Edexcel International A Level Chemistry Acid-base Equilibria Exam Questions

About Edexcel International A Level Chemistry Acid-base Equilibria

Acid-base equilibria in Unit 4 of Edexcel International A Level Chemistry takes acids and bases from simple neutralisation to quantitative equilibrium. You use the Bronsted-Lowry model and conjugate pairs, calculate pH for strong acids, strong bases and weak acids using Kw and Ka, and explain how buffer solutions resist changes in pH. You also interpret pH curves for different acid and base combinations and choose suitable indicators. It builds on Chemical equilibria and is one of the most calculation-heavy topics in the course. Examiners expect clear working, correct use of logs, and explanations of buffer action written with equations.

What you need to know

• Bronsted-Lowry theory An acid is a proton donor and a base is a proton acceptor. Conjugate acid-base pairs differ by one H+, for example CH3COOH and CH3COO-.
• pH of strong acids and bases pH = -log[H+]. Strong acids dissociate completely, so [H+] equals the acid concentration for a monoprotic acid. For strong bases use Kw = [H+][OH-] = 1.00 x 10^-14 mol2 dm-6 at 298 K.
• Weak acids Ka = [H+][A-] / [HA] and pKa = -log Ka. For a weak acid alone, assume [H+] = [A-] and that [HA] at equilibrium equals the initial concentration, so [H+] = square root of (Ka x [HA]).
• Buffers A buffer contains a weak acid and its conjugate base (or a weak base and its conjugate acid). Added H+ reacts with A-, added OH- reacts with HA, and the large reservoirs keep the ratio [HA]/[A-] nearly constant. [H+] = Ka x [HA] / [A-].
• Titration curves Learn the shapes for strong acid with strong base (vertical section about pH 3 to 11), weak acid with strong base (equivalence above 7), strong acid with weak base (equivalence below 7). At half-neutralisation of a weak acid, pH = pKa.
• Indicators Choose an indicator whose pH range lies within the vertical section of the curve. Methyl orange suits strong acid with weak base; phenolphthalein suits weak acid with strong base.
• Enthalpy of neutralisation For strong acids with strong bases it is about -57 kJ mol-1. For weak acids it is less exothermic because energy is needed to complete dissociation of the acid.

Common mistakes in Acid-base Equilibria

• Using the weak acid formula for a buffer. In a buffer [A-] comes from the salt, so use [H+] = Ka x [HA] / [A-].
• Forgetting to use Kw for strong bases and just taking -log of [OH-]. That gives pOH, not pH.
• Stating the assumptions for weak acid pH calculations incompletely. You need both: [H+] = [A-], and dissociation is so small that [HA] is unchanged.
• Describing buffer action without equations. Write the equilibrium HA and H+ + A- and explain which direction it shifts when acid or alkali is added.
• Rounding pH to one decimal place. Give pH to two decimal places unless told otherwise.

Try three Acid-base Equilibria exam-style questions