Strong & Weak Acids (WJEC GCSE Chemistry)

Proton Transfer, Strong & Weak Acids

What is a strong acid?

• Acids can be either strong or weak, depending on how many H⁺ ions they produce when dissolved in water

• Strong acids completely dissociate (or ionise) in water, producing solutions of a very low pH 

• Strong acids include HCl and H₂SO₄ 

• Example of a strong acid: hydrochloric acid

HCl (aq) → H⁺ (aq) + Cl^- (aq)

What is a weak acid?

• Weak acids partially dissociate (or ionise) in water and produce pH values which are closer to the middle of the pH scale, whilst still being below 7

• For weak acids, there is usually an equilibrium set-up between the molecules and their ions once they have been added to water

• Example of a weak acid: propanoic acid

CH₃CH₂COOH ⇌ H⁺ + CH₃CH₂COO^-

• The equilibrium lies to the left, indicating a high concentration of intact acid molecules, with a low concentration of H⁺ ions in the solution

• Another example of a weak acid is ethanoic acid which will react with alkalis such as sodium hydroxide to form ethanoate salts

ethanoic acid + sodium hydroxide → sodium ethanoate + water

CH₃COOH + NaOH → CH₃COONa + H₂O

The reactions of strong & weak acids 

• The pH of a strong acid is lower than a weak acid

• The number of hydrogen ions, H⁺, is greater in the strong acid so there are a higher number of reacting particles

• Therefore the reactions of strong acids with substances like metals, bases or carbonates will be more vigorous than with weak acids

• This can be seen by:

  • Increase in the rate of gas is formed (e.g. hydrogen with metals and carbon dioxide with carbonates)

  • Increase in the temperature change of the reaction

Strong and weak acids reacting with metal

When a strong acid reacts with a metal such as magnesium the reaction is more vigorous than if a weak acid was used

• These reactions are exothermic which means the temperature increases

• For a strong acid the increase in temperature is greater than that of a weak acid so is more exothermic

Graph to show the temperature change when a strong and weak acid react with a metal

When a strong acid reacts with an acid, the temperature change is greater than when a weak acid reacts with an acid

• A dilute acid has the acid molecules mixed with a large amount of water

• Concentrated acids have little to no water molecules mixed with the acid molecules

• Acids can be dilute or concentrated but specific acids are strong or weak

  • This is illustrated in the diagram below

Diagram to show the difference between dilute, concentrated, weak and strong acid

The concentration of the acids in the diagram is increasing from left to right. The top two beakers show a weak (partially ionised) acid and the bottom two beakers show a strong (fully ionised) acid