text: # 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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