Question

Propane (C3H8) is burned with excess air, both supplied at 25C. Assume that air is 21% oxygen, balance nitrogen.

Part 1 : If 1.0 mol of propane is burned with x mols of air, how many mols of oxygen appear in the combustion products?

A. 3.0 + 0.21x

B. 4.0 + 0.79x

C. 0.79x

D. 0.21x – 5.0 .

Part 2 : The standard heat of combustion for propane

A. depends on the state of the water in the products

B. depends on the amount of excess air

C. depends on temperature

D. depends on the completeness of combustion

Part 3 : Mean heat capacities for combustion gases are convenient but they cannot be used for

A. combustion above 1.0 bar

B. combustion with excess air

C. systems with a turbine

D. components which change phase

Answer 1

The reaction is C3H8+ 5O2----> 3CO2 + 4H2O

Oxygen required for one mole of C3H8= 5 moles

Moles of air required= 5/0.21= 23.81 mole of air

Supplied air =x moles oxygen in x moles of air =x*0.21

Oxygen requred= 5 moles

hence oxygen in the products= 0.21x-5 ( D is the correct answer)

part2 : the standard heat of combustion for propane depends on state of water since enthalpy of vapor and liquid are different and also on completeness of combustion since standard heat is expresed in j/mol

Part 3 : Mean heat capacities can't be use for components with change of phase since specific heat of vapor and liquid are different

and also combustion above 1 bar