Enthalpy Change and Chemical Equations

Question

The following reaction is exothermic.

1. H₂(g) + F₂(g)2 HF(g)    ΔH(1) = -542 kJ

Calculate the enthalpy change for the reaction of the elements to form one mole of HF(g).

1/2 H2(g) + 1/2 F2(g)HF(g)    ΔH(2) =  kJ

Answer 1

H₂(g) + F₂(g) ==> 2HF(g) ... ∆H = -542 kJ

Divide this equation by 2 to get

1/2 H₂(g) + 1/2 F₂(g) ==> HF(g) and divide original ∆H by 2 to get -542/2 = -271 kJ