Hess's Law - Enthalpy vs Equilibrium Constant

[Kelly Tang 1B]

Hello! I get that when we are working with Hess's Law and enthalpy, we calculate total ΔH of the reaction by adding up the ΔH of each reaction to get a sum, but how come we had to find the product rather than the sum when working with Hess's Law and equilibrium constants?

[Luke Iwai 3L]

If I understand the questions correctly, enthalpy and ΔH are a different topic than equilibrium constant.

Enthalpy is a state property and therefore to find total change of enthalpy it is possible to add up ΔH for multiple steps of a reaction
The equilibrium constant is a ratio between concentrations at equilibrium, therefore multiplication and division is required.

Basically, its like comparing apples or oranges, they are no the same thing.

[Rebecca Liu 2E]

Enthalpy changes are additive because it is a state function, which allows us to use Hess's Law: adding reactions to obtain a net reaction, and also adding the enthalpy change at each step in a multi-step reaction for the total enthalpy change. Equilibrium constants, from the other chapter, involve a products over reactant ratio with coefficients as powers, which is completely different than Hess's law.

[Hersh Nanda 2J]

Hess's law does not relate to equilibrium constants. Hess's law concerns a reaction's enthalpy, which is the transfer of energy (exothermic reactions with a negative deltaH would release energy while endothermic reactions with a positive deltaH would absorb energy). Equilibrium constants, on the other hand, concern substance rather than energy transfer; it relates to the ratio of products and reactants as well as the direction of a reaction.

[Jackson Crist 1G]

The simplified answer for what everyone else is saying is basically: heiss law is not related to equilibrium constants. Heiss law can be used because enthalpy is a state property so you can just add them all together to get the total.

[Suraj Kulkarni 2B]

Hello! Yes - basically these concepts are completely different and there shouldn't really be that much overlap. Hess's law has to do with all the details you discussed in your question, and then equilibrium constants have to do with our first week (and the concept also applies to the second week as well where we covered acidity and basicity constants).