Neutralisation is an acid reacting with a base
Definition
Neutralisation
The reaction in which hydrogen ions from an acid join with hydroxide ions from an alkali to form water.
Definition
Base
A substance that reacts with an acid to form a salt and water only.
- In a neutralisation an acid and a base react together to give a salt and water.
- Both starting substances lose their acidic and basic character as the reaction proceeds.
- An alkali is a soluble base, so an acid and an alkali neutralise each other in solution.
- The general pattern is acid plus base giving salt plus water.
- That pattern holds whichever acid and whichever base are used.
Key Idea
- Acid plus base gives a salt and water, and nothing else.
- An alkali is the soluble kind of base, which is why this reaction happens in solution.
Hydrogen ions and hydroxide ions combine to make water
Definition
Aqueous solution
A solution made by dissolving a substance in water, shown in an equation by the state symbol (aq).
- An acid in solution supplies hydrogen ions, H+(aq)\text{H}^{+}(aq)H+(aq).
- An alkali in solution supplies hydroxide ions, OH−(aq)\text{OH}^{-}(aq)OH−(aq).
- One of each holds exactly the atoms of one water molecule, so they react in a 1:11:11:1 ratio: H+(aq)+OH−(aq)→H2O(l)\text{H}^{+}(aq) + \text{OH}^{-}(aq) \rightarrow \text{H}_2\text{O}(l)H+(aq)+OH−(aq)→H2O(l)
- Removing those ions from the solution is what neutralisation means at the particle level.
- The equation is the same for every acid and alkali, because each supplies one of the two ions.
Example
- Hydrochloric acid and sodium hydroxide: HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l).
- The part that reacts: the H+\text{H}^{+}H+ from the acid and the OH−\text{OH}^{-}OH− from the alkali form the water.
The other ions are spectators
Definition
Spectator ion
An ion that is unchanged by the reaction and appears on both sides of the full ionic equation, so it is cancelled out.
Definition
Ionic equation
An equation that shows only the ions and substances that change during a reaction, with the spectator ions left out.
- Sodium ions and chloride ions are present before the reaction and afterwards, unchanged.
- Ions that do not change are left out of the ionic equation.
- What remains is the change that actually happens, between H+\text{H}^{+}H+ and OH−\text{OH}^{-}OH−.
- Those spectator ions stay in solution and become the salt once the water is evaporated.
- The full equation shows the salt, while the ionic equation shows the change.
Note
- The salt still forms, even though the ionic equation does not show it.
- State symbols matter here, because the water forms as a liquid among aqueous ions.
Writing about neutralisation
- Identify the acid and say that it supplies H+\text{H}^{+}H+.
- Identify the base or alkali and say that it supplies OH−\text{OH}^{-}OH−.
- State that those two ions react together to form water.
- Name the salt, which takes its name from the metal and from the acid.
- Give the ionic equation with state symbols whenever the question asks for the particle-level change.
Self review
- What is meant by a neutralisation reaction?
- What is the difference between a base and an alkali?
- Which ion comes from the acid and which from the alkali?
- What is the ionic equation for the neutralisation of an acid by an alkali?
- Why are sodium and chloride ions called spectator ions?
