The 3 Methods for Calculating the Enthalpy of a Reaction

[Thu Uyen Tran 1B]

Dr. Lavelle went over three methods to solve for the enthalpy of different reactions. How do you know which method should be used to solve problems, or can any method be applied and it is just a matter of what is given and preferred?

[Tim Foster 2A]

Hey! I know this much, rxn enthalpy is heat given off or taken up for the rxn, or just the enthalpy difference between the reactants and products. The enthalpy of formation of a compound is the enthalpy change between the reactants in their standard state and the product.
Hope that helps

[Justin Chang 2K]

It honestly depends on what’s given to you in the problem. For example, if you’re given the bond enthalpies of the compounds, then you should definitely use the bonds broken- bonds formed way of calculating enthalpy.

[Michelle Lee 2E]

It all depends on the information you're given. If you're given enthalpy changes and need to find a total enthalpy change, use Hess's Law. If you're given bond enthalpies, use the second method. If you're given standard reaction enthalpies, use the third method. This is not the only way to look at it though.

Question: Is there any conversion methods to convert information so we can use a specific method? (Like in 14A where we could change concentrations to partial pressures.) Will we ever be given something with multiple forms of the information? (like bond enthalpies and standard reaction enthalpies in the same problem)

[McKenna disc 1C]

I believe that it will be evident which method should be used based upon what is given in the problem.