For a chemical reaction to occur the reacting molecules must be activated.Usually this activation is carried out thermally i.e by increasing the reaction temperature.Increasing temperature results in increase in the energy of the molecules present in the system.Molecules which have acquired sufficient energy, react and lead to formation of products.These type of reactions are called thermal reactions.
However, there is another way of introducing energy in a system – by absorption of radiation(light). Photochemistry is the study of reactions caused by absorption of IR, visible or UV light. The study of photochemistry deals with the interaction between molecules and photons of IR, visible or UV light and the subsequent chemical changes.
Photochemistry is very fundamental to our existence. The sun is one of the main reasons why life exists on earth.The origin of life must be a photochemical process, as only solar energy was available for simple molecules like methane(CH4) and carbon-di-oxide (CO2) to react and form complex molecules like proteins and nucleic acids.These molecules are the basis of life. The primary source of energy for all animals is through food.Plants synthesize various complex organic molecules (which animals consume as food) , using solar energy.This process is called as photosynthesis.This is a photochemical process too.
In today’s world, where energy crisis is becoming a major problem, harnessing solar energy could be an effective alternative to solve this problem. If suitable photochemical reactions are discovered and devices for proper utilization of this abundant source of energy perfected, half the world’s energy problem can be solved. So lets begin our study on photochemistry to understand this topic better
Difference between thermal and photochemical reactions
Many reactions which cannot be carried out thermally, can be successfully carried out using electromagnetic radiation. What is the fundamental difference between thermal and photochemical reactions? In photochemical reactions, we can selectively excite specific reactants to get specific products. Thermal reactions lack this kind of selectivity.
As seen in the earlier post, different wavelengths of light have different energy. Wavelength and energy are inversely proportional to each other i.e if the wavelength(λ) of light is shorter, it has higher energy and vice versa.
When reactants react there is a change in their electronic structure. We know that electrons in different orbitals have different energies. So, in a photochemical process, if light of a specific wavelength is shone on the reactants, ONLY ELECTRONS THAT HAVE ENERGY EQUIVALENT TO THAT OF THE INCIDENT LIGHT WILL ABSORB AND GET EXCITED TO GIVE SUBSEQUENT PRODUCTS. Thus, we can selectively choose our radiation to bring about the desired reaction.
Photochemistry and radiation chemistry.
These two terms – photochemistry and radiation chemistry are different from each other.
Photochemistry is a term used to study reactions brought about by IR, visible and UV radiation.
Radiation chemistry deals not only with photochemical process but also with many more reactions brought about by other high energy radiations like α, β, γ, cyclotron, X-rays, or corpuscular beams like neutrons, protons or α-particles(obtained from nuclear reactions or electric accelerators). etc.
Thus, photochemistry is small branch of radiation chemistry which deals with low energy radiation in UV, visible and IR range. In our discussions on photochemistry we will be exclusively talking about reactions that use UV, visible or IR light to convert reactant to products.
In the next post we will try to understand the fundamental difference between these two kinds of reactions.Till then,
Be a perpetual student of life and keep learning….
Good day !