Chemical Kinetics

For a first-order reaction: After 10 min, 20% is complete 80% remains: . Hence For 75% completion, 25% remains: . Time required:

(i) Concentration:
According to the law of mass action, the rate of a chemical reaction is directly proportional to the concentration of reactants. Higher concentration more molecules per unit volume greater frequency of effective collisions higher reaction rate. For example, doubling the concentration of reactant in a first-order reaction doubles the rate.
(ii) Temperature:
Raising temperature increases the kinetic energy of molecules. This leads to: - More frequent collisions, and - A higher fraction of molecules possessing energy greater than the activation energy.
As per the Arrhenius equation, , the rate constant increases exponentially with temperature. Roughly, a rise in temperature nearly doubles the rate of many reactions.
(iii) Catalyst:
A catalyst provides an alternate reaction pathway with lower activation energy ( ). This does not affect the equilibrium position but significantly increases the rate at which equilibrium is achieved. Both homogeneous and heterogeneous catalysts enhance the velocity by lowering the energy barrier for effective collisions.

Molecularity:
- Molecularity of a reaction is the number of reactant species (atoms, ions or molecules) that collide simultaneously in a single step to bring about the reaction.
- It is always a whole number (1, 2, or 3).
- It is a theoretical concept based on reaction mechanism.
Example:
Unimolecular reaction: Decomposition of N O in a single step has molecularity = 1.
Order of reaction:
- The order of a reaction is the sum of the powers of the concentrations of reactants in the experimentally determined rate law expression.
- It can be zero, fractional, or whole number.
- Order is determined experimentally.
Example:
For decomposition of H O : Rate = k[H O ], order = 1.
Key Differences:
1. Molecularity is theoretical and cannot be zero or fractional, while order can be zero or fractional.
2. Molecularity refers to a single step, whereas order is obtained from the overall rate law.

Overall reaction on Pt: .
For a reaction : .
Zero order in NH on Pt means the rate of disappearance of NH is .
Hence .
Rate of formation of nitrogen: .
Rate of formation of hydrogen: .

(x) Order of a reaction and molecularity:
- Order of a reaction refers to the sum of the exponents of the concentration terms in the rate law. It indicates how the rate of reaction is affected by the concentration of the reactants. For example, in a reaction with the rate law , the order of the reaction is , where and are the orders with respect to reactants A and B, respectively.
- Molecularity refers to the number of reacting species (atoms, molecules, or ions) that collide in an elementary step of the reaction. Molecularity is an integer and can only be 1 (unimolecular), 2 (bimolecular), or 3 (termolecular) for most elementary reactions. Unlike order, molecularity is determined from the balanced equation of the elementary reaction, not from the experimental data.
Key Difference: The order of a reaction is determined experimentally, while molecularity is determined from the balanced equation of an elementary reaction.
(y) Average and instantaneous velocity:
- Average velocity is defined as the total displacement (change in position) divided by the total time taken. In terms of reaction rate, it can be defined as the total change in concentration of reactants or products divided by the total time interval.
- Instantaneous velocity is the velocity of an object at a specific instant in time. In terms of reaction rate, it refers to the rate of change of concentration at a particular moment, and it can be calculated by taking the derivative of the concentration with respect to time.
Key Difference: Average velocity is the total change over a time interval, while instantaneous velocity refers to the rate of change at a specific point in time.

Step 1: Define Zero Order Reaction
A zero-order reaction is one in which the rate of reaction is independent of the concentration of the reactants. Mathematically, it is expressed as: where is the rate constant. The concentration of reactants does not affect the rate, and the reaction rate remains constant over time.
Step 2: Example of Zero Order Reaction
An example of a zero-order reaction is the decomposition of hydrogen peroxide on the surface of platinum: In this reaction, the rate of decomposition is independent of the concentration of hydrogen peroxide. Final Answer: In zero-order reactions, the rate is constant and does not depend on the concentration of reactants. Correct Answer: Zero-order reactions have a constant rate, unaffected by reactant concentration.

Step 1: Define Molecularity
Molecularity refers to the number of reactant molecules involved in an elementary reaction step. It is the number of particles (atoms, molecules, or ions) that collide to produce a reaction.
Step 2: Types of Molecularity
- **Unimolecular**: A reaction involving the collision of a single molecule, e.g., the decomposition of N₂O₅: - **Bimolecular**: A reaction involving the collision of two molecules, e.g., the reaction between hydrogen and iodine: - **Trimolecular**: A reaction involving the collision of three molecules, e.g., a reaction between two nitrogen dioxide molecules and one oxygen molecule: Final Answer: Molecularity refers to the number of molecules or ions involved in an elementary reaction. Correct Answer: The number of molecules or ions involved in an elementary reaction.

Step 1: Rate Constant Equation
For a first-order reaction, the integrated rate equation is: Where:

• is the initial concentration.
• is the final concentration at time .
• is the rate constant.

Step 2: Half-Life Derivation
At half-life ( ), the final concentration becomes half of the initial concentration ( ). Substituting this into the equation: Since : Step 3: Conclusion
The formula contains only constant values and does not include the initial concentration term ( ). Therefore, the half-life of a first-order reaction is independent of the initial concentration.

Step 1: Understanding the Concept:
The order of a reaction is the sum of the powers (exponents) of the concentration terms in the rate law equation.
Step 2: Detailed Explanation:
Given rate law:
Here,
Order with respect to
Order with respect to
Total order of the reaction
Step 3: Final Answer:
The total order of the reaction is 2. Hence, it is a Second order reaction.