Addition in Hess's Law

[Ivy Zhang 2A]

When we are using Hess's method to calculate enthalpy change, why are we able to add the enthalpy change of each reaction step? In the lecture, it was mentioned that enthalpy is a state function, and therefore enthalpy changes are additive? Why is that?

[CaitlinDu_2A]

I think you can only apply Hess's Law if you wanted to find the difference in enthalpy, therefore you could add or subtract the initial enthalpy from the final.

[Zain Syed 1E]

They are additive because they are state functions. That means we only consider the endpoints and not the path to reach that certain state. Since we are only considering the endpoints of enthalpy, we are able to combine them fully without considering what was in between. Enthalpy is the amount of heat released or absorbed, so we just need to figure out the initial and final state to find the total enthalpy. But something like heat is not a state function since it is the energy that is transferred(meaning it depends on the path) into or out of the system.

[Mia Donato]

A state function does not depend on the path taken, but rather on the initial and final values. Hess's Law is that the total enthalpy is the sum of the reaction's enthalpy changes. We are able to add the enthalpy change of each reaction step because enthalpy is a state function.

[Nishanth Vemuri 2L]

Because we only want to look at the initial and final enthalpies of a reaction, we can add up the enthalpies of multiple reactions to get the total enthalpy change. Because enthalpy is a state function, it does not depend on the path taken, and can be added during any reaction.

[Madhavi Akella 2H]

The reason we can add enthalpy change is being enthalpy is a state function. Therefore to find the enthalpy, you can find the difference between the initial and final enthalpies of the reaction(through addition/subtraction).

[Elliana_Tsang_1L]

I believe that you are correct. Since enthalpy is a state function, it does not matter what the intermediate steps are. The energy change between from reactants to products is the only thing that determines enthalpy. Adding enthalpy from different reactions together in Hess' Law follows this, since only this difference in energy is important, they can be added together to get an overall change. Also, for some reactions, there may be an intermediate formed, so the energy to get from the reactants to the intermediate would be added to the energy to get from the intermediate to the products.

[Smita Ektare 2J]

Since enthalpy is a state function, it only matters what the difference between the initial and final enthalpies is, not the intermediate steps. Therefore, through Hess's Law, we can add the enthalpies of different reactions as long as the final sum is the same as the desired reaction.