Strong & Weak Acids (HT Only) (AQA GCSE Combined Science: Synergy: Physical Sciences): 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

  • For example, the general acid HX dissociates to form H⁺ and X^–

HX ⟶ H⁺ + X^–

What is a strong acid?

• Strong acids dissociate completely in water

  • This produces a solution with a high concentration of H⁺ ions 

  • The higher the concentration of H⁺ ions, the lower the pH

• Examples of strong acids include:

  • Hydrochloric acid, HCl

HCl (aq) → H⁺ (aq) + Cl^– (aq) 

• Nitric acid, HNO₃ 

HNO₃ (aq) → H⁺ (aq) + NO₃^– (aq)

• Sulfuric acid, H₂SO₄ 

H₂SO₄ (aq) → H⁺ (aq) + SO₄^2– (aq)

What is a weak acid?

• Weak acids partially dissociate (or ionise) in water

  • This produces a solution with a low concentration of H⁺ ions

  • At the same concentration, a weak acid will have a higher pH than a strong acid, though still 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

• Ethanoic acid for example dissociates as follows:

CH₃COOH ⇌ H⁺ + CH₃COO^–

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

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

• Examples of weak acids include ethanoic acid (CH₃COOH), citric acid, and carbonic acid (H₂CO₃)

Hydrogen ion concentration

Higher Tier Only

• 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

• A dilute acid or base solution is one that has much fewer acid or base molecules per dm³ of solution

• 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

Relative acidity

Higher Tier Only

• 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