Question

The average propane cylinder for a residential grill holds approximatly 18 kg of propane. How much energy (in kJ) is released by the combustion of 18.50 kilograms of propane in sufficient oxygen?

Answer 1

First let´s write the combustion reaction, we know that all hydrocarbons will produce CO2 and H2O in presence of oxygen, since the statement says you have sufficient oxygen this means you have a complete combustion

so propane has 3 carbons, therefore it has 3*2 +2 = 8

C₃H₈

reaction goes like this:

C₃H₈ + O₂ ====== CO₂ + H₂O, we have to balance the equation by multiplying by 3 the CO₂, because we have 3 carbons in the propane molecule

C₃H₈ + O₂ ====== 3 CO₂ + H₂O, now let´s balance the hydrogens, you have 8 on the left and 2 on the right, multiply the water molecule by 4

C₃H₈ + O₂ ====== 3 CO₂ + 4 H₂O, now balance the oxygens you have 6 + 4 on the right and you have only 2 on the left, let´s multiply oxygen by 5

C₃H₈ + 5 O₂ ====== 3 CO₂ + 4 H₂O, this is your balanced equation

so you have been asked for the energy release by the combustion of 18.5 kilograms, calculate the number of moles of propane

molar mass of propane is 44 g/gmol

moles = mass / molar mass

moles of propane = 18 500 g / 44 = 420.45 moles available

now let´s get the enthalpy combustion of propane, im getting a value online of -2207 KJ / mol

so we just have to multiply this value by the number of moles

Enthalpy is = -2207 * 420.45 = -927, 933.15 Kilo Joules

the negative sign is because heat is being released