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CBSE Class 12 Chemistry Notes Chapter 4 Chemical Kinetics FAQs
What is Chemical Kinetics?
Chemical kinetics is the branch of chemistry that studies the rates of chemical reactions and the factors that affect these rates. It involves understanding how different conditions (such as temperature, concentration, and catalysts) influence the speed of a reaction and the mechanisms by which reactions occur.
What is the Rate of Reaction?
The rate of reaction is the change in the concentration of reactants or products per unit time. It can be expressed in units like mol/L/s or M/s. The rate provides information on how quickly a reaction proceeds.
What is the Arrhenius Equation?
The Arrhenius equation describes how the rate constant (k) of a reaction depends on temperature.
What is Activation Energy (E_a)?
Activation energy is the minimum energy required for reactants to transform into products. It represents the energy barrier that must be overcome for a reaction to proceed.
What is the Activated Complex (or Transition State)?
The activated complex is a high-energy, unstable intermediate state during a chemical reaction where bonds are partially broken and formed. It exists for a very short time before converting into products or reverting to reactants.
CBSE Class 12 Chemistry Notes Chapter 4 Chemical Kinetics
Here we have provided CBSE Class 12 Chemistry Notes Chapter 4 Chemical Kinetics for the ease of students so that they can prepare better for their exams.
Ananya Gupta5 Sept, 2024
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CBSE Class 12 Chemistry Notes Chapter 4:
In Class 12 Chemistry you will learn advanced concepts that build upon the foundational knowledge gained in previous years. In Class 12 Chemistry the focus shifts towards understanding the intricate details of chemical reactions, properties of elements, and the principles governing them.
These notes are created by subject experts and provide a clear and concise summary of each chapter, ensuring you can easily grasp complex concepts. Whether you are revising for exams or simply want to deepen your understanding, these Chemistry Notes are an important resource throughout your studies.
CBSE Class 12 Chemistry Notes Chapter 4 Chemical Kinetics Overview
These CBSE Class 12 Chemistry Notes for Chapter 4 Chemical Kinetics have been prepared by subject experts of Physics Wallah. The notes provide a detailed overview of the chapter focusing on the study of reaction rates and the factors that influence them.
You'll find detailed explanations of key concepts such as the rate of reaction, the effect of concentration, temperature, and catalysts, and the significance of the Arrhenius equation.
CBSE Class 12 Chemistry Notes Chapter 4 PDF
For a detailed understanding of CBSE Class 12 Chemistry Chapter 4 Chemical Kinetics, you can access the complete notes in PDF format through the link provided below.
These notes cover all important topics, including reaction rates, the impact of various factors on these rates, and key concepts like the Arrhenius equation, reaction order, and molecularity.
CBSE Class 12 Chemistry Notes Chapter 4 Chemical Kinetics
Here we have provided CBSE Class 12 Chemistry Notes Chapter 4 Chemical Kinetics-
Chemical Kinetics
Chemical Kinetics
is a branch of chemistry that focuses on the rate of chemical reactions, the factors influencing these rates, and the mechanisms through which reactions occur. Understanding how quickly a reaction proceeds is crucial, and chemical kinetics provides the framework to analyze this.
Chemical reactions can be categorized based on their rate of reaction:
Fast/Instantaneous Reactions:
These are reactions that complete in less than 1 picosecond (10⁻¹² seconds). Due to their extremely rapid nature, measuring the speed of such reactions is nearly impossible. Examples include ionic reactions and certain organic substitution reactions.
Slow Reactions:
These reactions take a significantly longer time to complete, ranging from minutes to years. Examples include the rusting of iron and the transformation of diamond into graphite.
Moderately Slow Reactions:
These reactions fall between the fast and slow categories. They do not complete instantaneously but are faster than slow reactions, making them intermediate in speed.
Rate of Reaction
The
rate of reaction
refers to the change in concentration of a reactant or product per unit time. It is commonly expressed in units of mol L⁻¹ s⁻¹, M s⁻¹, or atm s⁻¹.
Rate of reaction = (decrease/increase in the concentration of reactant/product/time taken)
This measurement is known as the
average rate of reaction
(rₐᵥ). To calculate this, you take the difference in concentration of the reactant or product over a specific time interval.
Instantaneous rate of reaction
The
instantaneous rate of reaction
refers to the rate of a chemical reaction at a specific moment in time. Unlike the average rate, which measures the overall change in concentration over a period, the instantaneous rate provides a snapshot of the reaction rate at a particular point.
To measure the reaction rate, several methods can be employed:
pH Measurement:
Tracking changes in pH can indicate how the concentration of hydrogen ions is varying, which helps in determining the rate of reaction.
Change in Optical Activity:
For reactions involving chiral substances, measuring changes in optical rotation can provide information about the rate of reaction.
Change in Pressure:
For reactions involving gases, monitoring changes in pressure can be used to determine the rate of reaction.
Change in Conductance:
In reactions that involve ionic species, measuring changes in electrical conductance can help assess the reaction rate.
Factors Affecting Rate of Reaction
The rate of a chemical reaction is influenced by several key factors:
Nature and Concentration of Reactants:
The inherent properties of the reactants and their concentrations play a crucial role. For instance, reactions involving more reactive substances or higher concentrations of reactants typically proceed faster.
Temperature:
Increasing the temperature generally accelerates the rate of reaction. This is because higher temperatures provide more energy to the reactant molecules, increasing their collision frequency and the energy available for overcoming the activation energy barrier.
Surface Area of Reactants:
The surface area of the reactants affects the reaction rate. A greater surface area allows more reactant molecules to be exposed and interact, speeding up the reaction. For example, powdered solids react faster than larger chunks of the same material.
Radiations and Catalysts:
Certain types of radiation can affect reaction rates, especially in photochemical reactions. Catalysts, on the other hand, are substances that speed up a reaction without being consumed in the process. They work by providing an alternative reaction pathway with a lower activation energy.
Pressure of Gas:
For reactions involving gases, increasing the pressure generally increases the rate of reaction. This is because higher pressure increases the concentration of gas molecules, leading to more frequent collisions.
Rate Law Expressions
According to the law of mass action,
For a chemical reaction,
aA + bB → Products
Rate α [A]a
[B]b
= k[A]a
B]b
But experimentally, it is observed that the rate of reaction is found to depend upon ‘α’ concentration terms of A and ‘β’ concentration terms of B Then,
Rate α [A]α
[B]β
= k[A]α
[B]β
where, [A] and [B] molar concentrations of A and B respectively and k is the velocity constant
or rate constant. The above expression is known as rate law.
Rate Constant
In the above expression, k is called rate constant or velocity constant. Rate constant may be defined as the specific rate of reaction when the molar concentrations of the reactants is taken to be unity, i.e.,
Rate = k, if [A] = [B] = 1
Units of rate constant or specific reaction rate for a nth order reaction is given as K = (1/Time) x(1/[Conc.]n – 1)
Characteristics of rate constant
The rate of a chemical reaction is influenced by several key factors:
Nature and Concentration of Reactants:
The inherent properties of the reactants and their concentrations play a crucial role. For instance, reactions involving more reactive substances or higher concentrations of reactants typically proceed faster.
Temperature:
Increasing the temperature generally accelerates the rate of reaction. This is because higher temperatures provide more energy to the reactant molecules, increasing their collision frequency and the energy available for overcoming the activation energy barrier.
Surface Area of Reactants:
The surface area of the reactants affects the reaction rate. A greater surface area allows more reactant molecules to be exposed and interact, speeding up the reaction. For example, powdered solids react faster than larger chunks of the same material.
Radiations and Catalysts:
Certain types of radiation can affect reaction rates, especially in photochemical reactions. Catalysts, on the other hand, are substances that speed up a reaction without being consumed in the process. They work by providing an alternative reaction pathway with a lower activation energy.
Pressure of Gas:
For reactions involving gases, increasing the pressure generally increases the rate of reaction. This is because higher pressure increases the concentration of gas molecules, leading to more frequent collisions.
Pseudo First Order Reactions
Pseudo First Order Reactions
are chemical reactions that, despite involving more than one reactant, can be treated as first-order reactions under certain conditions. This simplification occurs when one of the reactants is present in such excess that its concentration remains nearly constant throughout the reaction. As a result, the reaction effectively behaves as if it were first-order with respect to the reactant that is not in excess.
In a pseudo first-order reaction, the rate law can be simplified to resemble a first-order reaction. This often happens in reactions where one reactant is present in large excess compared to the other.
For example, in the hydrolysis of an ester in the presence of a large amount of water, the concentration of water remains approximately constant, so the reaction rate depends predominantly on the concentration of the ester.
Arrhenius Equation
The Arrhenius Equation provides a quantitative relationship between the rate constant of a chemical reaction and the temperature. It helps in understanding how temperature affects the rate of reaction by relating it to the activation energy of the reaction.
Activated Complex (or Transition State)
The
activated complex
, also known as the transition state, is a high-energy, unstable intermediate that forms between the reactants and the products during a chemical reaction.
It represents the state where the reactant bonds are partially broken, and the product bonds are partially formed. The activated complex has a very short lifespan and quickly decomposes into the final products or reverts to reactants.
Threshold Energy (E_T)
The
threshold energy
(E_T) is the minimum amount of energy that reactant molecules must possess to convert into products. This energy must be reached for the reactants to undergo a successful reaction.
Activation Energy (E_a)
Activation energy
(E_a) is the additional amount of energy required by the reactants to reach the threshold energy and convert into products.
The Activated Complex Theory (or Transition State Theory)
The
Activated Complex Theory
or
Transition State Theory
provides a framework for understanding how chemical reactions occur. According to this theory:
Reactants
transform into
products
via an
activated complex
(or transition state), which is an intermediate state with partially broken and partially formed bonds.
This theory emphasizes that bond cleavage and bond formation happen simultaneously, and the reactants do not convert directly into products. Instead, they first form a high-energy intermediate state known as the activated complex.
Energy Barrier:
The reactants must overcome an energy barrier to reach the activated complex. The height of this barrier determines the
threshold energy
required for the reaction to proceed.
Photochemical Reactions
Photochemical reactions
are chemical reactions that occur when exposed to visible light or other forms of electromagnetic radiation. Key points include:
Intensity of Light:
The rate of a photochemical reaction is significantly influenced by the intensity of the light. Higher light intensity generally increases the reaction rate by providing more photons to drive the reaction.
Temperature:
Temperature has a relatively minor effect on photochemical reactions compared to other types of chemical reactions. The primary driving force is the light energy rather than thermal energy.
Quantum Yield (φ):
The quantum yield or quantum efficiency of a photochemical reaction measures how effectively light is used in the reaction.
Benefits of CBSE Class 12 Chemistry Notes Chapter 4 Chemical Kinetics
Comprehensive Understanding:
The notes provide a clear and concise explanation of key concepts in chemical kinetics, including rate laws, reaction mechanisms and the Arrhenius equation helping students grasp complex topics effectively.
Simplified Concepts:
Complex ideas such as the activated complex, transition state theory, and factors affecting reaction rates are broken down into simpler terms making them more accessible for students.
Revision Aid:
With concise summaries and key points these notes are an excellent resource for quick revision helping students consolidate their understanding and retain important information.
Expert Preparation:
The notes are prepared by subject experts ensuring that the content is accurate, relevant and aligned with the CBSE curriculum, providing students with reliable study material.
The Arrhenius Equation provides a quantitative relationship between the rate constant of a chemical reaction and the temperature. It helps in understanding how temperature affects the rate of reaction by relating it to the activation energy of the reaction.
The
activated complex
, also known as the transition state, is a high-energy, unstable intermediate that forms between the reactants and the products during a chemical reaction.
It represents the state where the reactant bonds are partially broken, and the product bonds are partially formed. The activated complex has a very short lifespan and quickly decomposes into the final products or reverts to reactants.
