Photochemical Reactions

A photochemical reaction is a chemical process initiated by the absorption of energy in the form of light—specifically visible, ultraviolet (UV), or infrared radiation. Unlike thermal reactions that rely on heat energy to overcome the activation energy barrier, photochemical reactions utilize the energy of discrete packets of light, known as photons. The fundamental law governing this process states that only the light absorbed by a system can be effective in bringing about a chemical change. The general representation is:

(Where hν represents the energy of an absorbed photon; h is Planck’s constant and ν is the frequency of light).

Mechanism of Photochemical Activation

Photochemical reactions progress through a distinct two-step pathway that differentiates them from conventional thermal reactions.

Primary Photochemical Process

This initial step involves the direct absorption of a photon by an atom, molecule, or ion. The energy of the absorbed photon prompts an electron to transition from a stable lower-energy ground state to a highly reactive, short-lived higher-energy excited state or leads to homolytic cleavage to generate free radicals. This primary step is independent of temperature.

Secondary Chemical Process

The highly reactive excited species or free radicals produced in the primary stage undergo subsequent spontaneous chemical transformations, collisions, or chain propagation steps to yield the final products. These secondary steps do not require light and can be influenced by temperature or concentration.

Distinct Features: Photochemical vs. Thermal Reactions

The table below contrasts the fundamental thermodynamic and operational differences between light-driven and heat-driven chemical reactions.

Parameter	Photochemical Reactions	Thermal (Dark) Reactions
Primary Energy Source	Absorption of photons (light energy).	Absorption of thermal energy (heat).
Temperature Co-efficient	Remarkably low; temperature has negligible effect on the primary step.	High; reaction rates are highly sensitive to temperature changes.
Free Energy Change (Δ G)	Can occur even if Δ G is positive (absorbs light to drive non-spontaneous processes).	Can only occur spontaneously if the change in free energy (Δ G) is negative.
Selectivity	Highly selective; excites specific molecular bonds corresponding to the light wavelength.	Non-selective; thermal energy distributes across all vibrational and rotational modes.

Major Classifications and Real-World Examples

Natural Photochemical Processes: Photosynthesis

Photosynthesis is the most critical natural photochemical reaction on Earth, sustaining global food chains. Chlorophyll pigments in plant cells absorb red and blue visible light to convert atmospheric carbon dioxide and water into glucose and oxygen.

Environmental Photochemistry: Stratospheric Ozone Formation

The ozone layer protects life on Earth from harmful solar UV radiation. Its continuous formation and destruction cycle in the stratosphere is driven entirely by photochemical mechanisms.

• Ozone Formation: High-energy UV radiation splits molecular oxygen into highly reactive oxygen free radicals, which then combine with intact oxygen molecules.
  O₂ + hν → O^• + O^•
  O₂ + O^• → O₃ (Ozone)

Photographic Chemistry: Silver Halide Decomposition

Traditional black-and-white photography relies on the light-induced decomposition of silver halides (usually silver bromide, AgBr) coated on photographic films.

• Reaction Mechanism: Exposure to light triggers a redox-based decomposition where bromide ions lose electrons and silver ions are reduced to precipitate dark metallic silver clusters, recording the latent image on the film.
  2AgBr(s) + hν → 2Ag(s) + Br₂(g)

Photochemical Decomposition of Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) is chemically unstable when exposed to ambient light. Photons cause the homolytic cleavage of its weak oxygen-oxygen single bond, decomposing it into water and oxygen gas. This is why hydrogen peroxide is strictly stored in dark, opaque amber-colored bottles to block light penetration.

Environmental Challenges: Photochemical Smog

Photochemical smog is a modern hazardous environmental phenomenon prevalent in urban areas with high automobile density. It forms when solar radiation interacts with primary pollutants trapped in the lower atmosphere.

Primary Reactants

The reaction requires nitrogen oxides (NO_x) and Volatile Organic Compounds (VOCs) emitted from internal combustion engines, alongside warm temperature and abundant sunlight.

Generation of Secondary Pollutants

Sunlight breaks down nitrogen dioxide (NO₂) into nitric oxide and nascent oxygen atoms. These oxygen atoms react with atmospheric oxygen to form ground-level (tropospheric) ozone (O₃). Further complex photochemical interactions between ozone, NO_x, and hydrocarbons produce secondary toxic pollutants like Peroxyacetyl Nitrate (PAN), acrolein, and formaldehydes, which cause severe respiratory issues and intense eye irritation.

Fact File and Prelims-Specific Trivia

• The Concept of Quantum Yield: Quantum yield (φ) is a critical metric measuring the efficiency of a photochemical reaction. It is defined as the ratio of the number of molecules reacting to the number of photons absorbed. For certain chain reactions, like the synthesis of hydrochloric acid from hydrogen and chlorine gas, a single absorbed photon can yield up to 10⁶ molecules due to a self-propagating free-radical chain mechanism.
• Bioluminescence Contrast: While photochemical reactions absorb light to trigger chemical changes, bioluminescence (seen in fireflies and deep-sea organisms) is the exact reverse process: a biochemical reaction where chemical energy is converted directly into cold light emission.
• Vitamin D Synthesis: Humans rely on a vital intra-cutaneous photochemical reaction. Exposure to solar Ultraviolet-B (UVB) radiation alters the chemical structure of 7-dehydrocholesterol present in human skin cells, initiating its isomerization into cholecalciferol (Vitamin D3).

Last Modified: May 25, 2026