Science — AI-generated practice question

(a) Type of Reaction and Energy:
This is a thermal decomposition reaction — a single compound (lead nitrate) breaks down into simpler substances. The energy that drives it is heat (thermal energy). It is written as a thermolytic decomposition.

(b) Exothermic or Endothermic:
This reaction is endothermic — heat energy is absorbed from the surroundings to break down lead nitrate. Energy must be continuously supplied to keep the reaction going.

(c) Verification of Law of Conservation of Mass:

| Element | LHS (Reactants) | RHS (Products) |
|---------|----------------|----------------|
| Pb | 2 | 2 (in 2PbO) |
| N | 4 | 4 (in 4NO₂) |
| O | 12 (6×2) | 2+8+2 = 12 |

Atoms of each element are equal on both sides, so mass is conserved. This confirms the Law of Conservation of Mass: matter is neither created nor destroyed in a chemical reaction.

Source: Chapter 1 – Chemical Reactions and Equations

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• (a): Examiners expect the words "thermal decomposition" and "heat energy." One reactant → multiple products = decomposition.
• (b): Since heat must be supplied to decompose Pb(NO₃)₂, it is endothermic. A common mistake is calling it exothermic because it produces gases.
• (c): Always show a table or element-by-element count. For oxygen on LHS: 2 molecules × 6 O atoms = 12. On RHS: 2(PbO) = 2, 4(NO₂) = 8, 1(O₂) = 2 → total 12. This atom-counting method is what earns full marks.