Chemistry, Third edition

PRODUCING IONS IN WATER BY CHEMICAL REACTION

The reaction between HCl gas and water makes hydrogen chloride extremely soluble

in water (see the fountain experiment, page 186). A considerable amount of heat is

also given out, although this is quickly absorbed by the water.

Reactions of hydrochloric acid

The reactions of hydrochloric acid are simply the reactions of the hydronium and

chloride ions. Concentrated and dilute hydrochloric acid display similar reactions,

but the concentrated acid reacts at a faster rate.

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Hydronium ion

In the reaction between HCl(g) and H 2 O(l), a proton (H) bonds to the water molecule by

sharing a pair of electrons from the oxygen atom in the water molecule. Do you remember

the name of this type of covalent bond?

Exercise 6D

Reaction of hydrogen bromide and water

Hydrogen bromide, HBr(g), dissolves in water in a similar way to HCl(g) to make

hydrobromic acid, HBr(aq). Represent this by an ionic equation.

Exercise 6E

The H(aq) ion is the ion that is responsible for the reactions of all acids. Such

reactions are known as acidic properties. One of the best known reactions of H(aq)

ions and, therefore, of acids in general, is their ability to cause acid–base indicators

to change colour. The commonest indicator is litmus (litmus is usually absorbed in

paper) which turns red in an acidic solution and blue in basic (alkaline) solution. It

follows that dry HCl gas does not affect dry blue litmus paper, but if the HCl gas or

the litmus is damp, then the litmus will turn red.

The presence of the chloride ion in hydrochloric acid may be confirmed by

adding silver ions, when a white precipitate of silver chloride (AgCl) is seen:

Ag(aq)Cl(aq)AgCl(s)

Silver nitrate solution is usually used as the source of silver ions. A precipitate is

obtained by mixing silver ions with anychloride and this is used as the basis for a test

to detect the presence of a chloride. In testing unknown solutions using silver nitrate,

it is usual to add dilute nitric acid to prevent the precipitation of other insoluble silver

compounds, such as silver carbonate and silver sulfite.

Before moving on, it is interesting to note that hydrogen chloride gas dissolves in

benzene (a non-polar solvent) without producing any ions. Such a solution contains

hydrogen chloride molecules weakly attracted (solvated) to the surrounding benzene

molecules. As we might expect, a solution of HCl in benzene does not conduct

electricity and it does not show any of the reactions of hydrochloric acid.