Consider the combustion reaction,
2c4h2+5o > 4 co2+2h2o
where c2 h2 hf = 227.4 kj/mol), co2 (hf = –393.5 kj/mol), and h2 o (hf = –241.82 kj/mol). what is the enthalpy of the reaction?

–2,512.4 kj
–1,256.2 kj
1,256.2 kj
2,512.4 kj

The chemical reaction should be:
2 C₂H₂ + 5 O₂ ---> 4 CO₂ + 2 H₂O
This is complete combustion of a hydrocarbon.
We are also given with
C₂H₂ Hf = 227.4 kJ/mol
CO₂ Hf = –393.5 kJ/mol
H₂O Hf = –241.82 kJ/mol

The enthalpy of the reaction is
4(-393.5) + 2(-241.82) - 2(227.4) = -2512.44 kJ/mol