Science (086) — AI-generated practice question

(a) Why the equation is unbalanced:

The given equation $\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2$ is unbalanced because the number of atoms of each element is not equal on both sides, violating the Law of Conservation of Mass.

| Element | LHS | RHS |
|---------|-----|-----|
| Fe | 1 | 3 |
| O | 1 | 4 |
| H | 2 | 2 |

Since Fe and O atoms differ on LHS and RHS, the equation is a skeletal (unbalanced) chemical equation.

(b) Fully balanced equation with state symbols:

Using the hit-and-trial method — balance O first (coefficient 4 for H₂O), then H (coefficient 4 for H₂), then Fe (coefficient 3):

$$3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \rightarrow \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)$$

The state symbol (g) is used for H₂O because water is used as steam in this reaction.

Source: Chapter 1, Sections 1.1.1 and 1.1.2

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• (a) Examiners expect you to explicitly count atoms of each element on both sides and show the mismatch in a table. Simply saying "it's not balanced" without evidence will lose marks.
• (b) Always include state symbols when asked — (s) for Fe and Fe₃O₄, (g) for H₂O (steam), and (g) for H₂. Forgetting (g) for steam is a common error. The balanced coefficients 3, 4, 1, 4 are the key numbers to remember.
• The method used here is the hit-and-trial method — start with the compound having the most atoms (Fe₃O₄), balance O → H → Fe in that order.