Answer to Question #117401 in Chemistry for Isabelle

Question

Calculate the enthalpy of combustion of ethyne (C2H2) from the data below: ΔHf (CO2) = -393kJ/mol , ΔHf (H2O) = -286kJ/mol and ΔHf (C2H2) = -229kJ/mol *

Answer 1

Solution:

The standard enthalpy of combustion is ΔH^∘_c. It is the heat evolved when 1 mol of a substance burns completely in oxygen at standard conditions.

The balanced chemical equation of combustion of ethyne (C₂H₂):

C₂H₂(g) + 2.5O₂(g) → 2CO₂(g) + H₂O(l)

To calculate ΔH^∘_c from standard enthalpies of formation:

ΔH^∘_c = ∑ΔH^∘_f (products) − ∑ΔH^∘_f (reactants)

The enthalpy of formation for oxygen (O₂) is zero.

ΔH^∘_f (C₂H₂) = +229 kJ/mol

ΔH^∘_f (CO₂) = -393 kJ/mol

ΔH^∘_f (H₂O) = -286 kJ/mol

Thus:

ΔH^∘_c = ΔH^∘_f (H₂O) + 2×ΔH^∘_f (CO₂) - ΔH^∘_f (C₂H₂)

ΔH^∘_c = (-286 kJ/mol) + 2×(-393 kJ/mol) - (+229 kJ/mol) = -1301 kJ/mol

Therefore, the enthalpy of combustion for ethyne is:

ΔH^∘_c = -1301 kJ/mol

Answer: The enthalpy of combustion of ethyne is -1301 kJ/mol.