Burning of magnesium in air and the thermal decomposition of calcium carbonate both involve a metal compound and oxygen, yet one is classified as a combination reaction and the other as a decomposition reaction. Write balanced chemical equations for both reactions and explain, with reasoning, the basis on which each is assigned to its respective category.
Generated by claude-sonnet-4-6 · 2026-06-26 00:55 · grounding rag
Model Answer
Burning of Magnesium (Combination Reaction):
$$2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)$$
Thermal Decomposition of Calcium Carbonate (Decomposition Reaction):
$$\text{CaCO}_3(s) \xrightarrow{\Delta} \text{CaO}(s) + \text{CO}_2(g)$$
Reasoning:
In the first reaction, two reactants (Mg and O₂) combine to form a single product (MgO) — this defines a combination reaction.
In the second reaction, a single reactant (CaCO₃) breaks down into two products (CaO and CO₂) on heating — this defines a decomposition reaction. The two reactions are therefore opposites of each other.
Source: Chapter 1, Section 1.2.1 (Combination Reaction); Chapter 1, Section 1.2.2 (Decomposition Reaction)
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Explanation
- The examiner awards marks for: (1) correct balanced equation for Mg burning, (2) correct balanced equation for thermal decomposition of CaCO₃, and (3) clear reasoning linking "two reactants → one product" = combination, and "one reactant → two products" = decomposition.
- The key phrase to remember: decomposition is the opposite of combination — the textbook states this explicitly.
- Always balance equations: Mg needs a coefficient of 2, giving 2MgO.
- The heat/delta symbol (Δ) above the arrow for CaCO₃ decomposition shows it requires heat (endothermic); include it for full credit.