he following table contains some standard enthalpies of formation, H f, at 298 K

Question

he following table contains some standard enthalpies of formation, Hf, at 298 K.

Many portable gas heaters and grills use propane, C3H8(g).

Using enthalpies of formation, calculate the quantity of heat produced when 14.0 g of propane is completely combusted in air under standard conditions. Assume that liquid water is forming.

Substance Formula Hf
(kJ/mol) Carbon dioxide CO2(g) 393.5 Carbon monoxide CO(g) 110.5 Methane CH4(g) 74.80 Propane C3H8(g) 103.85 Water H2O(l) 285.8 Water vapor H2O(g) 241.8

Explanation / Answer

The balanced equation for the combustion of propane is -

C3H8 (g) + 5 O2 (g) -----------> 3 CO2 (g) + 4 H2O (l)

From standard enthalpies of formation.

DHrxn = 3 DH(CO2) + 4 DH (H2O) - [ DH (C3H8 ) + 5 DH (O2)]

          = [3 (-393.5 kJ) + 4(-241.8 kJ) ] - [-103.85 kJ + 5 (0)]

          = -2043.85 kJ/mol

Number of mols of propane present in the reaction = 14 g /44 g/mol = 0.318 mol

Hence , the energy evolved out by complete combustion of 14 g of propane = (-2043.85 kJ/mol)(0.318 mol)

                                                                                                                        = -650.3 kJ

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