Chlorine and Its Inorganic Compounds

Lessons

Senior 3 · Chemistry

Chlorine and Its Inorganic Compounds

4 lessons

Lesson 3 of 4

Hydrogen Chloride and Hydrochloric Acid

25 min

An acid inside you

Your stomach makes about one to two litres of gastric juice a day, and its acid is hydrochloric acid, strong enough to kill most bacteria in food and to help digest proteins. The same acid is used in industry to clean rust from steel and scale from boilers. Hydrochloric acid is a solution of a gas, hydrogen chloride, in water. The gas and its solution behave very differently, and the difference shows what an acid really is.

Preparing hydrogen chloride

Hydrogen chloride is made by warming sodium chloride with concentrated sulfuric acid (Figure 3.1). As you saw in Unit 3, the involatile sulfuric acid drives out the volatile hydrogen chloride:

$\mathrm{NaCl(s) + H_2SO_4(l) \rightarrow NaHSO_4(s) + HCl(g)}$

With stronger heating, sodium sulfate forms instead: $\mathrm{2NaCl(s) + H_2SO_4(l) \rightarrow Na_2SO_4(s) + 2HCl(g)}$ .

The gas is dried with concentrated sulfuric acid, which does not react with it.
It is collected by downward delivery: it is denser than air ( $M_{r} = 36.5$ ) and extremely soluble in water.
To make hydrochloric acid, the gas is dissolved in water through an upside-down funnel whose rim just touches the water (Figure 3.2). The gas dissolves so fast that water would be sucked back into the hot flask up a narrow tube. With the funnel, any water that rises into it lowers the level in the beaker below the rim, so the water falls back.

Figure 3.1: Laboratory preparation of dry hydrogen chloride (a teacher demonstration).

Warning

Safety. Hydrogen chloride is corrosive and toxic: it attacks the eyes and lungs. Work in a fume cupboard with eye protection. Concentrated sulfuric acid causes severe burns (Unit 3). Never let a delivery tube dip into water while the flask is hot: suck-back can crack the flask.

Figure 3.2: Dissolving hydrogen chloride in water safely.

Properties of hydrogen chloride gas

A colourless gas with a sharp, choking smell. It fumes in moist air, because it dissolves in water vapour to form tiny droplets of hydrochloric acid.
Denser than air, and extremely soluble in water; like ammonia, it gives a fountain experiment.
Test: it forms dense white fumes of ammonium chloride with ammonia (a glass rod dipped in concentrated ammonia solution): $\mathrm{NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)}$ . It turns damp blue litmus red, but does not bleach it, unlike chlorine.

Dry hydrogen chloride is not an acid. It is a covalent gas made of HCl molecules. Dry gas does not turn dry blue litmus red, and a solution of it in methylbenzene (a solvent that is not water) does not react with magnesium or conduct electricity. Only when it dissolves in water does it ionise, forming hydrogen ions:

$\mathrm{HCl(g) \xrightarrow{\text{water}} H^+(aq) + Cl^-(aq)}$

It is the hydrogen ions that give hydrochloric acid its acidic properties.

Definition

Hydrochloric acid: a solution of hydrogen chloride gas in water. Hydrogen chloride ionises completely in water, so hydrochloric acid is a strong acid: $\mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)}$ . Concentrated hydrochloric acid contains about 36% hydrogen chloride by mass.

Reactions of hydrochloric acid

Table 3.1: Reactions of dilute hydrochloric acid

Reacts with	Products	Example
metals above hydrogen in the reactivity series	a chloride and hydrogen	$\mathrm{Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)}$
iron	iron(II) chloride (not iron(III)) and hydrogen	$\mathrm{Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g)}$
bases (metal oxides and hydroxides)	a chloride and water	$\mathrm{CuO(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l)}$
carbonates and hydrogencarbonates	a chloride, water and carbon dioxide	$\mathrm{NaHCO_3(s) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g)}$
manganese(IV) oxide (concentrated acid, warm)	chlorine (Lesson 1)	$\mathrm{MnO_2(s) + 4HCl(aq) \rightarrow MnCl_2(aq) + Cl_2(g) + 2H_2O(l)}$
silver nitrate solution	white precipitate of silver chloride (Lesson 4)	$\mathrm{AgNO_3(aq) + HCl(aq) \rightarrow AgCl(s) + HNO_3(aq)}$

Copper, silver and gold, which are below hydrogen in the reactivity series, do not react with hydrochloric acid. And, unlike nitric acid, hydrochloric acid is not an oxidising acid: its reactions with metals really do give hydrogen.

Uses of hydrochloric acid

Cleaning steel ("pickling") before it is galvanised or painted: it dissolves rust.
Removing scale (calcium carbonate) from kettles, boilers and pipes.
Making chlorides and other chemicals, and controlling the pH of water and swimming pools.
In the stomach, to kill germs in food and help digest protein.

Worked example

Problem. What mass of sodium chloride must react with concentrated sulfuric acid, forming sodium hydrogensulfate, to give $2.4\,\mathrm{dm^3}$ of hydrogen chloride at room temperature and pressure? (Na = 23, Cl = 35.5; molar volume $24\,\mathrm{dm^3\,mol^{-1}}$ )

Solution. $n(\mathrm{HCl}) = \frac{2.4}{24} = 0.10\,\mathrm{mol}$ . From $\mathrm{NaCl(s) + H_2SO_4(l) \rightarrow NaHSO_4(s) + HCl(g)}$ , 1 mol NaCl gives 1 mol HCl, so 0.10 mol NaCl is needed.

$M(\mathrm{NaCl}) = 23 + 35.5 = 58.5\,\mathrm{g\,mol^{-1}}$ , so the mass is $0.10 \times 58.5 = 5.85\,\mathrm{g}$ .

Worked example

Problem. Two gas jars contain colourless gases A and B. Both have a sharp, choking smell and turn damp blue litmus red. A fumes in moist air and gives dense white fumes with ammonia; B turns limewater milky and turns acidified potassium dichromate(VI) green. Identify A and B.

Solution. Both are acidic gases. The white fumes with ammonia (ammonium chloride) show that A is hydrogen chloride. B reduces dichromate(VI) and turns limewater milky, so B is sulfur dioxide (Unit 3). Neither is chlorine, which is yellow-green and would bleach the litmus.

Tip

No water, no acid. Hydrogen chloride only behaves as an acid once it has dissolved in water and formed $\mathrm{H^+(aq)}$ ions. And remember the inverted funnel whenever a very soluble gas is dissolved in water.

Royal Society of Chemistry: The reaction of ammonia and hydrogen chloride (4 min). Watch where the white ring of ammonium chloride forms in the tube, and think about which gas moves faster.

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Practice

Write the equation for the laboratory preparation of hydrogen chloride from sodium chloride.
Why is hydrogen chloride (a) dried with concentrated sulfuric acid, (b) collected by downward delivery?
Explain how the inverted funnel in Figure 3.2 prevents suck-back.
Describe the test for hydrogen chloride and write the equation.
A solution of hydrogen chloride in methylbenzene does not react with magnesium, but a solution in water does. Explain.
Write equations for the reactions of dilute hydrochloric acid with (a) zinc, (b) calcium carbonate, (c) sodium hydroxide.
Why does iron react with hydrochloric acid to form iron(II) chloride rather than iron(III) chloride, while it forms iron(III) chloride with chlorine?
What volume of hydrogen, at room temperature and pressure, is given off when 1.2 g of magnesium reacts with excess dilute hydrochloric acid? (Mg = 24; molar volume $24\,\mathrm{dm^3\,mol^{-1}}$ )

Show answersHide answers

$\mathrm{NaCl(s) + H_2SO_4(l) \rightarrow NaHSO_4(s) + HCl(g)}$ .
(a) It is an acidic gas and does not react with the acid. (b) It is denser than air ( $M_{r}$ 36.5 against 29) and far too soluble to collect over water.
If water rises into the wide funnel, the level in the beaker drops below the rim; air then enters and the water in the funnel falls back, so it never reaches the flask.
Hold a glass rod dipped in concentrated ammonia solution in the gas: dense white fumes form. $\mathrm{NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)}$ .
In methylbenzene hydrogen chloride stays as covalent HCl molecules, so there are no hydrogen ions to react with magnesium. In water it ionises to $\mathrm{H^+(aq)}$ and $\mathrm{Cl^-(aq)}$ , and the hydrogen ions react.
(a) $\mathrm{Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)}$ ; (b) $\mathrm{CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)}$ ; (c) $\mathrm{NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)}$ .
Hydrogen ions are only a mild oxidising agent, so they oxidise iron only to iron(II). Chlorine is a much stronger oxidising agent and oxidises iron all the way to iron(III).
$n(\mathrm{Mg}) = \frac{1.2}{24} = 0.050\,\mathrm{mol}$ , which gives 0.050 mol $\mathrm{H_2}$ ; volume $= 0.050 \times 24 = 1.2\,\mathrm{dm^3}$ .

Sources and credits

Written by Ganzaa.
Video: Diffusion in action: the reaction of ammonia and hydrogen chloride demonstration, Royal Society of Chemistry, embedded from YouTube.
The diagrams, table, worked examples and Practice items were written and drawn by Ganzaa.

1An acid inside you

2Preparing hydrogen chloride

3Properties of hydrogen chloride gas

4Reactions of hydrochloric acid

5Uses of hydrochloric acid

An acid inside you

Preparing hydrogen chloride

Properties of hydrogen chloride gas

Reactions of hydrochloric acid

Uses of hydrochloric acid