AS Level · Chemistry 9701 · Physical chemistry

Equilibria: AS Level Chemistry questions with answers

Free Equilibria questions for AS Level Chemistry (9701), part of Physical chemistry. Each question has the answer and the marking points an examiner would look for.

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Equilibria practice questions

Question 1Multiple choice · 1 markA* challenge

0.40 mol of H2 and 0.40 mol of I2 are heated in a 1.00 dm3 flask. At equilibrium 0.10 mol of I2 has reacted.
H2(g) + I2(g) ⇌ 2HI(g)
What is Kc?

  1. A 0.111
  2. B 0.250
  3. C 0.444
  4. D 2.22
Show answer

Answer: C

Question 2Multiple choice · 1 mark

Which statement describes Brønsted–Lowry acid?

  1. A If a change is made to a system in equilibrium, the position of equilibrium moves to minimise the change
  2. B A proton donor
  3. C A measure of the dispersal of energy or disorder in a system
  4. D The enthalpy change when one mole of ionic compound forms from its gaseous ions
Show answer

Answer: B

Question 3Multiple choice · 1 markA* challenge

NH3 + H2O ⇌ NH4+ + OH−
Which species act as Brønsted–Lowry acids in the forward and reverse reactions?

  1. A H2O and NH4+
  2. B NH3 and H2O
  3. C NH3 and OH−
  4. D OH− and NH4+
Show answer

Answer: A

Question 4Structured · 9 marksA* challenge

1.00 mol of ethanoic acid and 1.20 mol of ethanol are mixed with a little acid catalyst and left to reach equilibrium at constant temperature. 0.50 mol of ethyl ethanoate is formed.
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O

(a)

Write the expression for Kc and calculate its value.

[3]
(b)

Explain why Kc has no units and why the total volume is not needed.

[2]
(c)

State and explain the effect on the yield of ester of adding more ethanol, and on Kc.

[2]
(d)

State the role of the acid catalyst and its effect on the position of equilibrium.

[2]
Show answers and mark scheme
PartAnswer and marking pointsMarks
(a)0.714
correct expression B1
equilibrium amounts C1
0.714 A1
3
(b)equal numbers of moles on each side; volumes cancel
moles top and bottom equal B1
V cancels B1
2
(c)yield increases as equilibrium shifts right to oppose the change; Kc unchanged (temperature constant)
yield increases (shifts right) B1
Kc unchanged B1
2
(d)provides an alternative route with lower activation energy; no effect on position
lower activation energy pathway B1
no change in position (both rates increase equally) B1
2
Question 5Structured · 5 marks

Brønsted–Lowry theory describes acids and bases.

(a)

Define a Brønsted–Lowry acid.

[1]
(b)

Identify the conjugate acid–base pairs in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.

[2]
(c)

Explain the difference between a strong acid and a concentrated acid.

[2]
Show answers and mark scheme
PartAnswer and marking pointsMarks
(a)proton donor
a proton donor B1
1
(b)NH₄⁺/NH₃; H₂O/OH⁻
NH₄⁺ and NH₃ B1
H₂O and OH⁻ B1
2
(c)fully dissociated vs high concentration
strong: fully dissociates B1
concentrated: large amount of acid per unit volume B1
2
Question 6Structured · 10 marks

Ethanoic acid reacts with ethanol to form ethyl ethanoate and water:
CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l)
2.00 mol of ethanoic acid and 1.50 mol of ethanol were mixed with a little acid catalyst. At equilibrium 0.50 mol of ethyl ethanoate was present.

(a)

State two features of a system in dynamic equilibrium.

[2]
(b)

Write the expression for Kc for this reaction.

[1]
(c)

Calculate the value of Kc, and explain why the volume of the mixture is not needed.

[3]
(d)

State and explain the effect on the equilibrium yield of ester of adding more ethanol.

[2]
(e)

State the effect of the acid catalyst on the value of Kc and on the time taken to reach equilibrium.

[2]
Show answers and mark scheme
PartAnswer and marking pointsMarks
(a)forward and reverse rates equal; concentrations constant; closed system
rate of forward reaction equals rate of reverse reaction B1
concentrations of reactants and products remain constant (closed system) B1
2
(b)[ester][water]/[acid][alcohol]
Kc = [CH₃COOC₂H₅][H₂O] / [CH₃COOH][C₂H₅OH] B1
1
(c)0.167
equilibrium amounts: 1.50, 1.00, 0.50, 0.50 mol M1
Kc = 0.167 A1
equal numbers of moles on each side so V cancels B1
3
(d)increases: equilibrium shifts right to oppose increase in ethanol concentration
yield increases B1
the position of equilibrium shifts to the right to reduce the concentration of ethanol (Le Chatelier) B1
2
(e)no effect on Kc; reduces time
Kc is unchanged B1
equilibrium is reached more quickly B1
2

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Original Cambridge-style practice questions written for mentorriyaz.in. These are not Cambridge past paper questions, and this site is not affiliated with or endorsed by Cambridge University Press & Assessment. Updated 2026-11-04. Paper generator · mentorriyaz.in