Acids, alkalis and the pH scale; neutralisation; reactions of acids with metals, bases, alkalis and carbonates; making soluble salts; strong and weak acids.

What this dot point is asking

AQA wants you to describe the pH scale and acids and alkalis in terms of hydrogen and hydroxide ions, explain neutralisation, write the products of acids reacting with metals, bases, alkalis and carbonates, describe how to make a pure dry soluble salt, and explain the difference between strong and weak acids. The four reaction patterns and the salt-preparation practical are reliable Paper 1 questions.

Acids, alkalis and pH

Neutralisation

This ionic equation is the same for every acid-alkali neutralisation, whatever the salt produced, which is why the temperature change in any neutralisation is similar.

Reactions of acids

• Acid + metal $\rightarrow$ salt + hydrogen.
• Acid + metal oxide (base) $\rightarrow$ salt + water.
• Acid + metal hydroxide (alkali) $\rightarrow$ salt + water.
• Acid + metal carbonate $\rightarrow$ salt + water + carbon dioxide.

The salt made depends on the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates. The first part of the salt name comes from the metal (for example, magnesium sulfate from magnesium and sulfuric acid).

Making a soluble salt

To make a pure, dry soluble salt (a required practical):

1. Add excess insoluble base (or carbonate) to warm acid until no more reacts, so all the acid is used up.
2. Filter off the excess solid.
3. Crystallise by evaporating some water to the point of crystallisation, then leaving the solution to cool and form crystals; dry the crystals between filter paper.

Using an excess of base, then filtering, guarantees there is no leftover acid in the final crystals and no unreacted base in the solution.

Strong and weak acids

A decrease of one unit on the pH scale means the hydrogen ion concentration increases by a factor of $10$, so a pH 2 acid has ten times the hydrogen ion concentration of a pH 3 acid (Higher tier).

Try this

Q1. Write the products when hydrochloric acid reacts with calcium carbonate. [2 marks]

• Cue. Calcium chloride, water and carbon dioxide.

Q2. Explain the difference between a strong acid and a weak acid. [2 marks]

• Cue. A strong acid is fully ionised in water; a weak acid is only partly ionised.

Q3. Name the salt formed when sulfuric acid reacts with sodium hydroxide. [1 mark]

• Cue. Sodium sulfate.