Question

"In a photochemical reaction, each molecule of the reacting substance absorbs a single photon of radiation causing the reaction and is activated to form the products." This is the statement of (a) Lambert-Beer's law (b) Grotthus-Draper law (c) Stark-Einstein law (d) Lambert's law

Short answer

The correct answer is (c) Stark-Einstein law.

Step-by-step solution

Read through all options

Review all the provided options in order to select the correct one. It's important to have a clear understanding of what each law stands for. (a) Lambert-Beer's law relates the absorption of light to the properties of the material the light is traveling through; (b) Grotthus-Draper law states that only that light which is absorbed by a system can bring about a photochemical reaction; (c) Stark-Einstein law also known as the law of photochemical equivalence, states that each molecule in a photochemical process is activated by the absorption of one photon only and then undergoes a chemical reaction; (d) Lambert's law states that the absorbance of a solution is directly proportional to its thickness/pathlength.

Match the statement with correct law

The provided statement specifically mentions that each molecule absorbs a single photon which results in a reaction to form products. This aligns well with the Stark-Einstein law (option c) which says each molecule in a photochemical process is activated by the absorption of one photon only and then undergoes a chemical reaction.

Key concepts

Photochemical Reaction

A photochemical reaction occurs when light energy, typically in the form of photons, initiates a chemical change in a substance. This is an essential process in many areas of chemistry and biology. For instance, photosynthesis in plants is a well-known photochemical reaction where sunlight is converted into chemical energy. The process typically follows these key steps:

• The absorption of light by the reactant molecules.
• Excitation of these molecules to a higher energy state.
• The excited molecules destabilize and rearrange, forming new chemical products.

In general, photochemical reactions differ from thermal chemical reactions because they utilize light instead of heat as the energy source. They play a crucial role in creating energy-rich bonds and enabling processes that would otherwise require higher energy inputs. Whether occurring in a lab, nature, or industrial processes, understanding photochemical reactions helps in manipulating conditions to attain desired chemical transformations effectively.

Photon Absorption

Photon absorption is the primary step in any photochemical reaction. It involves a molecule taking in a photon, a particle of light, which corresponds to a certain amount of energy. This process can be visualized as storing light energy within the molecule, which is then used to drive the chemical reaction. When a photon is absorbed, an electron in the molecule moves from a lower energy state to a higher energy level. This elevation in energy is crucial as it transforms the molecule into an excited state, making it more reactive. Since molecules only absorb photons of specific energy levels, the wavelength of light plays a significant role. For example, chlorophyll absorbs light most efficiently in the blue and red regions of the spectrum during photosynthesis. In essence, without photon absorption, no photochemical reaction can occur because the energy required for the reaction is not introduced into the system.

Law of Photochemical Equivalence

The Stark-Einstein Law, also known as the law of photochemical equivalence, underlines the fundamental concept that each molecule in a photochemical reaction absorbs one photon. This absorption is what allows the molecule to reach an excited state, making it ready to undergo further chemical changes. This law posits that, on average, every absorbed photon is expected to cause one molecular photochemical event, such as a chemical bond breakage or formation. It establishes a direct relationship between the number of absorbed photons and the number of reacting molecules. However, it is important to note that not all absorbed photons will necessarily result in a photochemical event due to inefficiencies and competing processes like fluorescence or non-radiative decay. What this law emphasizes is the potential for each photon to initiate a reaction, reflecting the precise linkage between photons and molecular transformations. Understanding this law is essential for analyzing and optimizing photochemical processes, providing insight into how light energy is harnessed in chemical reactions.