Lightning exhibits some of the most fascinating phenomena on planet Earth. These massive strikes involve electrical potentials of 300 million volts¹ at 300,000 Amps and can generate temperatures up to 30,000 K.^2-4 In fact, lighting is known to emit both X-rays and gamma rays, and even kicks off some nuclear reactions.^2,3,5 It’s fun to imagine all the amazing and exotic chemical reactions that might be possible under such drastic conditions.

The atmosphere contains mostly nitrogen and oxygen gases, so it’s easy to see how lightning strikes can produce nitrogen oxides (NOx, Equations 1-2):^2-7

N₂(g) + O₂(g) à 2 NO(g)                   Equation 1

2 NO(g) + O₂(g) à 2 NO₂(g)             Equation 2

These reactions are not thermodynamically favorable under the conditions normally present in the atmosphere. But lighting strikes provide more than enough energy to drive these reactions to completion. In fact, it has been estimated that between 33 and 660 moles of NO are produced with every flash of lighting.^1,2 Considering that 9 million lightning strikes occur around the world every day,¹ lightning bolts account for as many as 6 billion moles of NO produced daily!  

The NO_x produced by lightning flashes further react with available water to produce nitrous and nitric acids (Equation 3):⁸

2 NO₂(g) + H₂O à HNO₂(aq) + HNO₃(aq)               Equation 3

As a result of all these processes, lightning strikes move nitrogen from the atmosphere into the soil.

I recently discovered a simple method to demonstrate the fact that lightning produces nitric and nitrous acids, which acidifies water. This method makes use of an Oudin coil⁹ (Video 1).

Video 1: The Chemistry of Lighting, Tommy Technetium YouTube Channel

It’s possible to connect Equations 1-2 to topics in chemical thermodynamics. For example, using standard thermodynamic values (Table 1), standard enthalpies, entropies, and Gibbs energies of the processes in Equations 1-2 can be determined (Table 2).

Table 1: Thermodynamic values for select compounds

Table 2: Thermodynamic values for processes described in Equations 1-2

It’s interesting to note that the processes described by Equations 1 and 2 are thermodynamic opposites of each other in many ways. For example, the process in Equation 1 is not spontaneous under standard conditions (DG is positive), but the reaction described by Equation 2 is (DG is negative). Furthermore, the formation of NO from nitrogen and oxygen (Equation 1) is not favored enthalpically (DH is positive) but is favored entropically (DS is positive). This situation is reversed for the formation of NO₂ (Equation 2), which is favored enthalpically (DH is negative) but not entropically (DS is negative).

Using the equation:

DG = DH – TDS                      Equation 4

it is seen that the formation of NO (Equation 1) is predicted to be spontaneous at higher temperatures but not lower ones. The temperature at which NO formation (Equation 1) becomes spontaneous can be estimated to be about 7250 K using the following equation and the values for DH and DS displayed in Table 2:

T ≈ DH^o/DS^o               Equation 5

Therefore, while the formation of NO (Equation 1) is nonspontaneous at lower temperatures, it becomes spontaneous under the high temperature conditions associated with a lightning strike.

Using Equation 4 it can also be seen that the formation of NO₂ (Equation 2) is predicted to be spontaneous at lower temperatures, but not higher ones. If we again use Equation 5 and the appropriate values in Table 2, it is seen that the formation of NO₂ becomes nonspontaneous at temperatures exceeding roughly 780 K. According to this estimated temperature, the formation of NO₂ likely proceeds after the air cools slightly following a blast of lighting. Overall, this analysis would predict that the initial blast probably generates high concentrations of NO (Equation 1) which then react with atmospheric oxygen (Equation 2) once the air cools. It should be kept in mind that these are simple thermodynamic estimates that ignore concentration effects and the incredibly high electrical potentials associated with a lightning strike. Nevertheless, it has been suggested that NO forms simultaneously with lighting strikes and NO₂ forms as the surrounding air cools thereafter.¹⁰

References

1. Price, C. Penner, J. NO_x from lightning. 1. Global distribution based on lighting physics. J. Geophys. Res. 1997, 102, 5929-5941.
2. Dwyer, J. R., Uman, M. A. The physics of lighting. Physics Reports, 2014, 534, 147-241.
3. Gibb, B. C. Lightning-fast chemistry. Nature Chemistry, 2019, 11, 677-679.
4. Hill, R. D.; Rinker, R. G.; Wilson, H. D. Atmospheric Nitrogen Fixation by Lightning. J. Atmos. Sci. 1980, 37, 179-192.
5. Enoto, T.; et. al. Photonuclear reactions triggered by lightning discharge. Nature, 2017, 551, 481-484.
6. Griffing, G. W. Ozone and Oxides of Nitrogen Production During Storms. J. Geophys. Res. 1977, 82, 943-950.
7. Although not covered here, lightning bolts also generate ozone gas (3 O₂ à 2 O₃), and ozone gas can react with NO to produce NO₂. See references 1, 4, and 6.
8. Zhu, R. S.; Lai, K.-Y.; Lin, M. C. Ab Initio Chemical Kinetics for the Hydrolysis of N₂O₄ Isomers in the Gas Phase. J. Phys. Chem. A. 2012, 116, 4466-4472.
9. For more experiments connecting lightning and atmospheric components, see Wagner, E. P.; Murray, B. E.; J. Chem. Educ. 2021, 98, 1361-1370.
10. Karabulut, E. Oxygen Molecule Formation and the Puzzle of Nitrogen Dioxide and Nitrogen Oxide during Lightning Flash. J. Phys. Chem. A. 2022, 126, 5363-5374.