Strong & Weak Acids (AQA GCSE Chemistry: Combined Science)

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 
  • This means that strong acids have a low pH, typically pH 1 - 3 
• 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
  • This means that weak acids have pH values that are closer to the middle of the pH scale, whilst still being below 7, i.e. pH 4 - 6
• For weak acids, there is usually 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, 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

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

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 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

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