Standard enthalpies of formation [ENDORSED]

[Michelle Dong 1F]

In what case would a standard reaction enthalpy be equal to the standard enthalpy of formation?

[Sohini Halder 1G]

If you are finding the standard enthalpy reaction of 1/2 N2(g) + 1/2 02(g) ==> NO(g), that will be the same as finding the enthalpy of formation of NO(g). Only when your reactants are in gas form and and pure substances that form 1 mole of a product are they the same.

[K Stefanescu 2I]

In general, the complexity of the chemical formulas given on homework problems indicates that for most answers, this would not be the case. However, even though bond enthalpies are averages and not specific to the given compounds, if you have time (or want more practice), you can check your answers by doing whichever method wasn't requested of you in the particular problem. (I've found it to be a very useful way to sanity-check my answers and prep for the exam. The answers should be close to the same value.)