Which of the following is a statement of Hess's law?

Answer 1

All the reactions in a path are added together.

Answer 2

The enthalpy of reaction does not depend on the steps taken in the reaction.(Apex)

Answer 3

Hess's law states that if a reaction takes place in several steps then its standard reaction enthalpy is the sum of the standard enthalpies of the intermediate reactions into which the overall reaction may be divided at the same temperature.

Answer 4

The concept behind Hess's law is that the change in enthalpy in going from an initial state to a final state is pathway-independent since enthalpy is a state function. Therefore, if we have a reaction in a series of steps, the change in enthalpy as when the reaction takes place in one step.

This may sound a bit abstract at the moment, but I can clarify it with a simple example.

Say you want to find the delta H1 of a reaction:

1. N2 (g) + O2 (g) --> 2NO2 (g)

However, you know the change in enthalpy for two other reactions (which we shall label 2 and 3 just to avoid confusion) :

2. N2 (g) + O2 (g) --> 2NO (g) delta H2 = 180 kJ

3. 2NO (g) + O2 (g) --> 2NO2 (g) delta H3 = -112 kJ

When you add reactions 2 and 3 together you get:

N2 (g) + O2 (g) + 2NO (g) + O2 (g)--> 2NO (g) + 2NO2

The 2NO(g) on each side cancel out and you are left with:

N2 (g) + O2 (g) --> 2NO2 (g)

which is the equation for which you would want to find delta H!

Therefore, using Hess's law, we can simply add delta H2 = 180 kJ and delta H3 = -112 kJ together to find our delta H for the first reaction since the second reaction and the third reaction add together to give us the first reaction.

Therefore delta H=180+(-112)=68 kJ