A-LevelChemistryEquilibria9701/42 · May/June 2017 · Q610 Marks

The table lists some organic acids and their pKa values. acid formula pKa ethanoic acid CH3CO₂H 4.76 chloroethanoic acid CICH₂CO₂H 2.86 aminoethanoic acid (glycine) H2NCH2CO2H 9.87

(a)(i)State the relationship between pKa and the strength of an acid.[1]
(a)(ii)State the mathematical relationship between pKa and the acidity constant Ka.[1]
(a)(iii)Give reasons for why the pKa value for chloroethanoic acid is smaller than that for ethanoic acid.[2]
(b)(i)Use the zwitterionic structure for aminoethanoic acid (glycine) in aqueous solution to write an equation for its dissociation giving H+(aq) ions.[1]
(b)(ii)Calculate the pH of a 0.100 moldm-³ solution of aminoethanoic acid. pH = ...[2]
(b)(iii)A 10.0 cm³ sample of 0.100 mol dm-3 aminoethanoic acid (glycine) was titrated with 0.100 mol dm-3 NaOH. After 20.0 cm³ of NaOH, an excess, had been added, the pH was found to be 12.5. Using the following axes, sketch a graph showing how the pH changes during this titration.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] the lower / smaller the pKa, the stronger the acid
[1] pKa = -log(Ka) or pKa = -lg(Ka) or Ka = 10⁻pKa
[1] (stronger than ethanoic acid because) Cl is electron-withdrawing
[1] and so stabilises the RCO2⁻ anion / conjugate base or weakens O-H bond (so H⁺ is more easily released)
[1] NH3⁺CH2CO2⁻ → NH2CH2CO2⁻ + H⁺ OR NH3⁺CH2CO2⁻ + H2O → NH2CH2CO2⁻ + H3O⁺
[1] Ka = 10⁻9.87 = 1.35 × 10⁻¹⁰
[1] [H⁺] = √(Ka.c) = 3.67 × 10⁻⁶ pH = 5.4 (5.43–5.44) min 2sf
[1] curve starts at 5.4 and continuous
[1] vertical portion (end point) at vol added = 10.0 cm³
[1] finishes at pH = 12.5 at 20 cm³ (and does not increase in pH)

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.