Bihar Board - Class 12 Chemistry - Chapter 4: Chemical Kinetics Handwritten Notes
Chemical kinetics is the branch of chemistry that deals with the study of the rate of chemical reactions and the factors that affect these rates. It helps in understanding how fast reactions occur and the mechanism by which reactants are converted to products. The study of chemical kinetics is essential for designing chemical processes in industries and for controlling the rates of reactions in various fields.
Key Points:
Rate of Reaction:
The rate of reaction refers to the change in concentration of reactants or products per unit time.
It can be measured by monitoring the change in the concentration of reactants or products over time.
Units of rate depend on the order of the reaction.
Factors Affecting the Rate of Reaction:
Concentration of Reactants: Higher concentration of reactants generally increases the rate of reaction.
Temperature: Increasing temperature increases the rate of reaction due to higher kinetic energy of molecules.
Catalysts: Catalysts speed up the reaction without being consumed in the process.
Surface Area of Reactants: A larger surface area increases the reaction rate, as more reactant particles are exposed for collisions.
Pressure (for gases): Increasing pressure generally increases the rate of reaction for reactions involving gases.
Order of Reaction:
The order of a reaction is the sum of the powers of the concentrations of reactants in the rate law equation.
It can be determined experimentally.
Zero-order, first-order, and second-order reactions are common types of reactions based on the order.
Rate Law:
The rate law expresses the rate of a reaction as a function of the concentration of reactants.
It is typically written as: Rate = k [A]^m [B]^n, where k is the rate constant and m, n are the orders with respect to reactants A and B.
Activation Energy (Ea):
Activation energy is the minimum energy required for a chemical reaction to occur.
It can be lowered by the presence of a catalyst, which provides an alternative reaction pathway with a lower activation energy.
Arrhenius Equation:
The Arrhenius equation describes the dependence of the rate constant on temperature and activation energy:
k = A e-Ea/RT
Where:
- k = rate constant
- A = frequency factor
- Ea = activation energy
- R = gas constant
- T = temperature in Kelvin
Half-Life (t1/2):
Half-life is the time required for the concentration of a reactant to decrease to half of its initial value.
The half-life depends on the order of the reaction. For first-order reactions, the half-life is constant.
Collision Theory:
The collision theory explains that for a reaction to occur, reactant molecules must collide with sufficient energy and in the proper orientation.
The number of effective collisions per unit time determines the rate of reaction.
Reaction Mechanism:
A reaction mechanism is the step-by-step sequence of elementary reactions by which overall chemical change occurs.
Each elementary reaction has its own rate law, and the rate-determining step controls the overall rate of the reaction.
Conclusion:
In conclusion, chemical kinetics plays a crucial role in understanding the speed and mechanisms of chemical reactions. By studying the factors that affect reaction rates, we can control and optimize chemical processes for industrial applications. The concepts of activation energy, reaction order, rate laws, and catalysts are fundamental in predicting and controlling chemical reactions. Knowledge of chemical kinetics is essential in fields such as pharmaceuticals, environmental science, and materials engineering. Understanding these principles allows scientists to design more efficient reactions, create new products, and improve existing processes.
