Question

Which one of the following is a correct formation reaction that properly defines a delta Hf value?

The answer is 1/2N2(g) + 2O2 = NO2(g) Please explain. I'm having a hard time understand this question.

The other answer options is

b) N2(g) + 2O2(g) = 2NO2

c) H2(g) + O(g) = H2O

d) 3H(g) + N(g) = NH3(g)

e) 2NH(g) = 3H2(g) + N2

I also don't understand why it's not the other options.

Answer 1

Definition of ΔH_f

The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the compound from its constituent elements, with all substances in their standard states at 1 atmosphere (1 atm or 101.3 kPa).

b.) N2(g) + 2O2(g) = 2NO2

2 moles are formed in this case and the state of NO2 is not given in the data.

c.)H2(g) + O(g) = H2O

O is not the standard state of oxygen. The standard state is O₂. The state of H2O is not given in the data.

d.)3H(g) + N(g) = NH3(g)

H and N is not the standard state of hydrogen and nitrogen. The standard state of H and N is H₂ and N₂.

e.)2NH(g) = 3H2(g) + N2

NH molecule does not exist.

a) The correct answer is: 1/2N2(g) + 2O2 = NO2(g).

Here one mole of NO₂ is fromed from N₂ and O₂ (The standard state of nitrogen and oxygen is N₂ and O₂).