To interpret and compare heats of reaction (Hrxn), standard states must be used. What are the standard states for a gas, an aqueous solution, and a pure substance? Why are they important?

Answer 1

I would think this would be covered in your text/notes. Here is a reference but IUPAC rules don't always follow through in chemistry texts/journals. For example, most U.S. based chem text use 1 atm as standard pressure but IUPAC suggest 100 kPa (1 bar).

Answer 2

http://en.wikipedia.org/wiki/Standard_state