What's the main difference between exothermic and endothermic reactions? Can you give simple examples?

The main difference between exothermic and endothermic reactions lies in how they exchange heat energy with their surroundings.

Exothermic reactions are those that release heat energy into the surroundings. This causes the temperature of the surroundings to increase, making the reaction vessel feel warm or hot. In these reactions, the products have lower energy than the reactants, and the excess energy is given off as heat.

• Examples: Burning of natural gas ($CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$), respiration in living organisms, the reaction of quicklime with water.

Endothermic reactions are those that absorb heat energy from the surroundings. This causes the temperature of the surroundings to decrease, making the reaction vessel feel cool or cold. In these reactions, the products have higher energy than the reactants, and the required energy is taken in from the surroundings.

• Examples: Photosynthesis ($6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$), dissolving ammonium chloride in water, melting ice.