Acids, bases and salts cont'd
Francine Taylor-Campbell, Contributor

Students from Waterford High School catch up on their study, at the Portmore Mall, recently. - Anthony Minott/Freelance Photographer

You should be able to:

Investigate the reactions of non-oxidising acids with metals, carbonates, hydrogen carbonates and bases.

Investigate the reaction of bases with ammonium salts.

Identify an appropriate method of salt preparation, based on the solubility of the salt.

Distinguish between acid salts and normal salts.

POINTS TO NOTE

• 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.

A salt is formed when all or part of the hydrogen in an acid is replaced by a metal ion or the ammonium ion. Acids generally consist of a negative ion from an acid and a positive ion from a base or metal. E.g. NaCl Na+ is from a metal or base (NaOH) and Cl- is from the acid (HCl). If all the replaceable hydrogen ions are removed, a normal salt is formed, e.g. NaCl. If only part of the replaceable hydrogen ions is removed, then an acid salt is formed, e.g. NaHCO3.

Let us examine the following questions:

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 marks)

(ii) Describe how ethanoic acid reacts with a named metal and 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 was used instead of ethanoic acid. (10 marks)

ANSWERS

1. 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 different numbers 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 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.

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 I

II - 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.

Neutralisation reactions in everyday life

1. Antacids - These are mild alkalis or basic salts used to neutralise the acidity on the stomach, which causes indigestion. Examples include calcium and magnesium carbonate, magnesium oxide and hydroxide. These substances are able to neutralise excess stomach acidity by reacting with the acid.

2. During baking, the use of baking powder allows the dough to rise. This happens because baking powder is a carbonate which, when heated, produces carbon dioxide gas which is responsible for raising the dough.

Francine Taylor-Campbell is an independent contributor.