Chemical Kinetics Class 12 Notes PDF Free Download
Are you looking for a comprehensive and easy-to-understand guide on chemical kinetics for class 12? If yes, then you have come to the right place. In this article, you will learn everything you need to know about chemical kinetics, which is one of the most important topics in physical chemistry. You will also get a free PDF download of the notes at the end of the article.
Chemical kinetics is the branch of chemistry that deals with the study of the rates of chemical reactions and the factors that affect them. It helps us to understand how fast or slow a reaction occurs, what are the conditions that influence its speed, and what are the steps involved in its mechanism. Chemical kinetics is essential for class 12 students because it helps them to prepare for competitive exams like JEE and NEET, where questions on this topic are frequently asked. It also helps them to appreciate the practical applications of chemistry in various fields like engineering, biology, medicine, and environmental science.
chemical kinetics class 12 notes pdf free download
Factors Affecting the Rate of a Chemical Reaction
The rate of a chemical reaction is defined as the change in concentration of reactants or products per unit time. It can be expressed as:
Rate = -d[R]/dt = d[P]/dt
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where [R] and [P] are the concentrations of reactants and products respectively, and t is time.
The rate of a chemical reaction depends on several factors, such as:
Concentration of Reactants
The concentration of reactants is directly proportional to the rate of reaction. This means that as the concentration of reactants increases, the rate of reaction also increases. This is because a higher concentration means more molecules per unit volume, which results in more collisions among them, and hence more chances for a successful reaction.
Temperature
The temperature is also directly proportional to the rate of reaction. This means that as the temperature increases, the rate of reaction also increases. This is because a higher temperature means more kinetic energy for the molecules, which results in faster movement, more collisions, and higher activation energy.
Catalyst
A catalyst is a substance that increases the rate of reaction without undergoing any permanent change itself. It does so by providing an alternative pathway for the reaction with lower activation energy. This means that a catalyst lowers the energy barrier for the reaction, which results in more molecules having enough energy to react.
Surface Area
The surface area is also directly proportional to the rate of reaction. This means that as the surface area increases, the rate of reaction also increases. This is because a larger surface area means more exposure of reactants to each other, which results in more collisions and more reactions.
Rate Law and Order of Reaction
The rate law is an expression that relates the rate of reaction to the concentration of reactants raised to some powers. It can be written as:
Rate = k[R1][R2]...
where k is the rate constant, [R1], [R2], ... are the concentrations of reactants, and m, n, ... are the orders of reaction with respect to each reactant. The overall order of reaction is the sum of the individual orders.
The rate law and the order of reaction can only be determined experimentally. They cannot be predicted from the balanced chemical equation. The order of reaction indicates how sensitive the rate of reaction is to the change in concentration of a reactant.
Definition and Examples of Rate Law and Rate Constant
The rate law is also called the rate equation or the differential rate expression. It shows how the rate of reaction depends on the concentration of reactants at any instant of time. For example, for the reaction:
2NO + O2 2NO2
The rate law is given by:
Rate = k[NO][O2]
This means that the rate of reaction is directly proportional to the square of the concentration of NO and the concentration of O2.
The rate constant, k, is a proportionality constant that depends on the nature of the reaction, the temperature, and the presence of a catalyst. It has a specific value for a given reaction at a given temperature. It does not depend on the concentration of reactants. The units of k vary depending on the order of reaction. For example, for a first order reaction, k has the units of s, while for a second order reaction, k has the units of L mol s.
Methods to Determine the Order of Reaction
There are two main methods to determine the order of reaction with respect to a reactant: the initial rate method and the integrated rate method.
The initial rate method involves measuring the initial rate of reaction at different concentrations of a reactant and keeping the other reactants constant. Then, a plot of log(rate) vs log(concentration) is made and the slope gives the order of reaction. For example, for the reaction:
A + B C + D
The initial rate method can be used to find the order with respect to A by varying [A] and keeping [B] constant.
The integrated rate method involves measuring the concentration of a reactant at different times during the course of a reaction and keeping the other reactants in excess. Then, a plot of concentration vs time is made and the shape of the curve gives the order of reaction. For example, for a first order reaction:
A B + C
The integrated rate method can be used to find the order with respect to A by plotting [A] vs t and getting a straight line with a negative slope.
Zero Order, First Order, and Second Order Reactions
A zero order reaction is one in which the rate of reaction is independent of the concentration of any reactant. It has a constant rate that is equal to the rate constant. The integrated rate law for a zero order reaction is:
[A] = -kt + [A]0
where [A] is the concentration of A at time t, [A]0 is the initial concentration of A, and k is is equal to the product of the collision frequency and the fraction of effective collisions. The fraction of effective collisions depends on the activation energy and the temperature, while the collision frequency depends on the concentration and the surface area of the reactants.
Conclusion
In this article, you have learned about chemical kinetics, which is the study of the rates of chemical reactions and the factors that affect them. You have learned about the following topics:
The factors affecting the rate of a chemical reaction, such as concentration, temperature, catalyst, and surface area.
The rate law and the order of reaction, which relate the rate of reaction to the concentration of reactants.
The Arrhenius equation and the activation energy, which relate the rate constant to the temperature and the energy barrier for the reaction.
The collision theory of reaction rates, which explains how the frequency and effectiveness of collisions between molecules determine the rate of reaction.
By learning these topics, you have gained a deeper understanding of how chemical reactions occur and how they can be controlled or optimized. You have also prepared yourself for higher-level studies in chemistry and related fields. You can download a free PDF of these notes by clicking on the link below.
FAQs
Here are some frequently asked questions and answers about chemical kinetics:
What is the difference between rate and rate constant?
The rate of a chemical reaction is the change in concentration of reactants or products per unit time. It depends on several factors, such as concentration, temperature, catalyst, and surface area. The rate constant is a proportionality constant that relates the rate of reaction to the concentration of reactants raised to some powers. It depends on the nature of the reaction, the temperature, and the presence of a catalyst.
What is the difference between activation energy and threshold energy?
The activation energy is the minimum energy required for a successful collision between molecules. It represents the energy barrier that must be overcome by the reactants to form products. The threshold energy is the minimum energy required for a collision between molecules. It represents the potential energy of the reactants in their ground state. The activation energy is always greater than or equal to the threshold energy. The difference between them is called the energy of activation.
What is the difference between molecularity and order of reaction?
The molecularity of a reaction is the number of molecules that collide with each other in a single elementary step. It can only have whole number values, such as 1, 2, or 3. The order of reaction is the sum of the powers of the concentration of reactants in the rate law. It can have any value, such as 0, 1, 1.5, or 2. The molecularity and the order of reaction are not necessarily the same. The molecularity can only be applied to elementary reactions, while the order of reaction can be applied to any reaction.
What is the difference between homogeneous and heterogeneous catalysis?
A homogeneous catalyst is a catalyst that is in the same phase as the reactants and products. It forms an intermediate complex with the reactants and lowers the activation energy for the reaction. A heterogeneous catalyst is a catalyst that is in a different phase from the reactants and products. It provides a surface for the reactants to adsorb and react on and lowers the activation energy for the reaction.
What is the difference between reversible and irreversible reactions?
A reversible reaction is a reaction that can proceed in both forward and backward directions. It reaches a state of equilibrium, where the rate of forward reaction is equal to the rate of backward reaction. The concentrations of reactants and products remain constant at equilibrium. An irreversible reaction is a reaction that can only proceed in one direction. It goes to completion, where all the reactants are converted into products. The rate of forward reaction is much greater than the rate of backward reaction. The concentrations of reactants decrease to zero and the concentrations of products increase to maximum at completion. 44f88ac181