Question

When 2 moles of C₂H_6(g) are completely burnt, 3129 kJ of heat is liberated. If ∆H_f for CO_2(g) and H₂O_(l) are −395 and −286 kJ per mole respectively, the heat combustion of C₂H_6(g) is ____________.

Options

• −167 kJ

• −83.5 kJ

• +85.98 kJ

• 334 kJ

Answer 1

When 2 moles of C₂H_6(g) are completely burnt, 3129 kJ of heat is liberated. If ∆H_f for CO_2(g) and H₂O_(l) are −395 and −286 kJ per mole respectively, the heat combustion of C₂H_6(g) is −83.5 kJ.

Explanation:

\[\ce{2C2H6_{(g)} + 7O2_{(g)} -> 4CO2_{(g)} + 6H2O_{(l)}}\]

∆H = −3129 kJ

∵ ∆H = `sum"H"_("f"("products")) - sum"H"_("f"("Reactants"))`

∴ −3129 kJ

= = [4 mol × (−3955 kJ mol⁻¹) + 6 mol × (−2865 kJ mol⁻¹)] − [2∆_f(C₂H₆) + 0]

= −1580 − 1716 − 2x

∴ 2x = −1580 − 1716 + 3129

= −167

∴ x =−83.5 kJ