Section 1 of 4Acid Behaviour
Carboxylic acids contain the -COOH group, which is responsible for their acidic properties.
Carboxylic acid group
- Carboxylic acid group Every carboxylic acid contains the -COOH group, and it is this group that gives the molecule its acidic behaviour.
Ionisation
- Ionisation In water, a carboxylic acid releases hydrogen ions, H⁺, into the solution as it ionises.
Weak acid
- Weak acid A weak acid is only partly ionised in solution, so it has fewer hydrogen ions and a higher pH than a strong acid of the same concentration.
Equilibrium
- Equilibrium Ethanoic acid ionises reversibly, CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq), so only some molecules release a hydrogen ion at any moment.
Section 2 of 4Reactions of Ethanoic Acid
Ethanoic acid reacts with the same types of substance as other acids, giving a salt in each case.
1
Ethanoic acid reacts with magnesium to give a salt and hydrogen gas.
2
Ethanoic acid reacts with sodium carbonate to give a salt, water and carbon dioxide gas.
3
Ethanoic acid reacts with sodium hydroxide in a neutralisation reaction to give a salt and water.
| Reactant | Products | Balanced symbol equation |
|---|---|---|
| Magnesium | Magnesium ethanoate and hydrogen | Mg(s) + 2CH₃COOH(aq) → (CH₃COO)₂Mg(aq) + H₂(g) |
| Sodium carbonate | Sodium ethanoate, water and carbon dioxide | 2CH₃COOH(aq) + Na₂CO₃(s) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g) |
| Sodium hydroxide | Sodium ethanoate and water | CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l) |
Section 3 of 4Strong and Weak Comparison
A comparison of equal concentrations of a strong and a weak acid shows the difference clearly.
Strong and Weak Comparison
Ionisation
- Ionisation Hydrochloric acid is fully ionised, while ethanoic acid is only partly ionised in water.
Hydrogen ions
- Hydrogen ions At the same concentration, hydrochloric acid has a much higher concentration of hydrogen ions than ethanoic acid.
Rate
- Rate Magnesium reacts faster with hydrochloric acid than with ethanoic acid of the same concentration.
pH
- pH Ethanoic acid has a higher pH than hydrochloric acid of the same concentration.
Section 4 of 4Exam Checklist & Tips
7.2.6 – Key Points for the Exam
- Carboxylic acid An organic acid containing the -COOH functional group.
- Weak acid An acid that is only partly ionised in aqueous solution.
- Ionisation
- The process of forming ions;
- for acids, releasing H⁺ ions into water.
- The process of forming ions;
- Salt A compound formed when the hydrogen in an acid is replaced by a metal.
- Reversible reaction A reaction that can proceed in both directions, shown by the symbol ⇌.
Common Mistakes
- Saying ethanoic acid is a strong acid because it is an organic compound;
- acid strength depends on how far it ionises, not on being organic.
- Writing a one-way arrow for the ionisation of ethanoic acid; the reaction is reversible, so ⇌ should be used.
- Forgetting that carbon dioxide is produced in the carbonate reaction, or leaving out the water.