[shylight200]

the question is

a. use the standard enthalpies of formation to calculate the standard enthalpy of the following reaction at 298 K 

2 NO2 (g) ---> N2O4 (g)

b. calculate the standard enthalpy at 373K

please help me get started

[Padfoot]

Use Hess's Law - http://en.wikipedia.org/wiki/Hess%E2%80%99s_law

1/2N2+O2 → NO2     dH= x
N2+2O2 → 2NO2      dH= 2x
2NO2 →  N2+2O2    dH= -2x

N2+2O2 → N2O4     dH= y

Using Hess’s Law:   2NO2→ N2O4    dH = -2x+y

x and y are heats of formation.