Standard enthalpy of formation

[Lea Chamoun 2J]

Today in lecture, Dr. Lavelle explained that the standard enthalpy of formation is for the formation of a substance from its elements in the most stable form. I was wondering how this is different from the ∆H we found using the first method, in which NO is formed from O2 and N2, both of which are in their stable states, or if it is different at all.

[Shrinidhy Srinivas 3L]

So, the first method, which is Hess's method, does not necessarily use the elements in their most stable form. The goal of Hess's method is to use the delta H from known reactions to find the delta H of an unknown reactions. However, when considering standard enthalpy of formation, it's specifically related to the most stable form of each element. I hope this helps and clarifies this a little!

[Yichen Fan 3A]

In the first method of using Hess's law, the delta H that is given is for the NO reaction. It doesn't specify that all the reactants and products are in the standard condition; therefore, it is possible that delta H of that given reaction is different from the standard enthalpy of formation for NO. But you are absolutely right about N2 and O2 being the reactants we will use in finding the standard enthalpy of formation for NO.

[Jiapeng Han 1C]

The standard enthalpy of formation for N2 and O2 is 0! Because they are in their naturally occurring state! That is why we use Hess's Law to calculate the delta H.

[Jamie2002]

Hess's Law has to be used in the first problem because it involves N2 and O2, and both their standard enthalpies of formation are 0. Hess's Law is the method that works with these elements.