Question

n-Butane is combusted completely in a furnace, and a stream of air is used as the oxygen source.

a) What is the minimum air/fuel (n-butane) molar ratio required for the complete combustion of n-butane?

b)What is that ratio when expressed on a mass basis?

c) Calculate the standard heat of reaction for the complete combustion of n-butane.

Answer 1

Complete combustion reaction of n-butane

C4H10 (g) + (13/2)(O2 + 3.76 N2) (g) = 4CO2(g) + 5H2O(g)

Part a

From the stoichiometry of the reaction

1 mol of C4H10 require = (13/2)*(1+3.76) = 30.94 mol of air

minimum air/fuel (n-butane)molar ratio required for the complete combustion of n-butane

= 30.94/1 = 30.94 mol air/mol fuel

Part b

minimum air/fuel (n-butane) mass ratio required for the complete combustion of n-butane

= (30.94 mol air x 29 g/mol) /(1 mol fuel x 58g/mol)

= 15.5 g air/g fuel

= 15.5 kg air / kg fuel

Part C

Heat of reaction = sum of heat of formation of products - sum of heat of formation of reactants

= (5* Hf H2O) + (4* Hf CO2) - (0) - (1* Hf C4H10)

= 5*(-241.8) + 4*(-393.5) - 0 - (-126.2)

∆H rxn = - 2656.8 kJ/mol