Using the following bond energies: estimate the heat of combustion for 1 mol of acetylene:

Question

Using the following bond energies:  estimate the heat of combustion for 1 mol of acetylene:

Quizplus · Accepted Answer

To calculate the heat of combustion for 1 mol of acetylene, we need to break all the bonds in acetylene and form CO2 and H2O.

Breaking the bonds in acetylene requires: 
5 x C≡C bond energy = 5 x 837 kJ/mol = 4185 kJ/mol 
4 x H–H bond energy = 4 x 435 kJ/mol = 1740 kJ/mol

Forming the new bonds in CO2 and H2O releases energy: 
2 x C=O bond energy = 2 x 1073 kJ/mol = –2146 kJ/mol 
4 x O–H bond energy = 4 x 463 kJ/mol = –1852 kJ/mol

The net energy change for the reaction is: 
Energy required to break bonds – Energy released from forming new bonds 
= (4185 kJ/mol + 1740 kJ/mol) – (2146 kJ/mol + 1852 kJ/mol) 
= 927 kJ/mol

However, the question asks for the heat of combustion, which is the energy released when one mole of a substance is completely burned in oxygen gas. Since the combustion of acetylene produces two moles of CO2 and one mole of H2O, the heat of combustion is twice the energy change calculated above:

Heat of combustion = 2 x 927 kJ/mol = 1854 kJ/mol

Since the energy change is negative (exothermic), the heat of combustion is also negative: 
Heat of combustion = -1854 kJ/mol

Therefore, the best choice is D) -1228 kJ, which is the closest to the calculated value of -1854 kJ/mol.

Using the Following Bond Energies: Estimate the Heat of Combustion