Read the following and answer the questions that follow:
A science teacher demonstrates an experiment where lead nitrate powder is heated in a boiling tube. Brown fumes are observed coming out of the tube, and a yellow solid remains at the bottom. The teacher then performs another experiment where a white solid, silver chloride, is placed in a china dish and kept in direct sunlight. After some time, the white solid turns grey, indicating the formation of silver metal and a greenish-yellow gas.
(i) Name the brown fumes produced when lead nitrate is heated and write the balanced chemical equation for this reaction. (1 mark)
(ii) Write the balanced chemical equation for the decomposition of silver chloride in sunlight. (1 mark)
(iii) What form of energy is responsible for the decomposition of silver chloride? How does this differ from the energy used to decompose lead nitrate? (1 mark)
(iv) Both reactions above are decomposition reactions. What is the general characteristic of all decomposition reactions in terms of energy? (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 00:55 · grounding rag
Model Answer
(i) The brown fumes are nitrogen dioxide (NO₂).
$$2\text{Pb(NO}_3)_2\text{(s)} \xrightarrow{\Delta} 2\text{PbO(s)} + 4\text{NO}_2\text{(g)} + \text{O}_2\text{(g)}$$
(ii) Decomposition of silver chloride in sunlight:
$$2\text{AgCl(s)} \xrightarrow{\text{sunlight}} 2\text{Ag(s)} + \text{Cl}_2\text{(g)}$$
(iii) Light energy (solar/photochemical energy) causes the decomposition of silver chloride.
In contrast, lead nitrate is decomposed by heat energy (thermal energy). Thus, one reaction is photochemical and the other is thermal decomposition.
(iv) All decomposition reactions are endothermic — they require energy to be supplied (in the form of heat, light, or electricity) to break down a single substance into two or more simpler substances.
Source: Chapter 1, Chemical Reactions and Equations
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Explanation
- (i): The yellow solid left behind is PbO (lead monoxide). Examiners expect the name "nitrogen dioxide" and a correctly balanced equation — check that atoms of Pb, N, and O balance on both sides.
- (ii): The greenish-yellow gas is Cl₂. The equation must show sunlight as the condition above the arrow, not as a reactant. Coefficient of 2 on both sides is essential for balance.
- (iii): The key distinction tested here is type of energy: light (photochemical) vs. heat (thermal). One line for each is enough.
- (iv): This is a direct concept from the chapter — decomposition reactions are endothermic because energy must be absorbed to break bonds. Contrast with combination reactions which are typically exothermic.