Strong & Weak Acids (OCR GCSE Chemistry A (Gateway)): Revision Note

Strong & Weak Acids

Higher Tier Only

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

• When added to water, acids ionise or dissociate to produce H+ ions:

Hydrochloric acid: HCl ⟶ H⁺ + Cl^–

Nitric acid: HNO₃ ⟶ H⁺ + NO₃^–

• Strong acids such as HCl and H₂SO₄ dissociate completely in water, producing solutions with a high concentration of H⁺ ions and thus a very low pH

• Weak acids such as ethanoic acid, CH₃COOH and hydrofluoric acid, HF, only partially ionise in water, producing solutions of pH values between 4 – 6

• This data is summarized in the table below:

Strong & Weak Acids Table

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

• Propanoic acid for example dissociates as follows:

CH₃CH₂COOH ⇌ H⁺ + CH₃CH₂COO^–

• The ⇌ symbol indicates that the process is reversible, as the products can react together forming the original reactants

• The equilibrium lies to the left, meaning there is a high concentration of intact acid molecules and therefore a low concentration of ions in solution, hence the pH is that of a weak acid and closer to 7 than a strong acid

Concentrated & Dilute Acids 

• A strong acid is not the same as a concentrated one, just like a dilute acid is not the same as a weak one

• Strong and weak refer to the ability of an acid to dissociate 

• Concentration refers to how many acid particles there are in a certain volume

• A concentrated solution will have more acid particles than a dilute one per dm³

• Solutions can be both concentrated and strong or weak and dilute 

• A dilute solution of a strong acid can have a lower pH than a concentrated solution of a weak acid, due to the stronger acid undergoing complete dissociation