Homework problem 8.59

[Shirley Wong 2E]

Use the data provided in Appendix 2A to calculate the standard reaction enthalpy for the reaction of pure nitric acid with hydrazine: 4HNO3(l) + 5N2H4(l) -> 7N2(g) + 12H2O(l)

For this problem, I understand that we add all the standard enthalpies of formation for the products minus the standard enthalpies of formation for the reactants. In the solutions manual, it doesn't add the standard enthalpy of formation of Nitrogen for the products. Why is that?

[Tiffany Chen 2E]

The Nitrogen is disregarded because N2 is an example of a gas in its standard state, which means that its standard enthalpy of formation is 0. Other gases in standard state and with a ΔHf of 0 include O2, F2, Br2, H2, and Cl2.

[Samantha Miceli 3J]

It doesn't add the standard enthalpy of N2 because it is equal to zero. In section 8.17 of the textbook, it says that for a element in its most stable form, the standard enthalpy of formation is zero.

[DamianW]

How do we know when an element is in its standard form?

[RussellChin_3A]

Dr. Lavelle gave some of the common standard states for elements such as the diatomic elements like N₂,O₂ would be in the standard states if they were gases and others like Br₂ would be standard states if it were in the liquid state. Other standard states he gave was solid Carbon as graphite.

[Nehal Banik]

Any gas at its most stable state is regarded as having an enthalpy of formation as equal to 0 because it becomes insignificant.