When lead nitrate is heated, brown fumes are produced along with a solid residue and oxygen gas. (i) Identify the brown fumes and the solid residue. (ii) Write the balanced chemical equation for this reaction and state the type of chemical reaction it represents, giving a reason for your answer.
Generated by claude-sonnet-4-6 · 2026-06-26 00:54 · grounding rag
Model Answer
(i) The brown fumes are nitrogen dioxide (NO₂). The solid residue is lead oxide (PbO).
(ii) Balanced chemical equation:
$$2\text{Pb(NO}_3)_2(s) \xrightarrow{\Delta} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)$$
This is a thermal decomposition reaction because a single reactant (lead nitrate) breaks down into simpler products (lead oxide, nitrogen dioxide, and oxygen) on the application of heat.
Source: Chapter 1, Section 1.2.2 — Decomposition Reaction, Activity 1.6
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Explanation
- (i) is worth 1 mark — name both products correctly.
- (ii) carries 2 marks — 1 for the balanced equation and 1 for naming the reaction type with a reason. Many students lose the second mark by just naming "decomposition" without explaining why (single reactant → multiple simpler products). Always state the reason when asked.
- The condition arrow must show heat (Δ or "Heat") for full credit.