Acids, alkalis and the pH scale, neutralisation, the reactions of acids with metals, bases and carbonates, making soluble salts, and electrolysis of molten compounds and solutions.

What this dot point is asking

Edexcel wants you to describe acids, alkalis and the pH scale in terms of ions, explain neutralisation, write the products of acids reacting with metals, bases and carbonates, describe how to make a soluble salt, and describe the electrolysis of molten compounds and solutions.

Acids, alkalis and pH

Neutralisation

This ionic equation is the same for every acid-alkali neutralisation, whatever salt is produced.

Reactions of acids

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

The salt depends on the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates.

Making a soluble salt

To make a pure, dry soluble salt (a core practical): add excess insoluble base (or carbonate) to warm acid until no more reacts; filter off the excess solid; then crystallise by evaporating some water and leaving it to cool. Using excess base ensures all the acid is used up.

Electrolysis

For a molten compound, the metal forms at the cathode and the non-metal at the anode (for example molten lead bromide gives lead and bromine). For a solution, hydrogen is produced at the cathode instead of the metal if the metal is more reactive than hydrogen, and at the anode a halogen forms if a halide is present, otherwise oxygen.

Electrolysis is widely used in industry. It is the only way to extract very reactive metals such as aluminium from their ores, because they are too reactive to be displaced by carbon. It is also used to electroplate objects with a thin layer of metal (for example coating steel with chromium for protection and appearance), and to purify copper. These uses all rely on the same principle: an electric current forces ions to gain or lose electrons at the electrodes.

Try this

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

• Cue. Calcium chloride, water and carbon dioxide.

Q2. Name the product at the cathode when molten sodium chloride is electrolysed. [1 mark]

• Cue. Sodium (metal).