Enthalpy reaction from enthalpy formation

Question

Using standard heats of formation, calculate the standard enthalpy change for the following reaction.

N₂(g) + O₂(g)2NO(g)

Answer: Kj

ALSO

Using standard heats of formation, calculate the standard enthalpy change for the following reaction.

2BrF₃(g)Br₂(g) + 3F₂(g)

Answer: Kj

Answer 1

You need to use the table provided for the standard heats of formation for the various compounds and then proceed as follows:

Sum ∆Hf products - sum ∆Hf reactants = ∆Hrxn

For N₂(g) + O₂(g) ==> 2NO(g) you would look up ∆Hf for NO and multiply that value by 2. Then subtract the ∆Hf for N₂ and for O₂, but since those values will be zero, the answer is just 2 x the ∆Hf for NO.

For 2BrF₃ ==> Br₂ + 3F₂ the ∆Hf is zero for both products so the answer will just be the tabular value for ∆Hf for BrF₃ x 2 with a minus sign.