Photochemical Reactions: Meaning, Laws, and Key Examples

Photochemical reactions are of immense importance as these are the basis of many such processes which are the basis of sustainable life on earth. For example, photosynthesis, the formation of vitamin D with sunlight, etc. are photochemical reactions. Photochemical reactions are studied or come under the branch of chemistry called photochemistry.  It is a branch of chemistry that deals with the chemical effects of light.

Trommsdorff described the first photochemical reaction in 1834. He observed the reaction on crystals of santonin. These crystals when exposed to sunlight turned yellow and burst. 

Those reactions which take place by absorption of light energy are called photochemical reactions. Generally, it takes place by the absorption of ultraviolet light, visible light, or infrared radiation. Wavelength of all these radiations are given below in the table –

Photochemical reactions proceed differently than temperature-driven reactions or thermal reactions. In paths of photochemical reactions, high energy intermediates are formed which cannot be formed thermally. In these reactions, large activation barriers are crossed in a short time. Some photochemical reactions are destructive such as photodegradation of plastics.

Examples of Photochemical Reactions 

Most common example of a photochemical reaction is photosynthesis. In photosynthesis, plants use sunlight and water to convert carbon dioxide into glucose (Carbohydrates) and oxygen. Reaction is given below –                 

\[ 6CO_{2} + 6H_{2}O \overset{sunlight}{\rightarrow} C_{6}H_{12}O_{6} + 6O_{2}\]

Some other examples of photochemical reactions are given below –

• Formation of vitamin D by exposure to sunlight. Reaction is given below –

• Bioluminescence reactions which occur mainly in marine animals. 

• Silver chloride absorbs light and decomposes it into silver and chlorine. Reaction is given below –

\[ 2AgCl \overset{sunlight}{\rightarrow} 2Ag + Cl_{2}\]

• Silver bromide also behaves as silver chloride in the presence of sunlight. Reaction is given below –

\[ 2AgBr \overset{sunlight}{\rightarrow} 2Ag +Br_{2}\]

• Many polymerization reactions use light energy. These are also photochemical reactions. In many polymerization reactions, free radicals are formed by photoinitiation. It is known as photolysis. Reaction is given below –

• Photodegradation of many substances takes place by photochemical reactions. For example, photodegradation of polyvinyl chloride. 

• Photodynamic therapy is based on photochemical reactions. Photochemical reaction takes place when doctors use light to destroy tumors. 

• Vision is initiated by the photochemical reaction of rhodopsin. 

• Photochemical reactions take place or are used for the production of anti-malarial drugs. 

• Photoalkylation is also an example of a photochemical reaction. In these reactions, alkyl groups are attached in the molecules by using light energy. 

• Electrocyclic reactions, radical reactions, photoisomerization, and Norrish reaction 1 and 2 are examples of photochemical organic reactions.  

• Industrial production of benzyl chloride is also a photochemical reaction. Reactions are given below –

\[Cl_{2} + hv\cdot  \rightarrow  2Cl\]

\[C_{6}H_{5}CH_{3} + 2Cl\cdot \rightarrow  C_{6}H_{5}CH_{2}\cdot + HCl\]

\[C_{6}H_{5}CH_{2}\cdot + Cl\cdot \rightarrow  C_{6}H_{5}CH_{2}Cl\]

• Free radical halogenation reactions are also examples of photochemical reactions.

• Coordination complexes and organometallic compounds are also photoreactive and show photochemical reactions. 

Laws of Photochemistry 

As we know, the arrival of a reactant to an excited state is the 1st step of photochemical processes. Which is called photoexcitation. With this photochemical reactions or processes follow the laws of photochemistry as well. There are two laws of photochemistry which are Grotthuss-Draper law and Stark–Einstein law. 

• Grotthuss - Draper law – This law states that light must be absorbed by a chemical substance in order to take place in a chemical reaction. This law was given by chemists Theodor Grotthuss and John W. Draper.  

• Stark – Einstein law – This law states that for each photon of light absorbed by a chemical system, no more than one molecule is activated for a photochemical reaction, as defined by the quantum yield. This law was given by physicists Johannes Stark and Albert Einstein. 

Tips to Study Photochemical Reactions

The above material gave you a thorough insight into the Photochemical Reactions with appropriate examples along with the laws of Photochemistry. 

Let’s understand the best way to prepare for a particular topic thoroughly. 

• Understand the Concepts 

The utmost important thing to do for students is to focus on understanding each and every concept that is a part of the course exam. No matter what level of examination you’re appearing for or how difficult it is, the only way to crack it is to be clear about every topic. 

• Make Notes

The art of making notes might seem a time-consuming task to do but it could help in unimaginable ways. Whether you think of the examination days or the preparation days, they come in handy and can be of the best help at all times. Hence, students are advised to prepare good revision notes for themselves to make their study swift and concise. 

• Revise Repeatedly 

To keep a track of whatever the students have understood, they need to revise the topics repeatedly. This never lets them lose their confidence. An important thing that a student shall remember at all times is that every bit of the syllabus is equally important and hence, leaving any of it can cost you more than you could ever imagine. 

• Attempt Mock Tests 

Another important part of your preparation strategy is to attempt some mock tests. It gives you an idea of what the questions would look like and what is the exam pattern that is followed. Once you understand it, it becomes easier for you to attempt the final paper. Practicing sample papers also helps you to boost confidence and charge you for the final days. 

• Measure your Progress 

An important skill that the students must have is to be able to measure their progress. Be your competitor, keep your previous results handy and compare them with the latest ones. This would give you an insight into how your preparation is going and what areas need more practice or attention. 

• Make an Effective Plan 

Once you’re done with observing your study patterns and habits, you’re required to make an effective, time-based, and realistic study plan which helps you upraise your preparation strategy. Students shall also understand that it is not only important to make a schedule but it is more important for them to stick to it. As much as you stay true to your goals and focus on achieving the targets, even the smallest ones, you get closer to your final destination. 

Self-doubt might stop you time and again but once you decide to work past them with a positive attitude, you're already halfway there! You might face a lot of challenges across the way but adopt a ‘Just Do It’ attitude and your dreams would turn into reality in no time!