Intermediate Topic 460 of 1102 Kinetics & Equilibrium
Photochemical Reactions
Reactions initiated by light absorption, such as photosynthesis and ozone formation.
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In-depth explanation of Photochemical Reactions
Photochemical reactions are initiated by the absorption of light, which provides the energy needed to excite a molecule to a reactive state even when thermal energy is insufficient. The absorbed photon must have an energy matching an electronic transition of the absorbing species, and the rate of the reaction is proportional to the intensity of the light absorbed. Many processes that are essential in nature and industry are photochemical, including photosynthesis, the formation of vitamin D and vision. In the stratosphere, ultraviolet light splits oxygen molecules, which leads to the formation of ozone.
Going deeper: Photochemical Reactions
Key ideas: The energy per photon is E = hc/lambda; for 400 nm this is about 300 kJ per mol of photons, enough to break many bonds. The Grotthuss-Draper law says only absorbed light is effective, and the Stark-Einstein law says one photon excites one molecule. Photochemical rates depend on light intensity, not usually on temperature, giving low apparent activation energies.
Common mistakes and connections: Students think that any light, even intense low-frequency light, will drive a reaction; the photon must have sufficient energy and be absorbed. Another mistake is to assume photochemical reactions are always uphill, when many are exergonic like H2 + Cl2. A third is ignoring that light is not a catalyst, since it is consumed. Related to quantum yield and chain reactions.
Visualization of Photochemical Reactions
Only photons with enough energy drive the reaction: wavelengths below about 242 nm split O2 and about 310 nm splits ozone, so UV light powers stratospheric chemistry. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Ozone is formed in the upper atmosphere when ultraviolet light splits O2 into oxygen atoms that then combine with more O2.
Frequently asked questions about Photochemical Reactions
What is Photochemical Reactions?
Reactions initiated by light absorption, such as photosynthesis and ozone formation.
Is Photochemical Reactions a beginner, intermediate or advanced topic?
Photochemical Reactions is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Photochemical Reactions?
Study these earlier topics in Kinetics & Equilibrium first: Autocatalysis, Chain Reactions, Oscillating Reactions.
What comes after Photochemical Reactions?
The next topics in Kinetics & Equilibrium are Quantum Yield and Reversible Reactions.
Can I practise Photochemical Reactions with quizzes?
Yes. The interactive lesson includes a self-check question on Photochemical Reactions, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Photochemical Reactions fits in Kinetics & Equilibrium
Photochemical Reactions is topic 41 of 91 in Kinetics & Equilibrium.