Question

"Only the fraction of incident light that is absorbed by the substance can bring about a chemical change." This is the (a) first law of photochemistry (b) second law of photochemistry (c) third law of photochemistry (d) none of these

Short answer

The correct answer is (a) first law of photochemistry.

Step-by-step solution

Analyze the statement

The given statement refers to the idea that for a chemical change to occur in a substance due to light (photochemical reactions), the light must be absorbed by the substance. This isn't speaking about the number of molecules participating in the reaction, just the necessity of absorption of light for the reaction.

Identify the law

By referring to the laws of photochemistry, it can be seen that the described scenario corresponds to the First law of photochemistry (Grotthuss–Draper law) which states: 'only that light which is absorbed by a system can bring about a photochemical change'.

Key concepts

Grotthuss-Draper Law

The Grotthuss-Draper Law serves as the foundational concept of photochemistry. This law states that only the light absorbed by a substance can initiate a photochemical reaction. Essentially, if light merely passes through or reflects off a material without being absorbed, it will not cause any chemical change. The law emphasizes the importance of light absorption for initiating reactions, highlighting that absorption is the primary step in any photochemical activity.
In simpler terms:

• If light is absorbed, a chemical reaction can occur.
• If light is not absorbed, there will be no reaction.

This understanding is crucial because it delineates the boundaries of possible reactions in photochemistry, ensuring that only molecules that actually absorb light can undergo chemical transformation.

Photochemical Reactions

Photochemical reactions are types of chemical reactions that are initiated by the absorption of light. These reactions can vary widely, involving processes such as photosynthesis or even the mechanisms behind photography. When a photon, or a particle of light, is absorbed by a molecule, it elevates the molecule to an excited state. This excited state is often more reactive, allowing it to participate in chemical reactions it normally wouldn't in its ground state.
In biochemistry, for example, the role of chlorophyll in plants is pivotal in capturing light energy, which is then used to convert carbon dioxide and water into sugars and oxygen. This illustrates the power of photochemical reactions in facilitating life-supporting processes.
Key characteristics of these reactions include:

• They require light to occur.
• The reactions involve the absorption of photons, which changes the energy state of molecules.
• These reactions can lead to diverse and complex outcomes, such as energy transfer or structural changes.

Understanding photochemical reactions opens up insights into how energy conversions and material changes occur entirely based on light exposure.

Light Absorption in Chemistry

Light absorption in chemistry refers to how light interacts with a substance at the molecular level. When light is absorbed, it causes changes in the electronic states of a molecule. This step is pivotal for any photochemical reaction to take place, as highlighted by the Grotthuss-Draper Law.
Different substances absorb different wavelengths of light. This is due to the unique molecular structures that dictate specific energy levels, meaning only certain photons match the energy required to elevate a molecule to its excited state.
Key factors affecting light absorption include:

• The chemical structure of the substance, which determines which wavelengths of light it can absorb.
• The concentration of the substance, as denser solutions typically absorb more light.
• The path length of light through the material; a longer path allows more opportunities for absorption.

Understanding light absorption is essential for designing experiments and interpreting results in photochemistry. It entails recognizing which parts of the electromagnetic spectrum are engaged by different materials, thereby facilitating targeted photochemical processes.