Edexcel A-Level Chemistry Paper 1, November 2020: Question 7

8 marks · Medium difficulty · Calculations

Calculate the equilibrium amounts for the esterification of ethanoic acid and ethanol, determine bond angles and units of Kp, and explain the effect of pressure on Kp.

Practise this question

Question

Exam question with two main parts about equilibrium and esters. Part (a) provides the equation for the esterification of ethanoic acid and ethanol, asking for the Kc expression and an equilibrium calculation starting from 1.2 mol of each reactant. Part (b) introduces methyl methanoate, with multiple choice questions on O-C-H bond angles, units of Kp, and an open response explaining the effect of pressure on Kp.
Question text

7 A mixture of ethanoic acid, ethanol and a catalyst was left for several days

to reach equilibrium.

CH3COOH(l) + CH3CH2OH(l) ⇌ CH3COOCH2CH3(l) + H2O(l)

The equilibrium constant, Kc, under these conditions, was 0.28.

(a) (i) Write the expression for the equilibrium constant, Kc.

(1)

(ii) The initial amounts of ethanol and ethanoic acid used were 1.2 mol of each reactant.

Use this information, your expression for the equilibrium constant, Kc,and the

value forKc, to find the amounts of each product at equilibrium, in moles.

(3)

Amount of CH3COOCH2CH3 = …

Amount of H2O = …

(b) Another ester, methyl methanoate, can be formed by the reaction between*P62668A01824*

methanol and carbon monoxide in the gaseous phase.

O H

CO(g) + CH3OH(g) H xC O yC H (g)

H

(i) The two O−C−H bond angles, x and y, in the ester are approximately

(1)

A 180° and 90°

B 120° and 90°

C 120° and 109.5°

D 109.5° and 109.5°

(ii) The reaction often forms an equilibrium mixture.

Which could be the units for the equilibrium constant, Kp?

(1)

A mol dm−3

B dm3 mol−1

C atm

D atm−1

(iii) Describe what effect, if any, increasing the pressure would have on the

equilibrium constant, Kp. Justify your answer.

(2)

(Total for Question 7 = 8 marks)

Mark scheme

Show the mark scheme Mark scheme providing answers and guidance for all parts of Question 7, including the Kc expression, the step-by-step ICE table calculation for equilibrium amounts yielding 0.42 mol, multiple choice answers for bond angles (C) and units of Kp (D), and points for explaining that Kp remains unchanged while equilibrium shifts with pressure.

How to answer it

Equilibrium Constants (Kc and Kp) & Bond Angles Study Guide

What this question tests

This question assesses your ability to construct equilibrium expressions (Kc), perform algebraic equilibrium calculations using initial moles and quadratic shortcuts, deduce shapes and bond angles using VSEPR theory, derive units for partial pressure equilibrium constants (Kp), and explain the fundamental rule that only temperature alters the value of an equilibrium constant.

Question 7 (a) (i)

Writing the Kc Expression

✅ Correct Answer

Kc = [CH₃COOCH₂CH₃(l)][H₂O(l)] / [CH₃COOH(l)][CH₃CH₂OH(l)]

Award (1) mark. Square brackets are mandatory. State symbols can be ignored.

❌ Common Errors

  • Using round brackets ( ) instead of square brackets [ ] .
  • Inverting the fraction (putting reactants on top instead of products).
Question 7 (a) (ii)

Equilibrium Amount Calculation

📐 Step-by-Step Calculation

  1. Set up equilibrium amounts: Let x be the moles of ester and water formed. Reactants at equilibrium = 1.2 - x .
  2. Substitute into Kc expression: Since volumes cancel out in this 1:1 molar ratio reaction, Kc = x² / (1.2 - x)² = 0.28 .
  3. Square root both sides: x / (1.2 - x) = √0.28 = 0.52915 .
  4. Rearrange and solve for x: x = 0.52915(1.2 - x) → x = 0.63498 - 0.52915x → 1.52915x = 0.63498 .
    x = 0.415 mol (or 0.42 mol).
Award (3) marks total: (1) for equilibrium amounts algebraic setup, (1) for substitution/rearrangement into Kc, (1) for final numerical answer. Correct answer with no working scores full marks.

🧠 Exam Technique & Traps

  • The Square Root Shortcut: Because both numerator and denominator are squared, taking the square root avoids expanding a complex quadratic equation!
  • Significant Figures: Give your answer to 2 or 3 significant figures (consistent with Kc = 0.28).
Question 7 (b) (i)

Bond Angles in Methyl Methanoate

✅ Correct Answer

C: 120° and 109.5°

Award (1) mark. Option C is correct.

💡 Key Knowledge

  • Angle x is around a carbonyl carbon ( C=O ) bonded to 3 regions of electron density (trigonal planar) = 120°.
  • Angle y is around a saturated sp³ carbon bonded to 4 single bonds (tetrahedral) = 109.5°.
Question 7 (b) (ii)

Units of Kp

✅ Correct Answer

D: atm⁻¹

Award (1) mark. Option D is correct.

📐 Derivation of Units

Equation: CO(g) + CH₃OH(g) ⇌ CH₃COOCH₃(g)

Kp expression = P(ester) / P(CO) × P(CH₃OH)

Substitute pressure units: atm / (atm × atm) = 1 / atm = atm⁻¹

Question 7 (b) (iii)

Effect of Pressure on Kp

✅ Correct Answer

  • Kp remains unchanged (1 mark).
  • Equilibrium shifts to the right-hand side to keep Kp constant, as pressure/concentration changes do not affect the value of equilibrium constants (only temperature does) (1 mark).
Award (2) marks. Maximum 1 mark if Kp is incorrectly stated as changing.

❌ Common Misconceptions

  • Students often confuse the position of equilibrium (which shifts with pressure) with the equilibrium constant itself (Kp/Kc), which is strictly temperature-dependent.

Topics

Physical Chemistry · Organic Chemistry · Topic 11: Equilibrium II · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.