thermochemistry help?

Question

Show the change in enthalpy calculation for the formation of NO from N2 and O2 using the following info:

NO (g) + .5O2 (g) --> NO2 (g)

change in enthalpy = -13.5 kcal/mol NO



i am so confused can someone help me please? (just telling me HOW to do it is fine)
THANK YOUUU <3

bleehhhhhhhhh i need help :(

Answer 1

I can not do it only with the data you gave. But I believe the necessary data can be found from your textbook.
For example, from the heat of formation ΔE for all three involved chemicals. This ΔE can be used to calculate the internal energy change ΔU: ΔU = sum of the ΔE's of the reactants subtracts the sum of the ΔE's of the products. Then you may get ΔH: ΔH = ΔU + PV. For a reaction in aquous solution, since the change in V can be approximated zero, ΔH = ΔU. For a gas phase reaction, you may not need to worry about change in PV as long as the total mole-number is the same from reactants to products (do you know why?). But in the current case you need to count the work done by the environment on the system, since 1.5 mole of reactants would produce only one mole of products (hence V would reduce under a constant P). Using ideal gas formula, PV = nRT. Therefore in this case ΔH = ΔU - 0.5RT, where -0.5 is the change in (total) moles, R = 8.314 the gas constant, and take T to be 298K.

I do not know why the problem only gives "change in enthalpy = -13.5 kcal/mol NO", but not for NO2?