Strong and Weak Acids (HT Only)

Section 1 of 6Strength: how far an acid ionises (HT only)

Strong acid

• Strong acid Ionises completely in water, so almost every molecule releases H⁺.
• Examples
  • hydrochloric, sulfuric and nitric acids.
  • HCl → H⁺ + Cl⁻.

Weak acid

• Weak acid Only partly ionises, so most molecules stay intact.
• Examples
  • ethanoic, citric and carbonic acids.
  • The reaction is reversible: CH₃COOH ⇌ CH₃COO⁻ + H⁺.

Section 2 of 6Strength and concentration are different

Property	Strength	Concentration
Meaning	Proportion of molecules that ionise	Amount of acid dissolved in a given volume
Unit or term	Strong or weak	mol/dm3, dilute or concentrated
Changed by	Only by choosing a different acid	Adding more acid or more water
Example	Ethanoic acid is weak	2.0 mol/dm3 is more concentrated than 0.1 mol/dm3

Section 3 of 6Effect on pH

1Same concentration

• Same concentration A strong acid gives more H⁺ ions than a weak one.

2Lower pH

• Lower pH The strong acid has the lower pH, for example 1 compared with about 3.

3Dilution by ten

• Dilution by ten Diluting a strong acid ten-fold raises its pH by 1 because H⁺ concentration falls to one tenth.

4Weak acid pH

• Weak acid pH A weak acid's pH is higher than a strong acid at the same concentration but it can still be corrosive if concentrated.

Section 4 of 6Comparing reactions

Same concentration and volume

Rate

• Rate The strong acid reacts faster with magnesium because it has more free hydrogen ions.

Amount of product

• Amount of product If the amounts of acid are the same, both can give the same final volume of gas since weak acid keeps ionising as H⁺ is used up.

Conductivity

• Conductivity A strong acid conducts better, because it has more ions.

Section 5 of 6Key terms

Term	Meaning
Strong acid	An acid that is completely ionised in aqueous solution.
Weak acid	An acid that is only partially ionised in aqueous solution.
Ionisation	The formation of ions when a substance dissolves in water.
Concentration	The amount of solute per unit volume of solution.
Reversible reaction	A reaction that can go in both directions, shown by the sign ⇌.

Section 6 of 6Exam technique

1

Using strong and concentrated as if they mean the same thing.

2

Thinking a weak acid is always dilute; it can be a concentrated solution.

3

Writing a single arrow for the ionisation of a weak acid instead of a reversible arrow.

4

Saying a ten-fold dilution changes the pH by ten rather than by one.