calculate the heat of combustion for the following reactions from the standard enthalpies of formation

Question

2H2(g) + O2(g) = 2H2O(l)
2C2H2(g) + 502(g) = 4CO2(g) + 2 H2O(l)

Answer 1

Part 1: a) ΔH^o = 2ΔH^o (H2O) − 2ΔH^o (H2) − ΔH^o (O2)

ΔH^o = (2)(−285.8 kJ/mol) − (2)(0) − (1)(0)
= −571.6 kJ/mol

Part 2: (b)ΔH^o = 4ΔH^o (CO2) + 2ΔH^o (H2O) − 2ΔH^o (C2H2) − 5ΔH^o (O2)

ΔH^o = (4)(−393.5 kJ/mol) + (2)(−285.8 kJ/mol) − (2)(226.6 kJ/mol− (5)(0)
= −2,599 kJ/mol