**CBSE Class–12 Subject Chemistry**

**NCERT Solutions**

**Chapter-4 : Chemical Kinetics**

**NCERT Solutions For Class 12 Chemistry Chapter 4 Chemical Kinetics**

**NCERT TEXTBOOK QUESTIONS SOLVED**

**4.1.For the reaction R—>P, the concentration of a reactant changes from 0.03 M to 0.02 M in 25 minutes. Calculate the average rate of reaction using units of time both in minutes and seconds.**

** Ans.**

**4.2.In a reaction, 2A —-> Products, the concentration of A decreases from 0.5 mol L ^{-1} to 0.4 molL^{-1} in 10 minutes. Calculate the rate during this interval?**

**Ans.**

**4.3.Fora reaction,A+B —> Product; the rate law is given by, r =****k [ A] ^{1/2} [B]^{2}**

**. What is the order of the reaction?**

**Ans.**Order of reaction. = 1/2+ 2 = 2

^{1/2}or 2.5

**4.4.The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y ?**

**Ans**.The reaction is : X—>Y

According to rate law,

rate = k[X]^{2}

If [X] is increased to 3 times, then the new rate is

rate’ = k[3X]^{2}

rate’ = 9 k [X]^{2} = 9 rate

Thus, rate of reaction becomes 9 times and hence rate of formation of Y increases 9-times.

**4.5. A first order reaction has a rate constant 1.15 x 10 ^{-3} s^{-1}. How long will 5 g of this reactant take to reduce to 3 g?**

**Ans.**

**4.6.Time required to decompose SO _{2}Cl_{2} to half of its initial amount is 60 minutes. If the decomposition is a first order reaction, calculate the rate constant of the reaction.**

**Ans.**For 1st order reaction,

**4.7. What will be the effect of temperature on rate constant?**

**Ans.**With the rise in temperature by 10°, the rate constant of a reaction is nearly doubled. The dependence of rate constant on temperature is given the Arrhenius equation, k = A e^{-Ea/RT} where A is the Arrhenius constant and Ea is activation energy of the reaction.

**4.8.The rate of the chemical reaction doubles for and increase of 10 K in absolute temperature from 298 K. Calculate E _{a}.**

**Ans.**

**4.9.The activation energy for the reaction, 2 HI(g) —-> H _{2}+I_{2} (g) is 209.5 k J mol^{-1} at 581 K.Calculate the fraction of molecules of reactants having energy equal to or greater than activation energy?**

**Ans**.Fraction of molecules having energy equal to or greater than activation energy is given by:

**NCERT EXERCISES**

**4.1 From the rate expression for the following reactions determine their order of reaction and the dimensions of the rate constants:**

** **

**Sol:**

**4.2 For the reaction, 2A + B ————> A _{2 }B, the rate = k [AJ[B]2 with k = 2.0 x 10^{-6} mol^{-2} L^{2} s^{-1}. Calculate the initial rate of the reaction when [A] = 0.1 mol L^{-1}, [B] = 0.2 mol L^{-1}. Calculate the rate of reaction after [A] is reduced to 0.06 mol L^{-1}.**

**Sol.**Initial rate of reaction = k [A] [B]

^{2}

= (20 x 10

^{-6}mol

^{-2}s

^{-1}) (0.1 mol L

^{-1}) (0.2 mol L

^{-1})

^{2}= 8 x 10-9molL

^{-1}s

^{-1}.

When [A] is reduced from 010 mol L

^{-1}to 0.06 molL

^{-1}, i.e., 0.04 mol L

^{-1}of A has reacted, the concentration of B reacted, is = 1/2 x 0.04 mol L

^{-1}= 0.02 mol L

^{-1}

Concentration of B, remained after reaction with A = 0.2 – 0-02=0.18 mol L

^{-1}

Now, rate=(20 x 10

^{-6}mol

^{-2}L

^{2}s

^{-1}) (0.06 mol L

^{-1}) (0.18 molL

^{-1})

^{2}

= 3-89 x 10

^{-9}mol L

^{-1}s

^{-1}

**4.3 The decomposition of NH _{3} on platinum surface is zero order reaction. What are the rates of production of N_{2} and H_{2} if Ar=2.5 x 10^{-4} mol-1 Ls^{-1}.**

**Sol.**

**4.4 The decomposition of dimethyl ether leads to the formation of CH _{4}, H_{2} and CO and die reaction, rate is given by **

**Rate=k [CH**

_{3}OCH_{3}]^{3/2}The rate of reaction is followed by increase in pressure in a closed vessel, so the rate can also, be expressed in terms of the partial pressure of dimethyl ether, i.e., Rate= k (PCH_{3}OCH_{3})^{3/2}**If the pressure is measured in bar and time in minutes, then what are the units of rate and rate constants?**

**Sol.**As the concentration in the rate law equation is given in terms of pressure,

**4.5 Mention the factors that affect the rate of a chemical reaction.**

**Sol.** Factors affecting the rate 6f chemical reaction are following:

(i) Natpre of reactants

(ii) Concentration of reactants

(iii) Surface area of reactants

(iv) Effect of temperature

(v) Presence of catalyst

(vi) Presence of light

**4.6 A reaction is second order with respect to a reactant How is the rate of reaction affected if the concentration of the reactant is (i) doubled (ii) reduced to** **half?**

**Sol.**

**4.7 What is the effect of temperature on the rate constant of a reaction? How can this effect of temperature on rate constant be represented quantitatively?**

**Sol.** The rate constant of a reaction increases with increase of temperature and becomes nearly double for every 10 °C rise of temperature. Quantitatively, this temperature effect on the rate constant is represented as, k = Ae^{-Ea/RT}

where E_{a } is the activation energy of the reaction, and A represents the frequency factor.

**4.8 In a pseudo first order hydrolysis of ester in water, the following results were obtained:**

**t/s 0 30 60 90**

**[Ester] mol L ^{-1} 0-55 0-31 0 17 0 085**

**(i) Calculate the average rate of reaction between the time interval 30 to 60 seconds.**

**(ii) Calculate the pseudo first order rate constant for the hydrolysis of ester.**

**Sol.**

**4.9 A reaction is first order in A and second order in B.**

**(i) Write the differential rate equation.**

**(ii) How is the rate affected on increasing the concentration of B three times?**

**(iii) How is the rate affected when the concentrations of both A and B is doubled?**

**Sol.**

**4.10 In a reaction between A and B, the initial rate of reaction (r _{0 }) was measured for different initial concentrations of A and B as given below:**

**What is the order of the reaction with respect to A and B?**

**Sol.**

.

**4.11 The following results have been obtained during the kinetic studies of the reaction.**

**2A+B ——–> C + D**

** **

**Determine the rate law and the rate constant for the reaction.**

**Sol.**

**4.12 The reaction between A and B is first order with respect to A and zero order with respect to B. Fill in the blanks in the following table:**

** **

**Sol.**

**4.13 Calculate the half-life of a first order reaction from their rate constants given below:**

**(i) 200 s ^{-1} (ii) 2 min^{-1}**

**(iii) 4 years**

^{-1}**Sol.**

**4.14 The half-life for radioactive decay of 14C is 5730 years. An archaeological artifact containing wood had only 80% of the 14C found in a living tree. Estimate the age of the sample.**

**Sol.** Radioactive decay follows first order kinetics.

**4.15 The experimental data for decomposition of N _{2}O_{5}**

**Sol.**