A student trying to balance the equation Fe + H₂O → Fe₃O₄ + H₂ changes the formula of water to H₂O₄ in order to equalise the oxygen atoms on both sides. (i) State why this approach is chemically incorrect. (ii) Using the correct method, write the fully balanced chemical equation for this reaction.
Generated by claude-sonnet-4-6 · 2026-06-26 00:53 · grounding rag
Model Answer
(i) This approach is chemically incorrect because we cannot change the formula of any reactant or product to balance an equation. H₂O₄ is not a real compound; altering a formula violates the law of conservation of mass and misrepresents the actual substances involved. Balancing is done only by placing coefficients (whole numbers) in front of the formulae.
(ii) Using the hit-and-trial method (placing coefficients only):
$$3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \rightarrow \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)$$
Verification:
| Element | LHS | RHS |
|---------|-----|-----|
| Fe | 3 | 3 ✓ |
| H | 8 | 8 ✓ |
| O | 4 | 4 ✓ |
Source: Chapter 1, Section 1.1.2 Balanced Chemical Equations
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Explanation
- Part (i) carries ~1 mark: state the rule clearly — formulae must never be changed; only coefficients are adjusted. The textbook explicitly says "we cannot alter the formulae of the compounds."
- Part (ii) carries ~2 marks: write the balanced equation with correct coefficients (3, 4, 1, 4). The verification table is good practice and shows the examiner you have checked your answer, but keep it brief. State symbols are a bonus if asked; include them if the question says "with state symbols."
- The key term to use: hit-and-trial method (as named in the NCERT text).