What does a positive enthalpy change indicate in a reaction?

A positive enthalpy change in a reaction indicates that the reaction is endothermic, absorbing heat from its surroundings.

In more detail, enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing volume and pressure. In the context of chemical reactions, the change in enthalpy (ΔH) is a crucial factor.

When ΔH is positive, it means the system has gained energy from its surroundings. This is characteristic of an endothermic reaction. In an endothermic reaction, the energy needed to break the bonds in the reactants is greater than the energy released when new bonds are formed in the products. This energy difference is absorbed from the surroundings, often in the form of heat. This is why endothermic reactions often cause a decrease in temperature of the surroundings.

Common examples of endothermic reactions include photosynthesis, the process by which plants convert carbon dioxide and water into glucose and oxygen using sunlight, and the process of evaporation, where liquid water becomes water vapour.

In contrast, if ΔH is negative, the reaction is exothermic, meaning it releases energy to its surroundings. This is because the energy released when new bonds are formed in the products is greater than the energy needed to break the bonds in the reactants.

Understanding whether a reaction is endothermic or exothermic is important as it can influence how the reaction is carried out. For example, endothermic reactions may require an external heat source to proceed, while exothermic reactions may need to be controlled to prevent them from becoming too vigorous or dangerous.