Edexcel A-Level Chemistry AS Paper 2, June 2023: Question 6

1 mark · Medium difficulty · Multiple Choice

Identify the correct expression for the equilibrium constant Kc for the thermal decomposition of solid ammonium carbamate into ammonia and carbon dioxide gases.

Practise this question

Question

Multiple choice question 6 showing the decomposition equation NH2COONH4(s) = 2NH3(g) + CO2(g) and four options A, B, C, and D for the equilibrium constant Kc expression, involving concentrations of ammonia and carbon dioxide.
Question text

6 Ammonium carbamate has the formula NH2COONH4. When a sample of

ammonium carbamate is placed in a sealed tube and heated, it partially decomposes

to the compounds used to synthesise it.

The equation for this reversible reaction is

NH2COONH4(s) 2NH3(g) + CO2(g)

Which is the expression for the equilibrium constant, Kc, for this reaction?

All concentration terms are at equilibrium.

NH3CO2

A

NH COONH24

2NH3CO2

B

NH COONH24

  2 

C NH3 CO2

  2 

NH3 CO2

D

NH COONH24

(Total for Question 6 = 1 mark)

Mark scheme

Show the mark scheme Mark scheme for question 6 indicating that the correct answer is C, with explanations for why options A, B, and D are incorrect based on stoichiometry and the exclusion of solids from Kc expressions.

Question

Answer Mark

Number

6 The only correct answer is C ([NH ]2[CO ]) (1)

A is not correct because the equation shows 2 mol NH3 so the term [NH3] must be squared, and ammonium carbamate is a

solid so it does not appear in the equilibrium expression

B is not correct because the equation shows 2 mol NH3 so the term [NH3] must be squared not doubled , and ammonium

carbamate is a solid so it does not appear in the equilibrium expression

D is not correct because ammonium carbamate is a solid so it should not be included in the equilibrium expression

(Total for Question 6 = 1 mark)

How to answer it

Writing Kc Equilibrium Expressions

Edexcel AS Level Chemistry • Multiple Choice

What this question tests

This question assesses your ability to construct an equilibrium constant expression (Kc) from a heterogeneous chemical equation, correctly handling stoichiometric balancing numbers as powers and omitting solid (s) or liquid (l) phases.

Question 6 Analysis

Decomposition of Ammonium Carbamate

✅ Correct Answer: C

The correct expression is [NH₃]²[CO₂] .

Ammonium carbamate is a solid (s), so its concentration is constant and it is omitted from the Kc expression. The coefficient of NH₃ is 2, which becomes a power.

💡 Key Knowledge

  • Heterogeneous equilibria: Only species with (g) or (aq) phase symbols appear in Kc expressions. Solids (s) and pure liquids (l) are excluded.
  • Stoichiometry: Balancing numbers in the chemical equation become powers (indices) in the equilibrium expression.

🧠 Exam Technique

  • Always check the state symbols (s) , (l) , (g) before writing any equilibrium expression.
  • Eliminate options containing the solid reactant right away to save time.

❌ Common Errors & Distractor Analysis

  • Distractor A: Includes the solid reactant in the denominator and fails to square the NH₃ term.
  • Distractor B: Multiplies the NH₃ concentration by 2 instead of raising it to the power of 2.
  • Distractor D: Correctly squares NH₃ and includes CO₂, but incorrectly keeps the solid reactant in the denominator.
Mark Allocation: (Total for Question 6 = 1 mark). Awarded for selecting option C.

Topics

Physical Chemistry · Topic 11: Equilibrium II

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.