How do you calculate the total enthalpy change in a multi-step reaction using Hess's Law?

To calculate the total enthalpy change in a multi-step reaction using Hess's Law, you add up the enthalpy changes of each step.

Hess's Law states that the total enthalpy change of a chemical reaction is independent of the route by which the reaction takes place, as long as the initial and final conditions are the same. This means that if a reaction can be expressed as the sum of two or more other reactions, the enthalpy change of the overall reaction is the sum of the enthalpy changes of those reactions.

To apply Hess's Law to a multi-step reaction, you first need to know the enthalpy changes for each individual step. These can be found in tables of standard enthalpies of formation, or they may be given in the problem.

Next, you need to ensure that the reactions are added together in such a way that the final reaction is the one you're interested in. This may involve reversing some reactions, which changes the sign of their enthalpy change, or multiplying them by a factor, which multiplies their enthalpy change by the same factor.

Once you have the reactions arranged correctly, you simply add up the enthalpy changes of each step to find the total enthalpy change for the overall reaction.

Remember, the key to using Hess's Law is to understand that the total enthalpy change for a reaction is the same, no matter what route is taken from reactants to products. This allows you to break down complex reactions into simpler steps, and calculate the enthalpy change for each step separately. By adding these together, you can find the total enthalpy change for the reaction, even if it's not possible to measure it directly.

In summary, using Hess's Law to calculate the total enthalpy change in a multi-step reaction involves knowing the enthalpy changes for each step, arranging the steps to give the overall reaction, and adding up the enthalpy changes.