Calculating delta H

[paulina_higashi_1G]

How do we know when to do (deltaH products - deltaH rectants), versus just the deltaH reactants and deltaH products combined?

[Sam Leistiko 2B]

I am not sure that I fully understand this question, but I will still try to answer.

If you have two reactions, and you want to combine the reactions, you add the ΔH for each reaction. This is Hess' law, and it comes from the fact that enthalpy is a state function. If two reactions are combined, and ΔH for the first is -100kJ, and ΔH for the second is +50kJ, then ΔH for the combined reaction would be -50kJ.

There are a few other ways to determine ΔH. This method can be somewhat confusing, because it requires you to do ΣH (reactants) - ΣH (products), while most other methods are some variation of H (products) - H (reactants). However, this is because bond enthalpy is defined as the energy required to break a bond, and is always positive. Therefore, energy being put into the system (positive) is the energy required to break the bonds in the reactants, and the energy going out of the system (negative) is the energy released when bonds are formed in the products, which is equal to the bond enthalpy of the products, but negative. Therefore, you can think of it like ΣH (bonds broken) - ΣH (bonds formed).

You can also use standard enthalpy of formation. Enthalpy of formation is the standard reaction enthalpy form the formation of a substance from its elements in their most stable form. Using these values, ΔH°= ΔH°(products) - ΔH°(reactants).

Hopefully this helps!