Endothermic Reactions


A chemical reaction that involves a gain of heat energy from the surroundings, resulting in a drop in the temperature of the surroundings is called an endothermic reaction.

  • Summarised version: A reaction that takes in heat from the surroundings.
  • The temperature of the surroundings decreases. (The reaction container becomes colder.)

The energy content of the products is higher than the energy content of the reactants.

enthalpy_endo

As heat energy is gained in an endothermic reaction, the $\Delta \text{H}$ is positive.

Examples of Endothermic Reactions

Formation of carbon disulphide is an endothermic reaction:

  • $\text{C}\left( \text{s} \right) + 2 \text{S} \left( \text{s} \right) \rightarrow \text{CS}_{2} \left( \text{s} \right)$, whereby $\Delta \text{H} =+100 \text{ kJ mol}^{-1}$.
  • This means that 100 kJ is required to form 1 mole of carbon disulphide from 1 mole of carbon and 2 moles of sulphur.
  • 1 mole of carbon disulphide has a higher energy content than 1 mole of carbon and 2 moles of sulphur.

Decomposition of calcium carbonate:

  • $\text{CaCO}_{3} \rightarrow \text{CaO} + \text{CO}_{2}$, whereby $\Delta \text{H} =+384 \text{ kJ mol}^{-1}$.

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