydrochloric acid is added to sodium sulphide.
- Concentrated hydrochloric acid is heated with manganese dioxide.
State what you would see in each of the following. [4]
- A rod moistened with ammonium hydroxide is held at the mouth of a jar of hydrogen chloride.
- Dilute hydrochloric acid is added to lead nitrate solution, and the mixture is then warmed.
- Silver chloride, freshly precipitated, is left on a watch glass in bright sunlight.
- Dilute hydrochloric acid is added to sodium thiosulphate solution.
How would you obtain each of the following from hydrogen chloride? Write the equation in each case. [3]
- Iron(II) chloride
- Ammonium chloride
- Silver chloride
A colourless liquid X gives a curdy white precipitate with silver nitrate solution. The precipitate is unaffected by dilute nitric acid but dissolves in ammonium hydroxide. When X is warmed with manganese dioxide, a greenish-yellow gas is evolved. Identify X, identify the gas, and write the equation for its formation. [3]
Refer to Fig. 8.3-1. [3]
- Name the liquid in the washer bottle and state its purpose.
- State why the lower end of the thistle funnel must dip below the surface of the liquid in the flask.
- State how you would know that the gas jar has filled with the gas.
Refer to Fig. 8.6-1 and Fig. 8.6-2. [3]
- Name the acid prepared by this arrangement.
- State the two functions of the inverted funnel.
- State the purpose of the empty flask placed before the trough.