Reactions that release energy and reactions that absorb energy are commonly referred to as exothermic and endothermic reactions, respectively. The key difference between these two types of reactions lies in the energy flow.
Exothermic Reactions: Exothermic reactions release energy into their surroundings. During an exothermic reaction, the reactants possess more energy than the products. As the reaction proceeds, the excess energy is released in the form of heat, light, or both. The energy released increases the temperature of the surroundings. Examples of exothermic reactions include combustion reactions (such as burning of wood) and many types of oxidation reactions.
Endothermic Reactions: Endothermic reactions, on the other hand, absorb energy from their surroundings. In these reactions, the reactants have lower energy than the products. As the reaction occurs, energy from the surroundings is absorbed to compensate for the energy difference. This absorption of energy usually causes the surroundings to cool down. Examples of endothermic reactions include photosynthesis, the process of converting water and carbon dioxide into glucose using sunlight, and the reaction between ammonium nitrate and water, which is used in instant cold packs.
In summary, exothermic reactions release energy to the surroundings, typically in the form of heat or light, whereas endothermic reactions absorb energy from the surroundings, resulting in a decrease in temperature.