NDA (II) 2025 General Ability Test · Question 108 of 147

Correct — D, thermal decomposition of lead(II) nitrate. Splitting 2Pb(NO₃)₂ into lead(II) oxide, nitrogen dioxide and oxygen needs a steady supply of heat to break the bonds of the solid nitrate; the reaction draws energy in from its surroundings and stops as soon as heating stops. A reaction that absorbs energy in this way is, by definition, endothermic. The tell-tale sign is the yellow-brown nitrogen dioxide gas that comes off only while the crucible is kept hot.

• (a)CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l) — This is the burning of methane, the reaction that heats a gas stove. Combustion releases a large amount of heat to the surroundings, so it is strongly exothermic, not endothermic.
• (b)C₆H₁₂O₆ (aq) + 6O₂ (g) → 6CO₂ (g) + 6H₂O (l) — This is the oxidation of glucose (respiration or combustion). It liberates energy that living cells harness, making it one of the most familiar exothermic reactions.
• (c)N₂ (g) + 3H₂ (g) → 2NH₃ (g) — The Haber synthesis of ammonia gives out about 92 kJ of heat per mole of reaction, so it is exothermic; that is why the process is run at moderate temperature to favour the forward reaction.

Chemical reactions are classified by their heat exchange with the surroundings. An endothermic reaction absorbs heat (the surroundings cool down), while an exothermic reaction releases heat (the surroundings warm up). Breaking bonds costs energy and making bonds gives energy back, so a reaction is endothermic overall when more energy goes into breaking bonds than is recovered on forming new ones.

Thermal decomposition reactions, where one compound is split by heat into two or more products, are the classic endothermic case because the solid must be heated continuously to keep the reaction going. Combustion, respiration and most synthesis reactions like the Haber process run the other way and give out heat. The trap is to assume that any reaction needing a flame or heat to start must be endothermic; combustion needs a spark to begin but is firmly exothermic once lit.

• Endothermic reactions absorb heat from the surroundings; exothermic reactions release heat to the surroundings.
• Thermal decomposition of most metal nitrates and carbonates is endothermic and needs sustained heating.
• Heating lead(II) nitrate gives lead(II) oxide, brown nitrogen dioxide gas and oxygen.
• Combustion, respiration and the Haber process are all exothermic — they give out heat.

• Do not assume that a reaction which needs heating to start is endothermic; combustion needs a spark but is exothermic.
• Photosynthesis absorbs energy and is endothermic, while its mirror image respiration releases energy and is exothermic — do not swap them.

Asked to pick the endothermic (or exothermic) reaction from a list, or to classify a given decomposition or combustion by its heat change.

• practice — not a real PYQ

  Which one of the following is an exothermic reaction?

  • (a)Decomposition of calcium carbonate on heating
  • (b)Burning of natural gas in air
  • (c)Photosynthesis in green leaves
  • (d)Electrolysis of water

  Answer(b) Burning of natural gas in air — combustion releases heat, so it is exothermic.

• practice — not a real PYQ

  The reaction CaCO₃ (s) → CaO (s) + CO₂ (g) on strong heating is an example of

  • (a)an exothermic combination reaction
  • (b)an endothermic thermal decomposition
  • (c)a displacement reaction
  • (d)a neutralisation reaction

  Answer(b) an endothermic thermal decomposition — limestone must be heated continuously to split into quicklime and carbon dioxide.