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The most recent topic in today's world is nuclear power. Have you ever wondered why some reactions, like nuclear explosions, happen within a fraction of second while some reactions take years to happen like rusting of iron? Well, this depends on the rate of reaction, and in this NCERT chapter of Chemical Kinetics you will learn everything about it. Similarly, Food preservation is something that we see every day. Fresh fruits and vegetables spoil due to interactions between microbes and oxygen in the air. Storing them in the refrigerator slows down these processes, allowing them to last longer. In medicine, how rapidly a drug dissolves in the body influences its effectiveness. All this involves study of rate of reaction, and hence, chemical kinetics.
Chemical Kinetics is the branch of Chemistry that investigates how quickly certain reactions occur. It investigates what influences the rate of reactions and how compounds transform into new products. It has the potential to improve industrial processes, reduce pollution, and potentially contribute to the development of life-saving drugs.
In the NCERT Notes of class 12 Chemical kinetics helps in scoring well in CBSE board exams by simplifying complex concepts and formulas. It is well-organised and concise, which makes it ideal for last-minute study and recapping before exams. These NCERT notes align with the updated CBSE syllabus covers all the important topics like Molecularity of reaction, Integrated rate equations, Temperature Dependence of the Rate of a Reaction, etc. This NCERT Chapter Chemical Kinetics includes some very important topics which are frequently asked in JEE and NEET, like Arrhenius equation, Temperature Dependence of Rate Constant, Rate Law and Order of Reaction.
Also, students can refer,
NCERT Notes Class 12 Chemistry |
NCERT Solutions for Class 12 Chemistry Chapter 3 Chemical Kinetics |
NCERT Exemplar Class 12 Chemistry Chapter 3 Solutions Chemical Kinetics |
In this chapter, you’ll learn about different factors affect reaction rates, Step-by-step derivations, Important formulas,Diagrams and graphs, how to calculate rate of reaction mathematically, and how to interpret experimental data using integrated rate laws. Below is a structured breakdown of each concept from the NCERT textbook to help you master the topic with clarity.
The rate of a reaction can be defined as the change (decrease or increase) in the concentration of a reactant or product in unit time. The rate of a reaction can be given as:
The average rate of reaction can be defined as the change in the concentration of a reactant or product in a definite time interval (Δt). It is denoted by the symbol
Consider a hypothetical reaction:
Reactant (M)
The rate of disappearance of reactant-
A minus sign indicates a decrease in concentration.
The rate of appearance of the product-
The instantaneous rate of reaction can be defined as the change in the concentration of a reactant or product at a particular instant of time. It is denoted by the symbol
For an infinitesimally small instant of time
Therefore,
For an overall rate of reaction, the rate of disappearance of any of the reactants (or the rate of appearance of products) is divided by their corresponding stoichiometric coefficients.
Such that for a reaction,
The overall rate is given as:
Or
Major factors that influence the rate of reaction are:
Concentration of reactants
Temperature
Catalyst
Rate Law
When the rate of reaction is represented in terms of the concentration of the reactants is known as rate law. It is also known as the rate equation or rate expression.
Consider the following reaction:
The rate law for this reaction is:
Rate
Where x and y represent order w.r.t to A and B and may or may not be equal to a and b.
The differential form of the rate law is:
The order of the chemical reaction is the summation of powers raised to the concentration of the reactants in the rate law.
For a reaction,
The rate law is:
Rate
Therefore, the order of the reaction is x+y.
The order of a reaction can be 0,1,2,3 and fraction.
Order can never be a negative number.
REACTION |
ORDER |
UNITS |
Zero-order reaction |
0 |
mol |
First-order reaction |
1 |
|
Second-order reaction |
2 |
|
Third-order reaction |
3 |
|
Molecularity | No. of reactants | Example Reaction |
---|---|---|
1 (Unimolecular) | One | |
2 (Bimolecular) | Two | |
3 (Termolecular) | Three |
Consider the reaction,
A → P
The rate law of this reaction is given by:
Rate = k[A]0
On integrating the above equation we get,
The graph of the above equation can be plotted as:
Consider the reaction,
A → P
The rate law of this reaction is given by:
Rate = k[A]
On integrating the above equation we get,
The exponential form of a first-order reaction is given as:
Let us consider a typical first order gas phase reaction
Let
If x atm be the decrease in pressure of A at time
Initial conditions (at
At time
The following equation can be used to plot graphs of first-order reactions:
The time in which the concentration of a reactant is decreased to one-half of its initial concentration is known as the half-life of a reaction. Half-life is represented as t1/2.
For zero-order reactions
Note that for zero-order reaction
The reactions in which the actual order is different from the expected rate order predicted using rate law are called pseudo-first-order reactions.
The rate law of the reaction:
Rate
Expected order = 2
The concentration of water does not get altered much during the reaction as it is in excess and can be considered constant.
Rate
Actual order = 1
Therefore, the reaction behaves as a first-order reaction, and such reactions are called pseudo-first-order reactions.
Order | Reaction Type | Differential Rate Law | Integrated rate law | Striaght line plot | Half-life | Units of k |
0 | ||||||
1 | time |
The fact that the rate of a chemical reaction is dependent on temperature can be explained by the Arrhenius equation.
The mathematical form of the Arrhenius equation is given below:
where A is the Arrhenius factor / the frequency factor / the pre-exponential factor.
According to Arrhenius, a reaction can take place only when two (or more) molecules collide to form an unstable intermediate. This intermediate exists for a very short time and then breaks up to form product(s).
The activation energy (
The minimum energy which the colliding molecules required to collide effectively is called threshold energy.
Threshold energy = Activation energy + Energy of the reactants
The rate of reaction depends on the temperature as it is found that with the rise in temperature by 10°, the rate constant is nearly doubled.
Temperature coeff icient
At a particular temperature, the fractions of molecules are plotted against corresponding kinetic energies, a graph given below is obtained.
The peak of the curve represents the kinetic energy possessed by the maximum fraction of molecules and is called the most probable kinetic energy.
Here, the yellow area shows the fraction of additional molecules which react at t+10 while the grey area shows the fraction of molecules reacting at t.
Taking ln on both sides of the following equation:
Arrhenius equation for two different temperatures can be modified as:
A catalyst is a substance that alters or changes the rate of a reaction without undergoing any permanent chemical change.
According to intermediate complex theory, a catalyst forms temporary bonds with the reactants, forming an intermediate complex. This is how a catalyst participates in a chemical reaction.
The catalyst reduces the energy difference known as activation energy between products and reactants which in turn lowers the potential energy barrier.
A catalyst catalyses to the same extent the forward as well as the backward reactions so that the equilibrium state remains same but equilibrium is reached faster.
Question: In a first order decomposition reaction, the time taken for the decomposition of reactant to one fourth and one eighth of its initial concentration are
(1)
(2)
(3)
(4)
Answer:
Hence, the correct answer is option (4).
Question: Half life of zero order reaction
(1) 0.5 hour
(2) 4 hour
(3) 15 min
(4) 60 min
Answer: For zero order reaction
Now
Hence, the correct answer is option (3).
Question: Which among the following is a false statement?
Answer: Statement (1). Rate of zero order reaction is independent of initial concentration of reactant.
This statement is correct because for zero order reaction
Rate
Means rate is independent of concentration.
Statement 2. Half-life of a zero order reaction is inversely proportional to the rate constant.
This statement is correct for zero order reaction because,
Statement 3. Molecularity of a reaction may be zero.
This statement is incorrect because Molecularity is the number of molecules involved in the reaction. It is always a positive integer. It can never be zero.
Statement 4: For a first order reaction,
This statement is true because for first-order reaction, the half-life is :
Statement (1). Rate of zero order reaction is independent of initial concentration of reactant.
This statement is correct because for zero order reaction
Rate
Means rate is independent of concentration.
Statement 2. Half-life of a zero order reaction is inversely proportional to the rate constant.
This statement is correct for zero order reaction because,
Statement 3. Molecularity of a reaction may be zero.
This statement is incorrect because Molecularity is the number of molecules involved in the reaction. It is always a positive integer. It can never be zero.
Statement 4: For a first order reaction,
This statement is true because for first-order reaction, the half-life is :
Hence, the correct answer is option (3).
The following are the points that can help you build a good approach to solve the questions effectively and efficiently:
1) Students first need to understand the concept of the Rate of Reaction.
2) Then, students need to determine the Rate Constant(k)
3) Plot graphs for different orders of reaction:
4) Analysis of Arrhenius Equation
5) Calculation of Half-life:
6) Read every question carefully and practice questions again and again.
7) Solve questions from NCERT textbook and NCERT exemplar as questions from these textbooks are asked directly in boards and competitive exams.
The hyperlinks of NCERT class 12 notes of each chapter are give below:
The hyperlinks of NCERT Exemplar Solutions of all subjects are given below:
The hyperlinks of NCERT Solutions of all subjects are given below:
NCERT Solutions for Class 12 Mathematics |
NCERT Solutions for Class 12 Chemistry |
NCERT Solutions for Class 12 Physics |
NCERT Solutions for Class 12 Biology |
Chemical kinetics is the branch of chemistry that deals with the rates of chemical reactions and the factors that affect these rates. It helps us understand how fast a reaction occurs and what influences that speed, such as concentration, temperature, and the presence of catalysts.
The rate of a chemical reaction can be determined by measuring the change in concentration of reactants or products over time. For example, if we start with a known concentration of a reactant and measure how much of it reacts in a specific time interval, we can calculate the reaction rate.
Several factors influence the rate of a chemical reaction, including:
1. Concentration of reactants, Higher concentration generally increases the rate.
2. As temperature increases, reactant particles move faster, increasing the rate.
3.A greater surface area of solid reactants leads to more collisions and a higher rate.
4. Substances that speed up reactions without being consumed in the process.
Catalysts are substances that increase the rate of a chemical reaction by lowering the activation energy required for the reaction to proceed. They help in forming transition states more easily, allowing the reaction to occur faster without being consumed in the process.
Activation energy is the minimum energy required for a chemical reaction to occur. It is the threshold that reactants must overcome to transform into products. The higher the activation energy, the slower the reaction tends to be, as fewer reactant molecules will have enough energy to initiate the reaction.
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