Enthalpy and Hess's Law

[Jinsiang_Ong_3H]

Can anyone simply explain the relationship between enthalpy and Hess's law and also how Hess's law is different from enthalpy of reaction?

[kamrin schultz 2J]

Hess's law can be used to calculate enthalpy changes. The law refers to the fact that the change in enthalpy of a reaction is the same whether the reaction takes place in one step or a series of steps. This means that enthalpy change at each step can be added to give a total enthalpy change, which is possible because enthalpy is a state function.

Some important characteristics of enthalpy change are:
-if a reaction reversed, the sign of ∆H is reversed
-if coefficients in a balanced equation are multiplied by a number, ∆H is also multiplied by that number

[Jennifer Liu 2A]

Hess's Law's relationship with enthalpy & enthalpy of reaction are the same--that they're additive. You can add/subtract (depending on the direction of the reaction and the given information of other enthalpies) reactions to obtain the net reaction and net enthalpy change.

Hess's Law is different from enthalpy of reaction because Hess's Law is describing the trait of enthalpy (that it's a state property) and enthalpy of reaction is quite literally the enthalpy of a given reaction.

[705995679]

What is the difference between standard enthalpy of formation and enthalpy of a reaction?
It was my understanding that you need the standard enthalpy of formation in order to perform any of the methods, so in order to find the enthalpy of a reaction or to use any of the methods the professor explained on Friday, you need the standard enthalpy of formation first right?

[andrew_bishay_1D]

Hess's Law is a method by which you can calculate the enthalpy change of a reaction. As enthalpy is a state function, enthalpies of different reactions can be added together when the reactions are added together. It's usually applied in the context of having to add a set of reactions together in order to find the change in enthalpy change for the desired rxn.

For ex, if
A + B --> C, delta H = x, and
C + D --> E, delta H = y, and say we want to find the delta H for A + B + D --> E, we can add the 2 rxn's together, and by Hess's law, the delta H of this rxn will be given as delta H = x+y

[Sandhya_Seetharaman_2A]

What is the difference between standard enthalpy of formation and enthalpy of a reaction?
It was my understanding that you need the standard enthalpy of formation in order to perform any of the methods, so in order to find the enthalpy of a reaction or to use any of the methods the professor explained on Friday, you need the standard enthalpy of formation first right?

I think you’re right in that you can use the standard enthalpies of formation for each reactant and product, add them up respectively, and then find the difference, but it’s a lot easier to just use the bond enthalpies, since you only need to look at the enthalpies of the bonds that are broken and those that are formed.