Which among the following reactions is endothermic?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Thermal decomposition
An endothermic reaction absorbs heat from its surroundings; an exothermic one gives heat out. In a thermal decomposition a compound breaks down only when heat is supplied, so the reaction takes in energy. For example, limestone breaks down into quicklime and carbon dioxide only on strong heating:
\(\ce{CaCO3}\xrightarrow{\text{heat}}\ce{CaO + CO2}\)
Why the other options are incorrect
Option (1) Respiration — Incorrect. Respiration oxidises glucose to carbon dioxide and water and releases energy, so it is exothermic.
Option (2) Burning of natural gas — Incorrect. Natural gas is mainly methane, and its combustion gives out heat: \(\ce{CH4 + 2O2 -> CO2 + 2H2O}\) + heat.
Option (3) Decomposition of vegetable matter into compost — Incorrect. Microbes break down the matter and release heat, which is why a compost heap feels warm.