Rates of Reaction: Endothermic and Exothermic Reactions

Endothermic and exothermic reactions are two types of chemical reactions that differ primarily by where energy is stored.
Exothermic reactions are those where energy is released, usually in the form of heat. Examples include combustion reactions or neutralisation reactions.
In an exothermic reaction, the energy required to break the initial bonds in the reactants is less than the energy released when the new bonds are formed in the products.
The temperature of the surroundings will increase during an exothermic reaction. The energy level diagram for an exothermic reaction shows that the products have less energy than the reactants.
Conversely, endothermic reactions are those where energy is taken in from the surroundings, usually again in the form of heat. These reactions will feel cold to touch. Photosynthesis is an example of this sort of reaction.
In an endothermic reaction, the energy required to break the initial bonds in the reactants is greater than the energy released when the new bonds are formed in the products.
The temperature of the surroundings will decrease during an endothermic reaction.
The energy level diagram for an endothermic reaction shows that the products have more energy than the reactants.
Energy changes in reactions can be measured using calorimetry. Knowing the mass of a substance and the temperature change it undergoes can allow one to calculate the amount of energy transferred in the reaction.
Activation energy refers to the minimum amount of energy needed for a reaction to occur. In exothermic reactions, this is the energy needed to break the bonds in the reactants, while in endothermic reactions, this is the energy needed to both break the bonds in the reactants and absorb additional energy to reach the product state.
Catalysts can decrease the activation energy of a reaction, making the reaction occur faster. They do not change the total amount of energy released or taken in by the reaction.
In revision sessions and textbooks, it’s common to see energy changes represented on energy diagrams. These diagrams show the energy of the reactants and products, and the activation energy as a ‘hump’ in between.
It is important to remember that, whether a reaction is endothermic or exothermic, it will only proceed if it leads to an arrangement of atoms with a lower overall energy. This is due to the principle that natural systems tend toward the lowest possible energy state.
An understanding of endothermic and exothermic reactions has applications in everyday life, from hand warmers to cold packs, and contributes to an understanding of larger processes like weather patterns and energy production.