How can you determine the enthalpy change for a reaction using Hess's Law?

The enthalpy change for a reaction can be determined using Hess's Law by summing the enthalpy changes of individual steps.

Hess's Law, also known as Hess's Law of Constant Heat Summation, states that the total enthalpy change for a chemical reaction is the same, regardless of how the process is carried out in stages. This is because enthalpy is a state function, meaning it only depends on the initial and final states of a system, not the path taken to get there.

To use Hess's Law to determine the enthalpy change for a reaction, you first need to break down the overall reaction into a series of individual steps. These steps should be reactions for which the enthalpy changes are known. These known values can often be found in tables of standard enthalpies of formation or standard enthalpies of reaction.

Once you have broken down the overall reaction into individual steps, you can then sum the enthalpy changes for each of these steps to find the total enthalpy change for the reaction. Remember to take into account the stoichiometry of the reaction - if a reaction needs to be multiplied by a factor to match the overall reaction, the enthalpy change for that step should also be multiplied by the same factor.

For example, if you have a reaction A + B → C and you know the enthalpy changes for the reactions A → D and D + B → C, you can add these two enthalpy changes together to find the enthalpy change for the overall reaction. If the reaction A → D needs to be reversed to match the overall reaction, the sign of the enthalpy change for that step should be reversed.

In summary, Hess's Law allows us to determine the enthalpy change for a reaction by breaking it down into a series of steps for which the enthalpy changes are known, and then summing these changes. This is a powerful tool in thermodynamics, allowing us to calculate enthalpy changes for reactions that may be difficult or impossible to measure directly.