Exothermic and Endothermic Reactions Notes

In an exothermic reaction, energy is transferred to the surroundings, usually as heat, and the temperature of the surroundings rises. The overall energy change is negative.

Examples: combustion of fuels, neutralisation, oxidation such as rusting, and respiration.

In an endothermic reaction, energy is taken in from the surroundings and their temperature falls. The overall energy change is positive.

Examples: thermal decomposition such as $\ce{CaCO3 -> CaO + CO2}$, citric acid with sodium hydrogencarbonate, and photosynthesis.

A reaction profile has energy on the vertical axis and reaction progress on the horizontal axis.

• Exothermic: products lower than reactants
• Endothermic: products higher than reactants
• The gap from the reactants up to the peak is the activation energy
• The gap between reactants and products is the overall energy change

Colliding particles with less than the activation energy simply bounce off. It is always positive and is the gap from the reactants up to the peak.

High activation energy means slow at room temperature; low means fast. A catalyst lowers the activation energy, so the reaction goes faster without the catalyst being used up.

Both types need activation energy. The difference is only whether energy is released overall or absorbed overall. When asked to compare, give the temperature change, the profile shape and an example.