Question

Ethyne (C2 H2 (g), mc030-1.jpgHf = 226.77 kJ/mol) undergoes complete combustion in the presence of oxygen to produce carbon dioxide (CO2 (g), mc030-2.jpgHf = –393.5 kJ/mol ) and water (H2 O(g), mc030-3.jpgHf = –241.82 kJ/mol) according to the equation below.

mc030-4.jpg

What is the enthalpy of combustion (per mole) of C2 H2 (g)?
Use mc030-5.jpg.
–2511.2 kJ/mol
–1255.6 kJ/mol
–862.1 kJ/mol
–431.0 kJ/mol

Answer 1

ΔHcomb = ΣΔHf(products) − ΣΔHf(reactants)

= [2(−393.50) + (−241.82)] − [226.77 + (5/2)(0)]
= [−787.00 − 241.82] − 226.77
= −1028.82 − 226.77
= −1255.59 ≈ −1255.6 kJ/mol

Answer: −1255.6 kJ/mol.