NCERT Solutions Class 12 Chemistry Chapter 4
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NCERT Solutions for Class 12 Chemistry Chapter 2: Chemical kinetics
Chemical Kinetics is a branch of chemistry that deals with chemical reaction, their factors, and mechanism. It is closely related to the chemical reaction and physical process. The principles of chemical kinetics apply to purely physical processes as well as to chemical reactions.
In 1864, Peter Waage and Cato Guldberg pioneered the development of chemical kinetics by formulating the law of mass action, which states that the speed of a chemical reaction is proportional to the quantity of the reacting substance.
The chapter also mentions the factors affecting the rate of a reaction, the integrated rate equation, Pseudo First Order reactions, and the collision theory of chemical kinetics.
NCERT Solutions Class 12 Chemistry Chapter 2 PDF
NCERT Solutions Chapter 4 Chemistry Class 12 plays a vital role in the fundamental development of the concepts of physical chemistry. Without appropriate guidance, it will create a problem for students as their knowledge of concepts can become vague. It will affect your marks in board exams. That’s why a student should refer to the Chemical kinetics Class 12 NCERT solutions.
Physical Chemistry which is a crucial part of the subject requires thorough reading and understanding of the concepts. Therefore, focusing on qualitative ways of preparation rather than quantitative measures would turn out to be fruitful for an aspiring board examinee.
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NCERT solutions for Class 12 Chemistry Chapter 4: Chemical kinetics Key Features
- The NCERT Solutions provide clear and precise answer.
- The columns are used wherever necessary.
- We follow the NCERT guidelines.
- These solutions help you to solve and revise the complete syllabus.
- Answers are explained with diagrams and illustration.
NCERT Solutions for Chemistry Class 12 Chapter 4: Chemical Kinetics Important Questions
Question:1 From the rate expression from the following reaction, determine their order of reaction and the dimension of the rate constants.
- 2NO(g) à N2O(g) Rate = k[NO]2
- H2O2(aq) + 3I–(aq) +2H+ à 2H2O(l) +I3– Rate = k[H2O2][I–]
- CH3CHO(g) à CH4(g) +CO(g) Rate = k[CH3CHO]3/2
- C2H5Cl(g) à C2H4(g) + HCl(g) Rate = k[C2H5C l]
- Given rate = k[NO]2
Therefore, order of the reaction = 2
K = Rate / [NO]2
Dimension of k = molL-1s-1 / (molL-1)2
= molL-1s-1 / mol2L-2
= L mol-1s-1
- Given rate k = [H2O2][I-1]
Therefore order of the reaction = 2
K = Rate / [H2O2][I-1]
Dimension of = mol-1s-1 / (mol/L)(mol/L)
- Given rate = k[CH3CHO]3/2
Therefore, order of reaction = 3/2
K = Rate/[CH3CH0]3/2
Dimension of = mol-1s-1/mol/L)3/2
- Given rate = k[C2H5Cl]
Therefore, order of the reaction = 1k = Rate / [C2H5Cl]
Dimension of = molL-1s-1 / molL-1
Question:2 For the reaction:
2A + B à A2B
The rate = k[A][B]2 with k = 2.0 x 10-6 mol-2 L2 / s. Calculate the initial rate of the reaction when [A] = 0.1 mol / L, [B] = 0.2 mol / L. Calculate the rate of reaction after [A] is reduced to 0.06 mol / L.
The initial rate of the reactionis
Rate = k [A][B]2
= (2.0 × 10-6 mol-2 L2 s-1) (0.1 mol L-1) (0.2 mol L-1)2
= 8.0 × 10-9 mol2 L2 s-1
When [A] is reduced from 0.1 mol L-1 to 0.06 mol-1, the concentration of A reacted = (0.1 – 0.06) mol-1 L-1 = 0.04 mol L
Therefore, concentration of B reacted= 1/2 x 0.04 mol L-1 = 0.02 mol L-1
Then, concentration of B available, [B] = (0.2 – 0.02) mol L-1
= 0.18 mol L-1
After [A] is reduced to 0.06 mol L-1, the rate of the reaction is given by,
Rate = k [A][B]2
= (2.0 × 10-6 mol-2 L2 s-1) (0.06 mol L-1) (0.18 mol L-1)2
= 3.89 mol L-1 s-1
Question:3 The decomposition of NH3 on platinum surface is zero order reaction. What are rates of production of N2 and H2 if k = 2.5 x 10-4 mol-1 L / s/
The decomposition of NH3 is
2NH3 → N2 + 3H2
Rate of reaction,
dx / dt = 1 / 2d[NH3] / dt = d[N2] / dt = 1 / 3d[H2] / dt = k
Where k is the rate constant. Since, reaction is of zero order,
Rate of reaction = dx/dt = d[N2]dt = k
= 2.5 x 10-4 M s-1
But, d[N2] / dt = 1/3d[H2] / dt
: d[H2] / dt = 3 d[N2] / dt
= 3 x 2.5 x 10-4 mol-1L s-1
= 7.5 x 10-4 M s-1
Question:4 The decomposition of dimethyl ether leads to the formation of CH4, H2 and CO and the reaction rate is given by
Rate = k[CH3OCH3]3/2
If the pressure is measured in bar and time in minutes, then what are the units of rate and rate constants?
If the pressure is measured in bar and time in minutes, then unit of rate = bar / min
Rate = k(p CH3OCH3)
k = Rate / (p CH3OCH3)3/2
Therefore, unit of rate constants (k) = bar min-1 / bar3/2
= bar-1/2 / min.
Question:5 Mention the factors that affect the rate of a chemical reaction.
On increasing concentration of reactants, the probability of colloisions of molecules increases hence rate of reaction increases.
On increasing temperature, the kinetic energy of molecules increases, hence, the number of collision increases. Therefore, the rate of reaction also increases.
On increasing pressure, the molecules of gases come closer to each other. As a result their collisions increase and hence rate of reaction increases.
- Surface Area of Reactants:
On increasing surface area of reactants, the rate of reaction increases. For example, the powdered metals react faster than the metals in a lump.
- Nature of reactants:
If the reactants are ionic in nature than the rate of reaction is faster than those in which reactants are molecular in nature.
Question: 6 A reaction in second order with respect to a reactant. How is the rate of reaction affected if the concentration of the reactant is
- Reduced to half?
Let the concentration of the reactant be [A] = a
Rate of reaction, R = k[A]2
- If the concentration of the reaction is double i.e., [A] = 2a then the rate of the reaction would be R = k(2a)2 = 4ka2 = 4R
Therefore, the rate of the reaction would increased by 4 times.
- If the concentration of the reactants is reduced to half i.e., [A] = ½ a the rate of the reaction would be
R = k(1/2a)2
Therefore, the rate of the reaction would be reduced to 1/4th .
Question: 7 What is the effect of temperature on the rate constant of a reaction? How can this temperature effect on the rate constant be represented quantitatively?
The rate constant is nearly doubled with a rise in temperature by 10° for a chemical reaction.
The temperature effect on the rate constant can be represented quantitatively by Arrhenius equation,
K = Ae-Ea / RT
where, k is the rate constant,
A is the Arrhenius factor or the frequency factor,
R is the gas constant,
T is the temperature, and
Ea is the energy of activation for the reaction
Question:8 In the pseudo first order hydrolysis of ester in water, the following results were obtained:
- Calculate the rate of reaction between the time interval 30 to 60 second.
- Calculate the pseudo first order rate constant for the hydrolysis of ester.
- Average rate of reaction between the time interval 30 to 60 seconds = d[Ester] / dt
= 0.31 – 0.17 / 60 – 30
= 0.14 / 30
- For a pseudo first order reaction,
K = 2.303 / t log [R]0 / [R]
For t = 30s, k1 = 2.303 / 30 log 0.55/0.31
= 1.91 x 10-1 / s
For t = 60, k2 = 2.303 / 60 log 0.55 / 0.17
= 1.96 x 10-2 / s
For t = 90, k3 = 2.303 / 90 log 0.55 / 0.085
= 2.07 x 10-2 / s
Then average rate constant, k = k1 = k2 + k3 / 3
1.98 x 10-2 / s
Question: 9 A reaction in first order in A and second order in B.
- Write the different rate equation.
- How is the rate affected on increasing the concentration of B three times?
- How is the rate affected when the concentration of both A and B are doubled?
- The different rate equation will be = d[r] / dt = k[A][B]2
- If the concentration of B is increased three times, then –d[R] / dt = k[A][3B]2 = 9.k[A][B]2
Therefore, the rate of reaction will becomes 9 times.
- When the concentration of both A and B are doubled, -d[R] / dt = k[A][B]2
Therefore, the rate of reaction will increase 8 times.
Question:10 In a reaction between A and B, the initial rate of reaction was measured for different initial concentration of A and B as given below:
|A/ mol / L||0.20||0.20||0.40|
|B/ mol /L||0.30||0.10||0.05|
|r0/ mol /L / s||5.07 x 10-5||5.07 x 10-5||1.43 x 10-4|
What is the order of the reaction with respect to A and B?
Let the order of the reaction with respect to A be x and with respect to B be y
ra = k[A]x[B]y
- x 10-5 = k[0.20]x[0.30]y….i
5.07 x 10-5 = k[0.20]x[0.10]y…..ii
1.43 x 10-5 = k[0.40]x[0.05]y…..iii
Dividing equation I by ii, we obtain
5.07 x 10-5 / 5.07 x 10-5 = k[0.20]x[0.30]y / k[0.20]x[0.10]y
= [0.30]y / [0.10]y
= (0.30/0.10)x = (0.30 / 0.10)y
Dividing equation iii. By ii, we obtain
1.43 x 10-5 / 5.07 x 10-5 = k[0.40]x[0.05]y / k[0.20]x[0.10]y
= 2.821 = 2x
Log2.821 = x log2[taking log on both sides]
X = log2.821 / log 2
= 1.5 (approx.)
Hence, the order of the reaction with respect of A is 1.5 and with respect to B is zero.
NCERT Solutions for Class 12 Chemistry Chapter 4: FAQs
- How to score good marks in Chemistry Chapter 4?
Ans. A student must prepare a study schedule and allot time for each chapter. Students have to solve various chemical equations to sharpen their problem – solving skill. They need to understand the theory thoroughly with complete information about the topic and must know where to apply the formulas. After solving the previous year question papers, they can prepare better for exams.
Students can also prepare for the board exam by sitting for an online exam on different websites. These portals provide affordable facilities like test evaluation, suggestion and marks fetching solutions.
- What are the sub topics in Chemical kinetic?
Ans. The Chemical kinetic chapter of class 12 is vital as well as the scoring chapter. It contains a wide range of sub topics which give detailed knowledge on the nature and mechanism of this branch of physical chemistry. The sub-topics include –
- Rate of chemical reaction.
- Dependence on the rate of concentration.
- Integrated rate equations.
- Molecularity and mechanism.
- Temperature dependence.
- Effect of catalyst.
- Collision theory of chemical reaction.
- Important formulas.
Apart from these topics, formulas like rate law, integrated rate law, units of the rate constant, linear plot to determine constant and half – line for zero, first, second and all order is also given.
- Explain the concept of Chemical Kinetics covered in the Chapter 4 of NCERT Solutions for class 12 Chemistry.
Ans. Chemical kinetics is a branch of chemistry which deals with the rate of chemical reaction, the mechanism of the reaction and the factors affecting it. According to the rate of reaction they are of three types – Slow reaction, Instantaneous reaction and moderately slow reaction. The reaction which completes in less than 1ps time is called a fast reaction. The reaction which occurs between fast and slow chemical reactions are called moderately slow reactions.