Edexcel A Level Chemistry
Edexcel A Level Chemistry Topic 12: Acid-base Equilibria Exam Questions
Free Edexcel A Level Chemistry exam questions for Topic 12: Acid-base Equilibria, with a mark scheme for every pack. Covers Bronsted-Lowry acids and bases, pH of strong and weak acids and bases, Ka, Kw, buffers, titration curves and indicators.
Topic 12: Acid-base Equilibria exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel A Level Chemistry Topic 12: Acid-base Equilibria
Topic 12 of Edexcel A Level Chemistry, Acid-base Equilibria, applies equilibrium ideas to acids and bases. You use Bronsted-Lowry theory and conjugate acid-base pairs, calculate the pH of strong acids, strong bases and weak acids using Ka and pKa, and use the ionic product of water, Kw. Buffer solutions are explained and calculated, including their use in controlling blood pH. You also interpret pH titration curves for different strong and weak combinations, choose suitable indicators and find pKa from the half-neutralisation point. Core practicals include finding Ka of a weak acid. It is assessed in Paper 1. Examiners focus heavily on pH calculations and clear buffer explanations.
What you need to know
- Bronsted-Lowry theory An acid is a proton donor and a base is a proton acceptor. In HA + H2O ⇌ H3O+ + A-, A- is the conjugate base of HA.
- pH of strong acids and bases pH = -log[H+]. For strong bases use Kw = [H+][OH-] = 1.00 x 10^-14 mol2 dm-6 at 298 K to find [H+] from [OH-].
- Weak acids Ka = [H+][A-]/[HA] and pKa = -log Ka. Assuming [H+] = [A-] and that [HA] is unchanged, [H+] = square root of (Ka x [HA]).
- Buffers A buffer resists change in pH when small amounts of acid or alkali are added, for example ethanoic acid and sodium ethanoate. [H+] = Ka x [HA]/[A-].
- Blood pH The carbonic acid and hydrogencarbonate system, H2CO3 ⇌ H+ + HCO3-, keeps blood pH close to 7.4.
- Titration curves Learn the shapes for strong acid-strong base, strong acid-weak base, weak acid-strong base and weak acid-weak base. The vertical section determines a suitable indicator, and at half-neutralisation pH = pKa.
- Indicators An indicator is a weak acid whose two forms have different colours. Choose one whose colour change range lies within the vertical part of the curve, such as phenolphthalein for weak acid-strong base titrations.
Common mistakes in Topic 12: Acid-base Equilibria
- Using the strong acid method for a weak acid, or forgetting to take the square root.
- Calculating the pH of a strong base as -log[OH-]; that gives pOH, so convert using Kw.
- Saying a buffer keeps pH constant; it resists change when small amounts of acid or alkali are added.
- Choosing an indicator for a weak acid-weak base titration; no sharp vertical section exists, so indicators are unsuitable.
- Doubling the concentration of a diprotic acid when it is not fully dissociated in both steps.
Try three Topic 12: Acid-base Equilibria exam-style questions
1. Calculate the pH of 0.0500 mol dm-3 sodium hydroxide at 298 K. (2 marks)
Show the mark scheme answer
[H+] = 1.00 x 10^-14/0.0500 = 2.00 x 10^-13 mol dm-3; pH = 12.70 (12.7).
2. A buffer contains 0.100 mol dm-3 ethanoic acid and 0.150 mol dm-3 sodium ethanoate. Ka = 1.74 x 10^-5 mol dm-3. Calculate its pH. (2 marks)
Show the mark scheme answer
[H+] = 1.74 x 10^-5 x 0.100/0.150 = 1.16 x 10^-5 mol dm-3; pH = 4.94.
3. Explain how the pKa of a weak acid can be found from its titration curve with sodium hydroxide. (2 marks)
Show the mark scheme answer
Find the volume of NaOH at the equivalence point and halve it; at half-neutralisation [HA] = [A-] so pH = pKa, read the pH from the curve.
Topic 12: Acid-base Equilibria FAQs
What does Edexcel A Level Chemistry Topic 12 cover?
It covers Bronsted-Lowry acids and bases, pH calculations, Ka, pKa and Kw, buffers, titration curves and indicators.
Which paper is Acid-base Equilibria examined in?
Topic 12 is assessed in Paper 1, and the related titration and pH skills appear in Paper 3.
Which equations do I need for acid-base equilibria?
pH = -log[H+], Kw = [H+][OH-], Ka = [H+][A-]/[HA], pKa = -log Ka, and [H+] = Ka x [HA]/[A-] for buffers.
