Acid Strength & Concentration (Edexcel GCSE Chemistry: Combined Science)

Strong & Weak

• 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 ionize 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

• A solution is formed when a solute is dissolved in a solvent
• A dilute solution contains a small amount of solute in a given volume of solution
• A concentrated solution contains a large amount of solute in a given volume of solution
• 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 therefore one that has much fewer acid or base molecules per dm³ of solution

Diagram illustrating how the concentration of a solution increases as more solute is added