The Chemistry of Lightning

pH changes occur in water exposed to high voltage sparks

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 NOx 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

Compound

DHfo / kJ mol-1

So / J mol-1 K-1

DGfo / kJ mol-1

N2(g)

0

191.6

0

NO(g)

90.3

210.8

86.6

NO2(g)

33.1

240.0

51.2

O2(g)

0

205.1

0

 

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

Reaction

DHo /

kJ mol-1

DSo /

J mol-1 K-1

DGo /

kJ mol-1

N2(g) + O2(g) à

2 NO(g)

+180.6

+24.9

+173.2

2 NO(g) + O2(g) à

2 NO2(g)        

-114.4

-146.7

-70.8

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 ≈ DHo/DSo               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. NOx 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.

 

 

Safety

General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

Other Safety resources

: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

 

Safety: Video Demonstration

Demonstration videos presented here are not meant as tools to teach chemical demonstration techniques. They are meant as a tool for classroom use. The demonstrations may present safety hazards or show phenomena that are difficult for an entire class to observe in a live demonstration.

Those performing the demonstrations shown in this video have been trained and adhere to best safety practices.

Anyone thinking about performing a chemistry demonstration should first read and then adhere to the  These guidelines are also available at ChemEd X.

NGSS

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.

Summary:

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.

Assessment Boundary:
Clarification:

Earth’s Systems, help students formulate an answer to the question: “How and why is Earth constantly changing?” The ESS2 Disciplinary Core Ideafrom the NRC Framework is broken down into five sub-ideas: Earth materials and systems, plate tectonics and large-scale system interactions, the roles of water in Earth’s surface processes, weather and climate, and biogeology. For the purpose of the NGSS, biogeology has been addressed within the life science standards. Students develop models and explanations for the ways that feedbacks between different Earth systems control the appearance of Earth’ssurface. Central to this is the tension between internal systems, which are largely responsiblefor creating land at Earth’s surface, and the sun-driven surface systems that tear down the land through weathering and erosion. Students begin to examine the ways that human activities cause feedbacks that create changes to other systems. Students understand the system interactions that control weather and climate, with a major emphasis on the mechanisms and implications of climate change. Students model the flow of energy between different components of the weather system and how this affects chemical cycles such as the carbon cycle. The crosscutting concepts of cause and effect, energy and matter, structure and function and stability and change are called out as organizing concepts for these disciplinary core ideas. In the ESS2 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and carrying out investigations, analyzing and interpreting data, and engaging in argument; and to use these practices to demonstrate understanding of the core ideas.

More information about all DCI for HS-ESS2 can be found .

Summary:

Earth’s Systems, help students formulate an answer to the question: “How and why is Earth constantly changing?” The ESS2 Disciplinary Core Ideafrom the NRC Framework is broken down into five sub-ideas: Earth materials and systems, plate tectonics and large-scale system interactions, the roles of water in Earth’s surface processes, weather and climate, and biogeology. For the purpose of the NGSS, biogeology has been addressed within the life science standards. Students develop models and explanations for the ways that feedbacks between different Earth systems control the appearance of Earth’ssurface. Central to this is the tension between internal systems, which are largely responsiblefor creating land at Earth’s surface, and the sun-driven surface systems that tear down the land through weathering and erosion. Students begin to examine the ways that human activities cause feedbacks that create changes to other systems. Students understand the system interactions that control weather and climate, with a major emphasis on the mechanisms and implications of climate change. Students model the flow of energy between different components of the weather system and how this affects chemical cycles such as the carbon cycle. The crosscutting concepts of cause and effect, energy and matter, structure and function and stability and change are called out as organizing concepts for these disciplinary core ideas. In the ESS2 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and carrying out investigations, analyzing and interpreting data, and engaging in argument; and to use these practices to demonstrate understanding of the core ideas.

Assessment Boundary:
Clarification:

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

*More information about all DCI for HS-PS1 can be found at  and further resources at .

Summary:

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Assessment Boundary:

Assessment is limited to chemical reactions involving main group elements and combustion reactions.

Clarification:

Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.

Students who demonstrate understanding can plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

*More information about all DCI for HS-PS1 can be found at  and further resources at .

Summary:

Students who demonstrate understanding can plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles. 

Assessment Boundary:

Assessment does not include Raoult’s law calculations of vapor pressure.

Clarification:

Emphasis is on understanding the strengths of forces between particles, not on naming specific intermolecular forces (such as dipole-dipole). Examples of particles could include ions, atoms, molecules, and networked materials (such as graphite). Examples of bulk properties of substances could include the melting point and boiling point, vapor pressure, and surface tension.

Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.

*More information about all DCI for HS-PS1 can be found at  and further resources at .

Summary:

Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.

Assessment Boundary:

Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.

Clarification:

Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.