What powers a backyard gas grill
Combustion of propane · Combustion · Redox · Exothermic
The rotten-egg smell of a gas grill is added on purpose; propane itself has no smell at all.
Reactants approach
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ΔH = -2,044 kJ · exothermic, releases heat
ΔH per mol C3H8, water formed as vapour.
Meet the molecules
- Reactantgas
C3H8
propane
- Reactantgas
O2
oxygen
- Productgas
CO2
carbon dioxide
- Productgas
H2O
water vapour
What you notice
Barbecue grills, camping stoves and patio heaters run on propane stored as a liquid under pressure in steel tanks. Open the valve and liquid propane boils into gas, which flows to the burners and mixes with air. Propane is popular away from gas mains because it liquefies at modest pressure, about 9 atmospheres at room temperature, so a standard 9 kg tank holds a lot of energy in a small space. Its strong smell comes from a sulfur compound, ethyl mercaptan, added so that leaks can be noticed; pure propane is odourless.
Try it
After a long grilling session on a humid day, look for a band of condensation or frost on the outside of the propane tank; it marks the level of the boiling liquid inside.
Safety: Never use a propane grill or heater indoors or in a closed garage, because incomplete combustion produces carbon monoxide.
Step by step
- 1
Liquid to gas
Opening the valve lowers the pressure, and liquid propane boils, absorbing heat; during long use the tank gets cold and can even frost over.
- 2
Mixing with air
Each propane molecule needs five O2 molecules, so the burner must draw in about 24 volumes of air for every volume of propane.
- 3
Rearranging atoms
All eight C–H bonds and both C–C bonds break. The three carbons end up in CO2 and the eight hydrogens in four H2O molecules.
- 4
Energy out
About 2044 kJ is released per mole (44 g) of propane, roughly 46 kJ per gram.