chemistry chapter chemical kinetics class 12

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What is Chemical Kinetics?

Chemical kinetics is a branch of physical chemistry that primarily focuses on studying the rate of chemical reactions, these rates, and the mechanism by which reactions proceed. It helps understand how fast reactants are converted into products and what steps are involved in the transformation.

Define Rate of a Reaction.

The rate of a reaction is defined as the change in concentration of a reactant or a product per unit time. It indicates how quickly the concentration of reactants decreases or the concentration of products increases over time. Mathematically, for a reaction R o P, the rate can be expressed as:

• Rate = - rac{ ext{d}[R]}{ ext{d}t} (decrease in reactant concentration)

• Rate = + rac{ ext{d}[P]}{ ext{d}t} (increase in product concentration)

The units for the rate of reaction are typically mol L^{-1} s^{-1} .

What are the factors affecting the rate of a chemical reaction?

The major factors that influence the rate of a chemical reaction include:

1. Concentration of Reactants: Generally, A as it increases the frequency of collisions between reacting molecules.

2. Temperature: Most reactions speed up with an increase in temperature because higher temperatures provide molecules with more kinetic energy, leading to more frequent and more energetic collisions.

3. Presence of a Catalyst: A catalyst is a substance that alters the rate of a reaction without being consumed in the reaction itself. It typically increases the reaction rate by providing an alternative reaction pathway with a lower activation energy.

4. Nature of Reactants: The physical and chemical properties of the reacting substances (e.g., bond strength, physical state, surface area) affect how easily they can react.

5. Surface Area of Reactants: For reactions involving solids, increasing the surface area of the solid reactant increases the rate of reaction as more sites are available for reaction.

Explain Rate Law (or Rate Equation).

The Rate Law is a mathematical expression that relates the rate of a reaction to the concentration of reactants. For a general reaction aA + bB o cC + dD, the rate law is typically expressed as:
Rate = k[A]^x[B]^y
Where:

• k is the rate constant, specific for a given reaction at a particular temperature.
• [A] and [B] are the molar concentrations of reactants A and B.
• x and y are the orders of reaction with respect to reactants A and B, respectively. They are determined experimentally and are not necessarily equal to the stoichiometric coefficients (a and b).

Differentiate between Order of Reaction and Molecularity of Reaction.

Order of Reaction

• Definition: The sum of the powers (exponents) of the concentration terms of the reactants in the experimentally determined rate law expression.

• ex: [A]2+2

• Determination: It is an experimentally determined quantity.

• Values: Can be a whole number, a fraction, or even zero.

• cant be an integer.

• Application: Pertains to the overall rate of reaction.

• Dependence: Depends on the reaction mechanism.

Molecularity of Reaction

• Definition: The number of reacting species (atoms, ions, or molecules) that collide simultaneously in an elementary reaction to bring about a chemical reaction. [collides simultaneously with a reaction to bring out chemical reaction]

• Determination: It is a theoretical concept, determined from the stoichiometry of an elementary reaction. [a theoritical concept determined from a stocheometric coefficient]

• Values: Must be a whole number (1, 2, or 3, as reactions involving simultaneous collision of more than three species are rare). [ any value but simultaneously collsion with more than 3 species rare]

• Application: Pertains only to elementary steps of a complex reaction, or to a simple one-step reaction.

• Dependence: Does not depend on the reaction mechanism in general, only on the number of molecules participating in an elementary step.