OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 6

1 mark · Medium difficulty · Multiple Choice

Identify which chemical process has the highest atom economy for preparing ethene among four given options.

Practise this question

Question

Multiple choice question 6 asking which process has the highest atom economy for preparing ethene C2H4, assuming ethene is the only product used. Four options are given with chemical equations: A is C10H22 yielding 3 C2H4 plus C4H10; B is C6H14 yielding 2 C2H4 plus C2H6; C is C2H5Cl yielding C2H4 plus HCl; D is C2H5OH yielding C2H4 plus H2O. An answer box is provided at the bottom.
Question text

6 Which process has the highest atom economy for preparing ethene, C2H4?

In each process, assume that ethene is the only product that is used.

A C10H22 3C2H4 + C4H10

B C6H14 2C2H4 + C2H6

C C2H5Cl C2H4 + HCl

D C2H5OH C2H4 + H2O

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme for question 6 showing the correct answer is option B.

6 B 1 AO1.2

How to answer it

Calculating Atom Economy for Ethene Preparation

What this question tests

This question assesses your understanding of atom economy in chemical synthesis (AO1.2). You are required to evaluate multiple chemical equations to determine which process incorporates the highest percentage of reactant atoms into the desired product (ethene, C₂H₄), treating all co-products as waste.

Question 6

Process Comparison & Solution

✅ Correct Answer: B

Process B has the highest atom economy for preparing ethene.

C₆H₁₄ → 2C₂H₄ + C₂H₆

💡 Key Knowledge

  • Definition: Atom economy measures the proportion of starting materials that become useful products.
  • Formula: Atom Economy = (Mr of desired product / Total Mr of all products) × 100%
  • Remember to multiply the molar mass of the desired product by its stoichiometric coefficient in the balanced equation!

🧠 Exam Technique

You don't always need to fully calculate every percentage to the final decimal place. Look for structural shortcuts:

  • Compare the total mass of reactants (or total products) relative to the mass of the desired ethene produced.
  • Process B converts a 6-carbon alkane into two 2-carbon ethene molecules and one 2-carbon ethane molecule (no mass is lost; 100% total mass conserved, and a large fraction goes to ethene).

❌ Common Errors

  • Forgetting to account for the stoichiometric balancing numbers (e.g., forgetting the multiplier 2 for 2C₂H₄ in options A and B).
  • Confusing atom economy with percentage yield. Atom economy is theoretical and derived purely from the balanced equation coefficients and molar masses.

📐 Step-by-Step Calculation Breakdown

Let's calculate and compare the atom economies using relative atomic masses (C = 12.0, H = 1.0, Cl = 35.5):

  1. Option A: C₁₀H₂₂ → 3C₂H₄ + C₄H₁₀
    Mass of desired product ( 3 × C₂H₄ ) = 3 × (28.0) = 84.0
    Total mass of products = 84.0 + 58.0 (for C₄H₁₀) = 142.0
    Atom Economy = (84.0 / 142.0) × 100 = 59.2%
  2. Option B: C₆H₁₄ → 2C₂H₄ + C₂H₆
    Mass of desired product ( 2 × C₂H₄ ) = 2 × (28.0) = 56.0
    Total mass of products = 56.0 + 30.0 (for C₂H₆) = 86.0
    Atom Economy = (56.0 / 86.0) × 100 = 65.1% (Highest value)
  3. Option C: C₂H₅Cl → C₂H₄ + HCl
    Mass of desired product ( C₂H₄ ) = 28.0
    Total mass of products = 28.0 + 36.5 (for HCl) = 64.5
    Atom Economy = (28.0 / 64.5) × 100 = 43.4%
  4. Option D: C₂H₅OH → C₂H₄ + H₂O
    Mass of desired product ( C₂H₄ ) = 28.0
    Total mass of products = 28.0 + 18.0 (for H₂O) = 46.0
    Atom Economy = (28.0 / 46.0) × 100 = 60.9%
Mark Scheme Award: 1 mark for selecting letter B (AO1.2).

Topics

Module 2: Foundations in chemistry · Module 4: Core organic chemistry · 2.1 Atoms and reactions · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.