How do you determine the enthalpy change for an indirect reaction using Hess's Law?

The enthalpy change for an indirect reaction is determined using Hess's Law by summing the enthalpy changes of direct reactions that give the same overall change.

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 principle is also known as the conservation of energy. In other words, the total energy of an isolated system remains constant if not acted upon by an external force.

To determine the enthalpy change for an indirect reaction, you first need to identify the direct reactions that, when added together, give the same overall chemical change as the indirect reaction. These direct reactions are often simpler and their enthalpy changes can be determined experimentally or found in tables of standard enthalpies of formation or combustion.

Once you have identified the direct reactions, you can calculate the enthalpy change for the indirect reaction by adding together the enthalpy changes for the direct reactions. This is where Hess's Law comes into play. It allows you to add together the enthalpy changes for the direct reactions just like you would add together the steps in a mathematical equation.

Remember, when using Hess's Law, you may need to reverse or multiply the direct reactions to match the overall chemical change of the indirect reaction. If you reverse a reaction, you must change the sign of its enthalpy change. If you multiply a reaction by a factor, you must also multiply its enthalpy change by the same factor.

In summary, determining the enthalpy change for an indirect reaction using Hess's Law involves identifying the direct reactions that give the same overall chemical change, and then summing their enthalpy changes. This method is particularly useful when the enthalpy change for the indirect reaction cannot be measured directly or is difficult to measure.