Example Question - 1

When 0.460 g of ethanol, C₂H₅OH, was burnt in a spirit burner, the temperature of 100 g of water in a copper can rose by 14.0 K. The specific heat capacity of water is 4.18 J g^-1 K^-1. Calculate the enthalpy change of combustion of ethanol.Step 1: heat energy transferred to the water, q = mcΔT = 100 × 4.18 × 14.0 = 5852 J = 5.852 kJ. Here m is the mass of water heated, not the mass of fuel burnt.Step 2: moles of ethanol burnt = 0.460 ÷ 46.0 = 0.0100 mol.Step 3: Δ_cH = -q ÷ n = -5.852 ÷ 0.0100 = -585 kJ mol^-1 (3 s.f.). The sign is negative because combustion is exothermic.The data book value is -1367 kJ mol^-1. The experimental value is much less exothermic because of heat lost to the surroundings, incomplete combustion and evaporation of ethanol from the wick.