How lightning fertilizes the soil
Nitrogen fixation by lightning · Synthesis · Endothermic · Equilibrium
A lightning bolt is hot enough to make the air itself react, turning nitrogen into plant food.
Reactants approach
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ΔH = +180.5 kJ · endothermic, absorbs heat
ΔH per mol N2.
Meet the molecules
- Reactantgas
N2
nitrogen
- Reactantgas
O2
oxygen
- Productgas
NO
nitric oxide
What you notice
A lightning bolt heats the air in its channel to around 30,000 °C for a fraction of a second, about five times hotter than the surface of the Sun. At that temperature the normally unreactive nitrogen and oxygen in air combine to form nitric oxide. In the atmosphere, NO is oxidized further to nitrogen dioxide and nitric acid, which rain carries into the soil as nitrate that plants can absorb. Lightning fixes a few million tonnes of nitrogen a year, far less than fertilizer factories. Hot car engines make NO by the same reaction.
Safety: During a thunderstorm, go indoors or into a hard-topped car; nowhere outdoors is safe from lightning.
Step by step
- 1
Too stable at room temperature
N2's triple bond is one of the strongest in chemistry, and the reaction is endothermic, so at 25 °C essentially no NO forms.
- 2
Extreme heat shifts the equilibrium
Because ΔH is positive, Le Chatelier's principle says higher temperature favours NO. In the lightning channel, N2 and O2 are torn into atoms that recombine partly as NO.
- 3
Freezing the products
The channel cools within milliseconds, too fast for NO to fall back to N2 and O2, so much of it survives.
- 4
On to nitrate
NO reacts with O2 to form brown NO2, which dissolves in water droplets as nitric acid and reaches the soil as nitrate ions.