Question

Calculate enthalpy of combustion of acetylene if 1950 kJ of heat is liberated when 39 g of acetylene undergoes combustion.

• A. -650 kJ mol⁻¹
• B. -975 kJ mol⁻¹
• C. -1300 kJ mol⁻¹
• D. -1600 kJ mol⁻¹

Answer 1

Answer: (C)

-1300 kJ mol⁻¹

Answer 2

1. Calculate moles of acetylene (C₂H₂):

   Molar mass = 2 × 12 + 2 × 1 = 26 g/mol

   Moles = 39 g / 26 g/mol = 1.5 mol

2. Determine heat liberated per mole:

   Given heat liberated = 1950 kJ for 1.5 mol

   Heat per mole = 1950 kJ / 1.5 mol = 1300 kJ/mol

   Since the reaction is exothermic, the enthalpy of combustion = –1300 kJ/mol

39 g of C₂H₂ corresponds to 1.5 mol. Dividing 1950 kJ by 1.5 gives 1300 kJ per mole. As combustion releases heat, the enthalpy is –1300 kJ/mol.