5.1.1 Energy Transfer During Exothermic and Endothermic Reactions

In chemical reactions, energy is conserved, which means that the total amount of energy in the universe remains constant. The energy can be transferred between the reactants, products, and the surroundings. Understanding the direction of energy transfer is important in classifying reactions as either exothermicA reaction that releases heat, where the products have less energy than the reactants. or endothermicA reaction that ‘absorbs’ heat, where the products have greater energy than the reactants..

Exothermic Reactions

An exothermic reaction is one that transfers energy from the reactants to the surroundings. As a result, the temperature of the surroundings increases. In exothermic reactions, the product molecules have less energy than the reactant molecules, and the excess energy is released into the surroundings.

Examples of exothermic reactions include combustion reactions, where substances react with oxygen and release heat and light. Many oxidation reactions, such as the rusting of iron, also fall into the category of exothermic reactions. Neutralisation reactions, where an acid and a base react to form a salt and water, are another common example.

Exothermic reactions have various practical applications. For instance, self-heating cans use exothermic reactions to warm up the contents of the can when a reactant is activated. Hand warmers, often used in cold weather, are also based on exothermic reactions that produce heat as a byproduct.

Endothermic Reactions

An endothermic reaction is one that absorbs energy from the surroundings. As a result, the temperature of the surroundings decreases. In endothermic reactions, the product molecules have more energy than the reactant molecules, and they gain this additional energy from the surroundings.

Examples of endothermic reactions include thermal decompositions, where a compound breaks down into simpler substances by absorbing heat energy. Another example is the reaction between citric acid and sodium hydrogen carbonate, which is used in certain sports injury packs to create a cooling sensation.

Conclusion

In an exothermic reaction, the surroundings become warmer, while in an endothermic reaction, the surroundings become cooler. This temperature change is observed in the surroundings, not necessarily within the reaction vessel.