Chemical changes: reactivity, acids and electrolysis — AQA GCSE Chemistry revision

The reactivity series

• Metals can be ranked by how readily they lose electrons to form positive ions.
• More reactive metals displace less reactive ones from their compounds in solution.
• Carbon and hydrogen are often placed within the series so metals can be classed as extractable by carbon or not.
• Metals above carbon usually need electrolysis for extraction; metals below carbon can be reduced using carbon.

Acids, bases and salts

• Acids release hydrogen ions (H+) when dissolved in water; alkalis release hydroxide ions (OH-).
• Neutralisation between an acid and a base produces a salt plus water.
• Acid + metal gives a salt plus hydrogen gas; acid + carbonate gives a salt, water and carbon dioxide.
• The name of the salt depends on the metal (or base) and the acid used, e.g. hydrochloric acid gives chloride salts.

Key terms

Displacement reaction

A reaction where a more reactive element takes the place of a less reactive one in a compound.

Oxidation

A process in which a substance loses electrons (or gains oxygen).

Reduction

A process in which a substance gains electrons (or loses oxygen).

Electrolyte

A molten or dissolved ionic substance that conducts electricity and is broken down during electrolysis.

Cathode

The negative electrode, where positive ions gain electrons.

Anode

The positive electrode, where negative ions lose electrons.

Common exam traps

• Do not say a metal 'loses' oxygen during oxidation — oxidation means losing electrons or gaining oxygen, it's easy to muddle the two.
• Remember it is the cathode that is negative in electrolysis, which can feel backwards compared to a normal cell.
• In electrolysis of aqueous solutions, don't assume the metal always forms at the cathode — if the metal is reactive, hydrogen forms instead.

Practice questions with answers

1. 1. Which gas is produced when an acid reacts with a metal?

   • • Oxygen
   • • Hydrogen
   • • Carbon dioxide
   • • Nitrogen

   Answer: Hydrogen

   Acid + metal produces a salt and hydrogen gas.

2. 2. What is produced when an acid reacts with a carbonate?

   • • Hydrogen
   • • Water only
   • • Carbon dioxide, water and a salt
   • • Oxygen

   Answer: Carbon dioxide, water and a salt

   Acid + carbonate gives salt + water + carbon dioxide.

3. 3. Name the products of neutralisation between an acid and an alkali.

   Answer: A salt and water

   This is the general pattern for acid–alkali neutralisation.

4. 4. At which electrode does reduction occur during electrolysis?

   • • Anode
   • • Cathode
   • • Both equally
   • • Neither

   Answer: Cathode

   Positive ions gain electrons (reduction) at the negative cathode.

5. 5. Which ion is responsible for acidity in aqueous solution?

   • • OH-
   • • H+
   • • Cl-
   • • Na+

   Answer: H+

   Acids release hydrogen ions in solution.

More free Chemistry revision

• All AQA Chemistry revision pages
• All GCSE Chemistry topics
• AQA Chemistry: Atoms, elements and the periodic table
• AQA Chemistry: Ionic, covalent and metallic bonding
• AQA Chemistry: Moles, masses and conservation
• AQA Chemistry: Rates of reaction and energy changes
• AQA Chemistry: Energy changes: exothermic, endothermic and bond energies
• AQA Chemistry: Reversible reactions and equilibrium
• Every subject on ReviseRush