Hess's Law

[Kyilah Terry]

Hey! So I was just going over some of the ways to find delta H, but got confused when reading the steps to Hess's Law. In the textbook it says that step one is to take a reactant a write a chemical equation in which it also appears as a reactant. and step two is select a product that also appears as a product. When I do these I really don't know what I'm looking for or if I'm even doing it right. Does someone know an easier method?

[Jessica Manzano 1B]

When using Hess's Law, you're basically manipulating two or more given chemical equations and their delta H values so that when they are added together, they give you the chemical equation and delta H of the reaction you are finding delta H for. The given equations are steps into which the equation you are finding delta H for can be divided into--- in summary, the overall enthalpy of a reaction can be found from the sum of the enthalpies of its steps.

For example, if the equations you are given include reactants or products that are not in the equation you want the delta H for, then you would want to reverse the equations so that those unneeded reactants or products cancel out when the given equations are added together. In this manipulation, because you reversed the direction of a reaction, you must change the sign of that reaction's delta H before you add the delta H values of all the reactions together.

Additionally, you may reverse the direction of an equation to get a reactant or product on the side required by the equation you are finding delta H for.

To get the desired equation from the given equations, you may also manipulate the given equations to have the correct amount of moles by using multiplication or division. When you use this manipulation, you must also multiply or divide the delta H of the equation you manipulated before you add the delta H values of all the reactions together.

In the end, you add all the manipulated given equations together and add all the manipulated given delta H values together to find the delta H you are looking for. The given equations must add up to the equation you are finding delta H for.

[Chem_Mod]

Problem 8.61 from the homework is a great example of how to do one of these types of problems if you're looking for more practice. The solutions manual gives a good description of how to go about solving the problem.