OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2024: Question 3

1 mark · Medium difficulty · Multiple Choice

Identify which chemical process for producing iron has the most sustainable atom economy.

Practise this question

Question

Multiple choice question asking which chemical process is the most sustainable in terms of atom economy for producing iron. Four chemical equations are given as options: A) Fe2O3 + 3CO -> 2Fe + 3CO2, B) Fe2O3 + 3H2 -> 2Fe + 3H2O, C) 2Fe2O3 -> 4Fe + 3O2, and D) 2Fe2O3 + 3C -> 4Fe + 3CO2. A blank box is provided for the student's answer.
Question text

3 Which chemical process is the most sustainable in terms of the atom economy of the iron

produced?

A Fe2O3 + 3CO 2Fe + 3CO2

B Fe2O3 + 3H2 2Fe + 3H2O

C 2Fe2O3 4Fe + 3O2

D 2Fe2O3 + 3C 4Fe + 3CO2

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 3 is C, worth 1 mark.

3 C 1

How to answer it

Sustainable Extraction and Atom Economy

OCR A-Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your understanding of atom economy in industrial chemical processes. You need to recall the formula for atom economy, interpret balanced chemical equations, and apply stoichiometric mass ratios to evaluate chemical sustainability without necessarily performing full numerical calculations.

Question Analysis

Identifying the Core Task

You must determine which of the four given reduction equations for iron(III) oxide ( Fe₂O₃ ) produces iron with the highest atom economy.

✅ Correct Answer

C ( 2Fe₂O₃ → 4Fe + 3O₂ )

Mark Awarded: 1 / 1

💡 Key Knowledge

  • Atom Economy Formula: (Mass of desired product / Total mass of all reactants) × 100%.
  • A process with a 100% atom economy has only one product (all reactants are converted into the desired product).
  • Decomposition reactions that yield only the metal and oxygen gas often have very high atom economies compared to reduction processes requiring external reducing agents (like CO, H₂, or C) which introduce extra reactant mass.

🧠 Exam Technique

Look at the stoichiometry of the products. In equation C, the only by-product is oxygen gas ( O₂ ), and importantly, no reducing agent is needed as a reactant. This drastically reduces the total molar mass of the reactants, maximizing the atom economy.

❌ Common Errors

  • Assuming equation D is best because carbon is cheap or commonly used in the blast furnace. Cost and feasibility are not the same as atom economy!
  • Forgetting to account for stoichiometric balancing numbers (e.g., using Fe₂O₃ instead of 2Fe₂O₃ ) when scaling molar masses.

📐 Why Equation C Wins (Step-by-Step Logic)

  1. Examine Reactants: Options A, B, and D require extra reactants ( 3CO , 3H₂ , or 3C ). Their total reactant mass includes both the iron ore and the reducing agent.
  2. Examine Equation C: The reactant side contains only iron(III) oxide ( 2Fe₂O₃ ). Therefore, the total mass of reactants equals the mass of the iron ore input.
  3. Evaluate Atom Economy: Since every single gram of reactant input contributes to forming either iron or oxygen, and no extraneous reducing agent mass is added to the denominator, Equation C achieves the highest possible percentage conversion by mass for this set of options.

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.