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Chemical kinetics explores the dynamics of chemical reactions, investigating reaction rates, the impact of variables, and the formation of intermediates.
- Reaction rates measure the change in reactant concentration or product formation over time, expressed as instantaneous or average rates.
- Factors such as temperature, reactant concentration, and catalysts influence reaction rates, governed by the rate law.
- Determined experimentally, the order of a reaction reflects the concentration's power in the rate equation, while the rate constant defines the proportionality factor.
- Molecularity, applicable to elementary reactions, aligns with the reaction's order.
- The Arrhenius equation illuminates temperature's effect on rate constants, with activation energy (Ea) representing the barrier to reaction initiation.
- Lowering Ea, facilitated by catalysts, accelerates reactions.
- Steric factors, per collision theory, impact molecular orientation, influencing effective collisions and modifying the Arrhenius equation accordingly.
Chemical kinetics thus delves into the intricacies of reaction dynamics, from elementary steps to overall rates, governing chemical transformations.
As per the CBSE Syllabus 2023-24, the Class 12 Electrochemistry chapter is now Chapter 3 with a weightage of 7 marks in the Class 12 Chemistry Exam.
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| Additional Resources for Preparation | |
|---|---|
| Chemical kinetics | NCERT Solutions for Class 12 Chemistry Chapter 4 |
| Chemical kinetics MCQs | Chemical kinetics Questions |
Class 12 Chemistry Chapter 4 Notes – Chemical Kinetics
Rate of a Chemical Reaction
- The rate of a chemical reaction measures the change in concentration of reactants or products per unit of time.
- It can be expressed as the instantaneous rate at a specific moment or the average rate over a period.
Rate of Reaction
Factors Influencing Rate of a Reaction
- Temperature: Higher temperatures generally increase reaction rates by providing more energy for successful collisions between reactant molecules.
- Concentration of Reactants: Higher concentrations of reactants lead to more frequent collisions, thus increasing the reaction rate.
- Presence of Catalysts: Catalysts provide an alternative reaction pathway with lower activation energy, facilitating faster reactions without being consumed in the process.
Factors Influencing Rate of a Reaction
Dependence of Rate on Concentration
- The rate of a chemical reaction often depends on the concentration of reactants.
- In many reactions, an increase in reactant concentration leads to a corresponding increase in reaction rate.
- This relationship is described by the rate law, which expresses the rate as a function of reactant concentrations.
Rate Expression and Rate Constant
- The rate expression represents the mathematical relationship between the rate of a chemical reaction and the concentrations of reactants.
- It is determined experimentally and provides insight into how the rate of reaction changes with varying reactant concentrations.
- The rate constant (k) is a proportionality constant in the rate expression, representing the rate of reaction under specific conditions.
αA + αB → αC + αD
\(Rate = - \frac{1}{a} \times \frac{\Delta [A]}{\Delta t}\)\(= - \frac{1}{b} \times \frac{\Delta [B]}{\Delta t}\)\(= - \frac{1}{c} \times \frac{\Delta [C]}{\Delta t}= - \frac{1}{d} \times \frac{\Delta [D]}{\Delta t}\)
Formula for Rate of Reaction
Order of a Reaction
- The order of a reaction is the sum of the powers of concentrations of reactants in the rate law equation.
- It indicates how the rate of a reaction depends on the concentration of reactants.
- The order can be zero, first, second, or even a fraction, reflecting the relationship between concentration and reaction rate.
| Reaction Order | Differential Rate Law | Integrated Rate Law | Characteristic Kinetic Plot | Slope of Kinetic Plot | Units of Rate Constant |
|---|---|---|---|---|---|
| Zero | \(\frac{- d [A]}{dt}\) = k | [A] = [A]0 - k t | [A] vs t | - k | mole L-1 sec-1 |
| First | \(\frac{- d [A]}{dt}\) = k [A] | [A] = [A]0 e-kt | in [A] vs t | - k | sec-1 |
| Second | \(\frac{- d [A]}{dt}\) = k [A]2 | [A] = \(\frac{[A]^0}{1 + kt [A]^0}\) | 1 / [A] vs t | k | L mole -1 sec-1 |
Molecularity of a Reaction
- Molecularity refers to the number of molecules that participate as reactants in an elementary chemical reaction.
- It is a characteristic of elementary reactions only, ranging from unimolecular (one molecule) to bimolecular (two molecules) to termolecular (three molecules).
- Molecularity directly influences the order of the reaction, which determines how the concentration of reactants affects the reaction rate.
Integrated Rate Equations
- Integrated rate equations provide relationships between reactant concentrations and time, aiding in the determination of reaction kinetics.
- These equations are derived from the rate laws of chemical reactions and allow for the calculation of reaction rates at specific times.
- They are particularly useful in studying the order of reactions and determining rate constants experimentally.
Integrated Rate Equations
Zero Order Reaction
- In a zero-order reaction, the rate of the reaction remains constant over time.
- The rate of the reaction is independent of the concentration of the reactant.
- The reaction proceeds at a constant rate until the reactant is completely consumed.
First Order Reaction
- First-order reactions involve only one reactant and proceed at a rate directly proportional to its concentration.
- The reaction rate follows the equation Rate =k[A], where k is the rate constant and [A] is the concentration of the reactant.
- The half-life of a first-order reaction remains constant throughout the reaction and is independent of the initial concentration of the reactant.
Second Order Reaction
- In a second-order reaction, the overall reaction order is 2, which means the rate of the reaction is proportional to the square of the concentration of one reactant or the product of two reactant concentrations.
- Second-order reactions can exhibit various kinetic behaviours, including dependence on one or multiple reactants, and can involve unimolecular or bimolecular processes.
- The integrated rate law for a second-order reaction depends on the specific reaction mechanism and can take different forms, such as the linear or nonlinear expression involving initial concentrations and time.
Half-Life of a Reaction
- The half-life of a reaction is the time required for the concentration of a reactant to decrease to half its initial value.
- It is a characteristic property of the reaction and varies with the order of the reaction.
- Half-life calculations are useful in determining reaction rates and kinetics, as well as in studying decay processes in radioactive substances.
Half-Life of a Reaction
Pseudo-First Order Reaction
- Pseudo-first-order reactions appear to follow first-order kinetics, despite being higher-order reactions.
- These reactions often involve one reactant present in large excess, so its concentration remains practically constant.
- By considering the limiting behavior of the reactant in excess, the reaction rate equation simplifies to resemble a first-order reaction, hence the term "pseudo-first order."
Pseudo-First Order Reaction
Temperature Dependence of the Rate of a Reaction
- The rate of a reaction typically increases with temperature.
- Elevated temperatures provide more kinetic energy to molecules, promoting more frequent and energetic collisions.
Arrhenius Equation
- According to the Arrhenius equation, rate constants generally rise exponentially with temperature, reflecting the temperature dependence of reaction rates.
- k = A.e–Ea/RT
- Ea is the activation energy
- A is Arrhenius factor
Arrhenius Equation
Effect of Catalyst
- Catalysts increase the rate of chemical reactions by providing an alternative reaction pathway with lower activation energy.
- They remain unchanged in quantity and chemical composition at the end of the reaction, making them highly efficient.
- Catalysts enhance reaction selectivity, facilitating the formation of desired products while minimizing unwanted side reactions.
Collision Theory of chemical reactions
- Molecular Collisions: Chemical reactions occur when molecules collide with sufficient energy and proper orientation.
- Activation Energy: Molecules must overcome a minimum energy barrier, known as activation energy, for a reaction to proceed.
- Effective Collisions: Not all collisions lead to a reaction; only those with proper orientation and energy higher than the activation energy result in a chemical change.
Collision Theory of chemical reactions
Formula for Activation Energy
The formula for Activation energy is given by:
K = Ae-Ea/RT
- Where K indicates the constant rate
- A indicates Arrhenius Constant
- Ea indicates Activation energy
- R determines Gas constant = 8.34 J/K/mol
Formula for Activation Energy
There are Some important List Of Top Chemistry Questions On Chemical Kinetics Asked In CBSE CLASS XII
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