Why do some reactions require energy input to occur?

Some reactions require energy input to overcome the activation energy and break the bonds in the reactants.

In more detail, chemical reactions involve the breaking and forming of bonds between atoms to create new substances. The energy needed to break these bonds is known as the activation energy. This is the minimum amount of energy required for a chemical reaction to occur. If the reactants do not have enough energy to overcome this barrier, the reaction will not proceed.

The energy can be supplied in various forms such as heat, light or electricity. For example, the reaction between hydrogen and oxygen to form water releases a large amount of energy. However, this reaction will not occur unless a small amount of energy is first supplied, for instance by a spark, to get the reaction started. This initial energy input is used to break the bonds in the hydrogen and oxygen molecules, after which the reaction can proceed and energy is released.

In an endothermic reaction, the energy required to break the bonds in the reactants is greater than the energy released when the new bonds are formed in the products. This means that these reactions absorb energy from their surroundings. For example, the process of photosynthesis in plants is an endothermic reaction. It requires energy from sunlight to convert carbon dioxide and water into glucose and oxygen.

In contrast, exothermic reactions release more energy when the new bonds are formed than is needed to break the bonds in the reactants. This means that these reactions give out energy to their surroundings, often in the form of heat. The burning of fuels is a common example of an exothermic reaction.

In summary, the requirement of energy input for some reactions is a fundamental aspect of chemical reactions, linked to the concept of activation energy and the nature of endothermic and exothermic reactions.