3 Methods to Find Enthalpy

[Alysha Dijamco 3E]

Hello!

I am still a bit confused on when to use the 3 methods of finding enthalpy. Are there any keywords I should be looking out for when reading a question in order to determine which method I should be using? Thank you!

[Brady Judson Section 2B]

If you are given multiple reactions and their respective enthalpies, then you should use method one (Hess' Law). If you are simply given an reaction and bond enthalpy values, you should use method two. Lastly, if you are pretty much given nothing but a reaction, you should use method three.

[Ashley Chu 3C]

The first method is using Hess's law to add up the enthalpies of each step to have a total enthalpy change. Usually, this is used when there are multiple parts of the equation given with their enthalpy of reactions. Method two uses the bond enthalpies of molecules to add up and find the total enthalpy of the reaction, but it is not always accurate since some bond energies are averages measured experimentally. The third method uses the standard enthalpies of the products and reactants to find the difference between them to find the enthalpy of reaction.

[Andrew Dinh 2L]

If you are trying to find the enthalpy of a reaction, and are given the enthalpies of other, similar related reactions, use Hess's Law (Method 1). If you are given bond enthalpies in any which way, use method 2 (subtracting enthalpies of bonds broken - bonds formed). If you are given reactants and products in their standard states, use the standard enthalpies of formation (method 3).

[Kelly Tang 1B]

HI! Just like the others said, Hess's Law (method 1) is usually used whenever you are given multiple equations/steps (to make the final equation) and have to find the total sum of enthalpies of those steps. The second method should be used when you are given bond enthalpies because the only way you could use bond enthalpies is through the equation ΔH = ΣΔH(bonds broken) - ΣΔH(bonds formed). Method 3 will be used whenever you are simply provided standard enthalpies of formation of the reaction.

[705586182]

I was also wondering if standard enthalpies (change in Hf) table is given to us or do we have to memorize everything?

[Anabelle Pham 2C]

When you are given the enthalpies of each reaction then use Hesse's law. When you are given the bond enthalpies then use that. When you basically are given nothing except the standard enthalpy then use the last method.

[Eleanor_C_1I]

I think the easiest way to distinguish between which to use is based on the information given in the problem. For example, if we are given a series of reactions with their associated enthalpies, we can use Hess' law to alter (if needed) and sum them, whereas if we are only given one reaction we can add up the individual bond enthalpies.