# Question #36d12

Dec 15, 2014

${C}_{3} {H}_{8} + 5 {O}_{2} \rightarrow 3 C {O}_{2} + 4 {H}_{2} O$

$\Delta {H}_{c} = - 2220 k J . m o {l}^{- 1} .$

${C}_{8} {H}_{18} + 8 \frac{1}{2} {O}_{2} \rightarrow 8 C {O}_{2} + 9 {H}_{2} O$

$\Delta {H}_{c} = - 5530 k J . m o {l}^{- 1}$

${M}_{r} {C}_{3} {H}_{8} = \left(3 \times 12 + 8 \times 1\right) = 44$

So 44g gives 2220kJ

So 1g gives 2220/44 = 50.45 kJ

${M}_{r} {C}_{8} {H}_{18} = \left(8 \times 12 + 18 \times 1\right) = 114$

So 114g gives 5530 kJ

So 1g gives 5530/114 = 48.51 kJ.

So we can see that propane gives slightly more energy per gram.

From an environmental point of view the amount of $C {O}_{2}$ produced per gram is about the same.

The other big factor is cost. The cost of crude oil, hence gasoline, is falling due a drop in demand and the increase in shale gas production in the US. I'm not sure how propane compares currently. If anyone knows perhaps they could update.

Dec 15, 2014

Propane

The heat of combustion of propane is -2202 kJ/mol.

Energy per gram = $\text{2202 kJ"/"1 mol" × "1 mol"/"44.10 g}$ = 49.9 kJ/g

Octane

The heat of combustion of octane is -5430 kJ/mol.

Energy per gram = $\text{5430 kJ"/"1 mol" × "1 mol"/"114.2 g}$= 47.5 kJ/g

a. Efficiency

You get about 5 % more heat energy from a gram of propane than from a gram of octane.

b. Cost

In American units, the international cost of propane at the time of writing is $0.51/gal. The average international cost of gasoline is$4.43/gal. Propane is much cheaper on a volume basis.

a. Difficulty in handling

Propane must be handled as a pressurized liquid.

b. Danger

Propane vapour is denser than air. If there is a leak, the gas will sink into any enclosed area and cause a risk of explosion and fire.

c. Limited availability

Propane is less readily available to consumers than gasoline. Buses can refuel at the terminal, but consumers have fewer options.

Most personal vehicles need expensive conversions to handle propane

d. About 18 % less driving distance per litre of fuel

Because liquid propane is less dense than gasoline.

e. Fuel tanks can be filled to only 80 % of capacity

Because liquid propane expands as it warms from -42 °C to ambient temperature.
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