Strong & Weak Acids (AQA GCSE Chemistry)

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 summarised 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

• A concentrated solution of either an acid or a base is one that contains a high number of acid or base molecules per dm³ of solution so would produce pH values below 4 and above 10
• A dilute acid or base solution is therefore one that has much fewer acid or base molecules per dm³ of solution, hence the pH value would lie between 5 and 9
• It does not necessarily mean that the acid or base is strong as it may be made from a weak acid or base which does not dissociate completely but a lot of it was added to the solution
• For example, a dilute solution of HCl will be more acidic than a concentrated solution of ethanoic acid, since most of the HCl molecules dissociate but very few of the CH₃COOH molecules do

• We have already seen that pH is a measure of the concentration of H⁺ ions in solution
• The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10
• Therefore an acid with a pH of 3 has ten times the concentration of H⁺ ions than an acid of pH 4
• An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H⁺ ions than an acid with a pH of 4
• From this we can summarise that for two acids of equal concentration, where one is strong and the other is weak, then the strong acid will have a lower pH due to its capacity to dissociate more and hence put more H⁺ ions into solution than the weak acid