Acids, bases and salts III
Francine Taylor-Campbell, Contributor

Let us examine the following questions.

1. (i) Ethanoic acid is a weak acid. Metals, carbonates and alcohols are some substances with which it reacts. Explain the term 'weak acid'. (2 marks)

(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 marks)

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 concentration, for example 1M HCl and 1M CH3COOH, but different pH values since they have a 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) + H2 (g)

sodium ethanoate

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

calcium ethanoate

(iii) As stated before, ethanoic acid is a weak acid, while hydrochloric acid is a strong one. 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.

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 marks)

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

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

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

ANSWERS

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

(b) (i) pH 2 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.

These two peer into a microscope at the recent opening of the McGrath High School Centre of Excellence. - Ian Allen/Staff Photographer

Francine Taylor-Campbell is an independent contributor.