Spectator Ions in Neutralisation Reactions (SQA National 5 Chemistry): Revision Note
Exam code: X813 75
Spectator ions
What is a spectator ion?
A spectator ion is an ion that does not change its state or charge during a chemical reaction
They are present in the solution but are not involved in the formation of the product
How to find spectator ions
Start with a full, balanced chemical equation with state symbols
Break down all aqueous (aq) ionic compounds into their separate ions
Do not split up any substances that are solids (s), liquids (l), or gases (g)
Identify the ions that appear in the exact same form on both the reactant (left) and product (right) sides of the equation
These are the spectator ions
The equation written without them is called a net ionic equation
Worked Example
The balanced equation for the neutralisation of hydrochloric acid by sodium hydroxide is shown below:
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
Identify the spectator ions and write the final ionic equation for this reaction.
[2]
Answer:
The full, balanced chemical equation is:
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
Break down aqueous compounds into ions:
H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq) → Na+ (aq) + Cl- (aq) + H2O (l)
(Note: H2O (l) is a liquid, so it is not split into ions)
Identify spectator ions:
The Na+ (aq) ion is on both sides, unchanged
The Cl- (aq) ion is on both sides, unchanged
So, the spectator ions are Na+ (aq) and Cl- (aq) [1 mark]
Write the ionic equation (without spectator ions):
H+ (aq) + OH- (aq) → H2O (l) [1 mark]
This ionic equation shows the real chemical change that is happening, the formation of water
For National 5, you need to know the ionic equations for acids reacting with different types of bases
The acid always supplies the H+ (aq) ions
Type of base | General reaction | Key ionic equation |
|---|---|---|
metal hydroxides (e.g. NaOH) | acid + hydroxide → salt + water | H+ (aq) + OH- (aq) → H2O (l) |
insoluble metal oxides (e.g. CuO) | acid + oxide → salt + water | 2H+ (aq) + O2- (s) → H2O (l) |
soluble metal carbonates (e.g. Na2CO3) | acid + carbonate → salt + water + CO2 | 2H+ (aq) + CO32- (aq) → H2O (l) + CO2 (g) |
insoluble metal carbonates (e.g. CaCO3) | acid + carbonate → salt + water + CO2 | 2H+ (aq) + CO32- (s) → H2O (l) + CO2 (g) |
Examiner Tips and Tricks
The only difference between the ionic equations for soluble and insoluble carbonates is the state symbol on the carbonate ion, (aq) for soluble and (s) for insoluble.
The rest of the equation is identical
Worked Example
The balanced equation for the reaction of nitric acid with solid copper(II) oxide is shown below:
2HNO3 (aq) + CuO (s) → Cu(NO3)2 (aq) + H2O (l)
Identify the spectator ions and write the final ionic equation for this reaction.
[2]
Answer:
The full, balanced chemical equation is:
2HNO3 (aq) + CuO (s) → Cu(NO3)2 (aq) + H2O (l)
Break down aqueous compounds into ions:
2H+ (aq) + 2NO3⁻(aq) + CuO (s) → Cu2+ (aq) + 2NO3- (aq) + H2O (l)
(Note: solid (s) and liquid (l) chemicals are not split into ions)
Identify spectator ions:
The NO3-(aq) ion is on both sides, unchanged
So, the spectator ion is nitrate (NO3-) [1 mark]
Write the ionic equation (without spectator ions):
2H+ (aq) + CuO (s) → Cu2+ (aq) + H2O (l) [1 mark]
Worked Example
The balanced equation for the reaction of hydrochloric acid with solid calcium carbonate is shown below:
2HCl (aq) + CaCO3 (s) → CaCl2 (aq) + H2O (l) + CO2 (g)
Identify the spectator ions and write the final ionic equation for this reaction.
[2]
Answer:
The full, balanced chemical equation is:
2HCl (aq) + CaCO3 (s) → CaCl2 (aq) + H2O (l) + CO2 (g)
Break down aqueous compounds into ions:
2H+ (aq) + 2Cl- (aq) + CaCO3 (s) → Ca2+ (aq) + 2Cl- (aq) + H2O (l) + CO2 (g)
(Note: solid (s) and liquid (l) chemicals are not split into ions)
Identify spectator ions:
The Cl-(aq) ion is on both sides, unchanged
So, the spectator ion is chloride (Cl-) [1 mark]
Write the ionic equation (without spectator ions):
2H+ (aq) + CaCO3 (s) → Ca2+ (aq) + H2O (l) + CO2 (g) [1 mark]
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