Properties of acids & bases

8.2 Properties of acids and bases (1 hour)

Pause for thought

The characteristic properties of acids are generally well known. Reactions of acids with reactive metals, metal oxides, hydroxides, carbonates and hydrogencarbonates are all obvious examples. Other less obvious examples which are not technically on the IB programme are reactions with sulfides and sulfites. You should also include their characteristic reaction with acid-base indicators such as litmus, phenolphthalein, methyl orange and universal indicator. It is less obvious to list the characteristic properties of bases. Firstly a base needs to be defined and although the previous sub-topic, 8.1 : Theories of acids & bases covered the Brønsted-Lowry definition I think what is meant here by a base is simply a substance that neutralises an acid. Consequently there are only three characteristic properties of bases:

1. They neutralize acids.

2. If they are soluble in water they give a characteristic colour with acid-base indicators indicating a pH value greater than 7.

3. Hydroxides react with ammonium salts when heated to form ammonia.

e.g. NH₄Cl(s) + NaOH(aq) → NH₃(g) + NaCl(aq) + H₂O(l)

Ionically the ammonium ion is behaving as a Brønsted-Lowry acid and the hydroxide ion is behaving as a Brønsted-Lowry base:

NH₄⁺(s) + OH^–(aq) → NH₃(g) + H₂O(l)

Utilization of the reverse of this reaction is made in Option D: Medicinal chemistry as many drugs contain an basic amine (substituted ammonia) functional group and can be made much more polar and hence more soluble in aqueous solution by reacting them with hydrochloric acid to convert them into a salt.

The magical diving submarine that works on baking powder

One toy from the past is a baking powder submarine. These were once given away in cereal packets. Baking powder or bicarbonate of soda is essentially sodium hydrogencarbonate. In water or slightly acidic solution the hydrogencarbonate ion behaves as a base and reacts to form carbon dioxide.

HCO₃^–(s) + H₃O⁺(l) → CO₂(g) + 2H₂O(l)

When the submarine containing a small amount of baking powder was placed in water the gas evolved caused the submarine to surface then it would dive as the gas was expelled and then rise up again - magic! - and a really nice way to show a characteristic reaction of a particular type of base.

Nature of Science

This sub-topic is an example of how obtaining evidence for theories arises from experimental work. The modification of acid–base theories has developed from the observable properties of the reactions of acids and bases.

Learning outcomes

After studying this topic students should be able to:

Understand:

Most acids react with reactive metals, metal oxides, metal hydroxides, hydrogen carbonates and carbonates in an observable and characteristic way.
Neutralisation reactions are exothermic and produce salt and water.

Apply their knowledge to:

Balance chemical equations for the reaction of acids.
Identify the acid and base required to make different salts.
Perform acid-base titrations with different indicators.

Clarification notes

Bases other than hydroxides, such as ammonia, soluble carbonates and hydrogencarbonates should also be covered.
Different indicators together with their colour changes are given in Section 22 of the data booklet.

International-mindedness

Nothing is listed on the syllabus under this heading.

Teaching tips

This is a fun topic to teach as you can cover most of it through practical work. Probably by now your students will already have done several acid-base titrations to cover some of the material in Topic 1: Stoichiometric relationships and often they will have prepared salts in classes lower down the school.

I give them the reactions in the practical Common chemical reactions and this not only shows the test tube reactions but also covers how to test for some basic gases such as hydrogen, carbon dioxide and hydrogen chloride. Get them to explain as much as they can including writing the relevant equations. Also impress upon them the ionic equations which better explain the underlying acid-base chemistry. If you have time it is nice to demonstrate the fountain experiment which not only shows the basic properties of ammonia but also its extreme solubility in water.

If you can get hold of a baking powder submarine from your local toy shop they will enjoy playing with it!

Study guide

Page 59

Questions

For ten 'quiz' multiple choice questions with the answers explained see MC test: Properties of acids & bases.

For short-answer questions which can be set as an assignment for a test, homework or given for self study together with model answers see Properties of acids & bases questions.