Why are enthalpy changes affected by the route taken for a reaction?

Enthalpy changes are affected by the route taken for a reaction due to the law of Hess.

The law of Hess, also known as Hess's Law, states that the total enthalpy change for a chemical reaction is independent of the route by which the chemical reaction takes place. It is only dependent on the initial and final states. This is a direct consequence of the law of conservation of energy. Energy cannot be created or destroyed, only transferred or transformed. Therefore, the total energy change in a system is the same, regardless of the path taken.

In a chemical reaction, the enthalpy change is the heat absorbed or released at constant pressure. It is a state function, meaning it only depends on the initial and final states of the system, not on the path taken to get from one state to another. This is why the route of a reaction affects the enthalpy change. If a reaction can occur in one step or several steps, the total enthalpy change will be the same.

For example, consider the combustion of methane. This can occur in one step, where methane reacts with oxygen to form carbon dioxide and water, releasing a certain amount of heat. Alternatively, it could occur in two steps, where methane first reacts with oxygen to form carbon monoxide and water, releasing some heat, and then the carbon monoxide reacts with more oxygen to form carbon dioxide, releasing more heat. The total heat released in the two-step process is the same as the heat released in the one-step process.

This principle is very useful in calculating the enthalpy changes for reactions that are difficult to carry out in a laboratory. By using Hess's Law, we can calculate the enthalpy change for a reaction by adding up the enthalpy changes for a series of reactions that, together, give the overall reaction. This is why the route of a reaction affects the enthalpy change.