Acids, bases and salts (II)
Francine Taylor-Campbell, Contributor

Students preparing to sit CXC exams in May and June arrive at a Gleaner-sponsored techniques seminar at the Girl Guides Association of Jamaica headquarters on Waterloo Road, St. Andrew. - Junior Dowie/Staff Photographer

IMPORTANT POINTS

Strong acids and weak acids differ in the number of molecules that react or dissociate to form ions. For a strong acid, a vast majority of the molecules form ions, while only a small proportion of ions are formed from a weak acid.

Dibasic and tribasic acids can form both normal and acid salts.

Soluble salts can be prepared from the action of an acid on a metal, carbonate and/or base. Insoluble salts are obtained from ionic precipitation reactions between two soluble salt solutions. Anhydrous salts must be prepared by direct combination.

Let us examine the following questions:

Question 1

1. (a) Ethanoic acid is a weak acid. Metals, carbonates and alcohols are some substances with which it reacts.

(i) Explain the term 'weak acid'. (2 mks)

(ii) Describe how ethanoic acid reacts with a named

... metal
... carbonate.

State one balanced equation for each reaction.

(iii) Explain how the reactions with the named metal and carbonate would differ if dilute hydrochloric acid were used instead of ethanoic acid. (10 mks)

ANSWERS

(I) A weak acid is one which ionises in solution to produce relatively few ions; namely hydrogen ions.

Comment: Two acids can have the same oncentration, for example, 1M HCl and 1M CH 3 COOH, but different pH values since they have different number of dissociated ions in solution.

(ii) Ethanoic acid will react relatively slowly with sodium and calcium carbonate since it is a weak acid. The reactions will produce aqueous salt solutions along with evolution of hydrogen and carbon dioxide gases.

COMMENT: Note that although it is a weak acid, ethanoic acid will still behave as a typical acid, forming salt solutions with metals and carbonates.

2Na (s) + 2CH3COOH (aq) = 2CH3COONa(aq) + H3(g)
sodium ethanoate

CaCO3(s) + 2CH3COOH(aq) = (CH3COO)2Ca(aq) + H2O(l) + CO2(g )
calcium ethanoate

As stated before, ethanoic acid is a weak acid, while hydrochloric acid is a strong acid. Hydrochloric acid would therefore react more vigorously with sodium and calcium carbonate. This would be seen in a faster evolution of hydrogen and carbon dioxide.

Question 2

2. You are given four solutions I, II, III and IV which have the pH values listed below.

I pH 2 II pH 7 III - pH 8 IV pH 13

(a) Classify each according to one of the following descriptions: neutral. strongly acidic or alkaline and slightly acidic or alkaline. (4 mks)

(b) (i) Which solution would most likely liberate hydrogen when reacted with magnesium metal? (1 mk)

(ii) Write an ionic equation for a likely reaction of the solution selected in (b) (i) above with magnesium. (2 mks)

(c) In which of the solutions I to IV would you say that the solute is almost completely ionised? (2 mks)

ANSWERS

2 . I strongly acidic II neutral III - slightly alkaline IV strongly alkaline

(b) (i) pH2 solution would most likely liberate hydrogen when reacted with magnesium metal. Metals react with acidic solutions to liberate hydrogen and form a salt.

(ii ) Mg(s) + 2H+ (aq) = Mg2+ (aq) + H2 (g)

An acidic solution is represented by H+ ions. The solid metal goes into solution as ions, while the H+ ions are displaced to form hydrogen gas.

(iii) The solute is almost completely ionised in solutions I and IV. These are strongly acidic and alkaline solutions.

TRY THIS QUESTION

A student made the following remark: "To prepare lead nitrate, add dilute nitric acid to lead carbonate and to prepare lead chloride, add dilute hydrochloric acid to lead carbonate." To what extent do you agree or disagree with the methods described above. Give reasons for your answer. Give full details of how you would prepare solid samples of lead nitrate and lead chloride starting from lead carbonate. (11 mks)

Francine Taylor-Campbell is an independent contributor.