Acetylene (C₂H₂) undergoes combustion in excess oxygen to generate gaseous carbon dioxide and water. Given $\Delta$H^$\circ$_f[CO₂(g)] = -393.5 kJ/mol, $\Delta$H^$\circ$_f[H₂O(g)] = -241.8 kJ/mol, and $\Delta$H^$\circ$_f[C₂H₂(g)] = 226.6 kJ/mol, how much energy is released (kJ) when 10.5 moles of acetylene is burned A) 2,510.8 kJ B) 26,400 kJ C) 13,200 kJ D) 52,700 kJ E) 9,050 kJ

Question

Quizplus · Accepted Answer

The balanced chemical equation for the combustion of acetylene is:
2C2H2(g) + 5O2(g) -> 4CO2(g) + 2H2O(g)
From this equation, it is clear that 2 moles of acetylene are required to produce 4 moles of carbon dioxide and 2 moles of water. Therefore, to calculate the energy released when 10.5 moles of acetylene are burned, we first need to find the energy released when 2 moles of acetylene are burned, and then multiply that by 5.25 (i.e. 10.5/2).
The energy released when 2 moles of acetylene are burned can be calculated using the enthalpy change of formation (ΔH°f) values given:
ΔH°rxn = ΣnΔH°f(products) - ΣnΔH°f(reactants)
ΔH°rxn = [4ΔH°f(CO2(g)) + 2ΔH°f(H2O(g))] - [2ΔH°f(C2H2(g))]
ΔH°rxn = [(4 x -393.5 kJ/mol) + (2 x -241.8 kJ/mol)] - [2 x 226.6 kJ/mol]
ΔH°rxn = -2603.2 kJ/mol
This means that 2 moles of acetylene release 2603.2 kJ of energy when they are burned. Multiplying this by 5.25 gives us the energy released when 10.5 moles of acetylene are burned:
Energy released = 2603.2 kJ/mol x 5.25 mol = 13,200 kJ (to three significant figures)

Acetylene (C2H2) Undergoes Combustion in Excess Oxygen to Generate Gaseous Δ\DeltaΔ

Acetylene (C₂H₂) Undergoes Combustion in Excess Oxygen to Generate Gaseous $\Delta$