Problem 43 - A Complete Resource Book in Chemistry for JEE Main

Question

For the reaction \(\mathrm{H}_{2}+\mathrm{Cl}_{2} \stackrel{\text { sunlight }}{\longrightarrow} 2 \mathrm{HCl}\) taking place on water, the order of reaction is (a) 0 (b) \(\underline{1}\) (c) 2 (d) 3

Step-by-Step Solution

Answer

The order of the reaction is (a) 0.

Step 1: Understand the Reaction Context

The given reaction is \( \mathrm{H}_{2} + \mathrm{Cl}_{2} \rightarrow 2 \mathrm{HCl} \), which requires sunlight as a catalyst. It indicates that the reaction takes place involving molecules in the presence of photons.

Step 2: Define Chemical Reaction Order

The order of a reaction is defined by the sum of the exponents of the concentration terms in the rate law expression. For a simple bimolecular reaction, the rate law expression is determined experimentally.

Step 3: Analyze the Role of Sunlight

In this reaction, sunlight acts as a catalyst to initiate the reaction. Generally, for photochemical reactions like this one, the reaction is zero-order with respect to the reactants because the intensity of light is the actual determining factor.

Step 4: Consider the Role of Catalysts

In photochemical reactions, especially the decomposition or formation reactions initiated by light, the reaction order mainly depends on the presence of light rather than concentration of reactants. This tends to mean a zero-order reaction.

Step 5: Conclusion from Theory and Practice

Since this is a photochemical reaction where light initiates the process, the typical order is zero-order with respect to the concentrations of \( \mathrm{H}_{2} \) and \( \mathrm{Cl}_{2} \). Therefore, the order of reaction is 0.

Key Concepts

Photochemical Reactions

Photochemical reactions are a fascinating class of reactions that rely on light to proceed. Unlike typical chemical reactions driven by heat or mixing, photochemical reactions harness the energy of photons to instigate chemical changes. In the given exercise, the reaction of hydrogen and chlorine to form hydrochloric acid is initiated by the presence of sunlight, a common feature in photochemical processes. Importantly, these reactions do not depend on the concentration of reactants, such as hydrogen and chlorine, in the traditional sense. Instead, they rely on the intensity and presence of light, which acts as a catalyst by providing the necessary energy to break chemical bonds and form new ones.

• Light as a Catalyst: Sunlight or artificial light sources provide photons that excite reactant molecules to a higher energy state, facilitating their transformation into products.
• Independence from Concentration: Unlike thermal reactions, where concentration plays a critical role, photochemical reactions are often zero-order due to their dependence on light intensity rather than reactant concentrations.

This unique reliance on light makes photochemical reactions a subject of significant interest, especially in fields like photochemistry and environmental science.

Rate Law Expression

The rate law expression is an equation that links the rate of a chemical reaction to the concentrations of its reactants. For most reactions, the rate law is determined experimentally and can be represented as:\[ \text{Rate} = k[A]^m[B]^n \]where \(A\) and \(B\) are reactants, \(m\) and \(n\) are the reaction orders with respect to each reactant, and \(k\) is the rate constant. The overall reaction order is the sum of \(m\) and \(n\). However, this equation can vary significantly in photochemical reactions.In the context of the given reaction, which is aided by the presence of sunlight, the standard understanding of rate laws slightly shifts.

• Zero-order in Reactants: The reaction doesn't rely on the concentration of hydrogen or chlorine but on the light intensity instead.
• Experimental Determination: The peculiarities in photochemical reactions necessitate experimental setup to adequately determine their rate laws, often leading to a zero-order conclusion for reactions relying on light.

Therefore, although the standard reactions might depend on reactant concentrations, photochemical reactions often feature a rate law independent of such concentrations due to their reliance on an external energy source like light.

Chemical Catalysis

Catalysis is the process by which a substance, known as a catalyst, increases the rate of a chemical reaction without undergoing permanent chemical change itself. In the case of the photochemical reaction under consideration, light acts as a unique catalyst. Instead of a chemical substance altering the pathway of a reaction, it's the energy from sunlight that facilitates the process. Catalysts lower the activation energy of reactions, making it easier for the reactants to achieve the transition state necessary for bonding. However, in photochemical reactions:

• Energy Input via Photons: Instead of traditional catalysts, photons provide the necessary energy to drive the reaction forward.
• Specificity of Light: Different wavelengths of light can selectively catalyze different reactions, offering precise control over chemical processes.

This photochemical catalysis is a potent reminder of the versatility of catalytic processes and how they can be adapted using various energy forms. Unlike typical catalysis relying on chemical substances, photochemical catalysis opens pathways that are otherwise difficult to achieve, showcasing the innovative ways chemistry integrates with natural phenomena like light.