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Chemistry LibreTexts

Solar Energy

Solar energy is energy from the sun. The most common forms of this energy is ultraviolet, visible, and infared energy. This energy is the activation energy used to drive chemical reactions throughout the planet.


Activation energy is the energy required to start a chemical reaction. It is also used in unending cycles such as the Calvin cycle, water cycle, nitrogen cycle and carbon cycle. The Calvin cycle is used in photosynthesis, while the water cycle depicts how water is evaporated from the oceans and plants, and returns to Earth's surface as rain or snow. The carbon cycle is a process of how carbon dioxide is released into the atmosphere and absorbed by plants. As plants and animals decompose, carbon dioxide is released back into the atmosphere.

Nitrogen Cycle

In the nitrogen cycle, nitrogen fixing bacteria converts the nitrogen in the atmosphere into ammonia or ammonium ions. N2---> NH3 or NH4+

Nitrogen fixing bacteria that live in legume root nodules or soil process nitrogen out of the air and convert it to nitrates, which can be absorbed by plants. During storms, lightning causes nitrogen to bond with rainwater creating the nitrate.

NH3- or NH4+---> NO2-

NO2-+ H2O ---> NO3-+ 2H

Plants absorb the nitrogen, which are eaten by herbivores, and then carnivores consume the herbivores. After animals and plants die, a second type of soil bacteria breaks down wastes and decaying matter and releases the nitrogen back into the atmosphere.

NH3 or NO3- or NO2- ---> N2

Nitrogen Cycle.jpg


Photosynthesis is a two stage process that plants use to convert carbon dioxide (CO2) and water (H2O) into a sugar, glucose (C6H12O6), and oxygen (O2). In the first phase, chlorophyll in plant leaves capture the energy of light and use it to make high-energy molecules. When sunlight hits a plant's leaf, electrons in the chlorophyll become excited and set off a chain reaction that produces two high energy molecules called adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate hydrogen (NADPH).

During the second phase, the these two molecules are used to break down carbon dioxide molecules and make a precursor to the glucose molecule.

3CO2+ 9ATP + 6NADPH + 6H+ ---> C3H6O3+ 9ADP + 8Pi+ 6NADP++ 3H2O

A simplified equation of the whole process is:

6CO2+ 12H2O+ photons ---> C6H12O6+ 6O2+ 6H2O


Evaporation is a process in the water cycle where water experiences a phase change from liquid to vapor. Solar energy drives the water cycle by causing evaporation of from the soil, oceans, and other bodies of water. The gas condenses in clouds, forming liquid, known as condensation, which falls back to Earth as rain or snow. The rain or snow runoff is collected in rivers, oceans and soil, which is then evaporated again to repeat the process. The energy from the sun gives water in the oceans, and other sources of water the required energy to vaporize. In some cases, ice can sublimate, experiencing a phase change from solid to vapor. The reason that leaving wet objects out in the sun to dry works even though the temperature is below boiling point is because of humidity and pressure. It's possible to calculate the pressure or temperature that water will evaporate at using the Clausius-Clapeyron relation.

H2O(l) ---> H2O(g)

Water Cycle.jpg

Solar Energy Applications

Energy from the sun is a renewable energy source that is absorbed in large quantities. The amount of energy absorbed by the earth from the sun is about 3850 zettajoules (1021 Joules). Humans have improved technology to use solar energy to provide electricity to cities, cars, and power plants. A solar cell, also called a photovoltic cell converts light energy into electricity through the photoelectric effect. This energy is stored in batteries seen in solar powered cars, homes, and lights. The solar panels on lights, cars and homes, absorb the light energy and store the electricity in batteries for later use. The most common source of energy for satellites is light energy, there are solar panels on the satellite that absorb the light energy and store it in a battery or batteries, to keep the satellite functioning for a long period of time. Solar water disinfection is a process that can be done by leaving bottles of water out in the sun for several hours.

Another method of using solar energy is by concentrating it, or converting light energy into Thermal energy. Large parabolic dishes focus light energy to a specific point, the focal point, causing this point to heat up. The same effect can be done by holding a magnifying glass at the right height and angle away from the ground to burn things. At this point there is usually a working fluid that is heated up and used for a power generation system.


  1. Petrucci, et al. General Chemistry: Principles & Modern Applications. 9th ed. Upper Saddle River, New Jersey 2007.
  2. Denise Kiernan and Joesph D'Agnese, Science 101: Chemistry. Irvington, NY 2007
  3. George Ochoa, Science 101: Biology. Irvington, NY 2007 


  • Kevin Fong