What happens in a gas stove flame
Combustion of methane · Combustion · Redox · Exothermic
Every blue flame on a gas stove is turning one invisible gas into two others.
Reactants approach
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ΔH = -802 kJ · exothermic, releases heat
ΔH per mol CH4, water formed as vapour.
Meet the molecules
- Reactantgas
CH4
methane
- Reactantgas
O2
oxygen
- Productgas
CO2
carbon dioxide
- Productgas
H2O
water vapour
What you notice
Natural gas piped to a kitchen stove is mostly methane, CH4. When you turn the knob, gas mixes with air at the burner holes and a spark lights it. A well-adjusted flame burns blue because the methane gets enough oxygen to burn completely; a yellow, sooty flame means too little air, so some carbon is left unburned as glowing particles. Hold a cold metal spoon above the flame for a second and it fogs with water, one of the two products. The other, carbon dioxide, is invisible.
Try it
With an adult present, hold a cold steel spoon by its handle above a blue gas flame for one or two seconds and watch water condense on it; let it cool before touching the bowl.
Safety: Incomplete combustion makes carbon monoxide, an odourless poison, so gas appliances need ventilation and a carbon monoxide alarm nearby.
Step by step
- 1
Bonds break first
Methane's four C–H bonds and oxygen's O=O double bonds must be pulled apart. The spark supplies the first push; after that, the flame's own heat keeps the reaction going.
- 2
Carbon finds oxygen
The carbon atom bonds to two oxygen atoms, forming a linear CO2 molecule held together by two strong C=O double bonds.
- 3
Hydrogen finds oxygen
The four hydrogen atoms pair up with the remaining two oxygen atoms to make two bent H2O molecules.
- 4
Net energy out
The new C=O and O–H bonds are stronger than the C–H and O=O bonds that broke, so about 802 kJ is released for every mole (16 g) of methane burned.