Exothermic and Endothermic Reactions

An exothermic reaction is one in which energy is released typically in the form of heat. This occurs when the total energy of the products is less than the total energy of the reactants. The symbol ΔH is used to represent the change in enthalpy or total energy. For an exothermic reaction, ΔH will be negative.
Heat is evolved in an exothermic reaction. This means that the surrounding environment will get warmer. An example is the combustion reaction where fuels like hydrocarbons react with oxygen to produce carbon dioxide, water, and energy.
Examples of exothermic reactions include combustion, neutralisation, formation of a gas from separate elements e.g, H2 + Cl2 → 2HCl.
In contrast, an endothermic reaction absorbs energy, usually as heat, from its surroundings. This is because the total energy of the reactants is less than the total energy of the products. The symbol ΔH is positive for an endothermic reaction.
Heat is absorbed in an endothermic reaction causing the surrounding environment to get cooler. An example is the process of photosynthesis in which plants absorb sunlight (and carbon dioxide) to produce glucose and oxygen.
Examples of endothermic reactions include photosynthesis, thermal decomposition (such as the breakdown of limestone into lime and carbon dioxide), and the dissolution of ammonium chloride in water.
As regards to equilibrium, an increase in temperature favours the formation of products in an endothermic reaction, as described by Le Chatelier’s Principle. Conversely, a decrease in temperature favours the formation of products in an exothermic reaction.
Le Chatelier’s principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.
These temperature changes can have a significant effect on industrial processes where reactions need to be maintained at a certain temperature to ensure high yield of product.
Understanding exothermic and endothermic reactions, and how changes in conditions can affect equilibrium, is key to controlling and optimising chemical processes, especially in the context of sustainable development and energy efficiency.