Please refer to the below Chemical Kinetics important questions for Class 12 Chemistry. These questions and answers have been prepared as per the latest NCERT Book for Class 12 Chemistry. Students should go through chapter wise Class 12 Chemistry Important Questions designed as per the latest examination pattern issued by CBSE.
Very Short Answer Questions :
Question. With the help of diagram, explain the physical significance of energy of activation (Ea) in chemical reactions.
Answer : The energy required to form activated complex is called activation energy. It is the difference between the threshold energy and the average energy possessed by the reacting molecules.
Activation energy (Ea) = Threshold energy – Average energy possessed by reacting molecules.
Question. Express clearly what do you understand by ‘rate expression’ and ‘rate constant’ of a reaction
Answer : Rate expression is a way of expressing rate of reaction in terms of concentration of reactants, e.g., for a general reaction, aA + bB → cC + dD
Rate = k[A]x [B]y
Rate constant (k) is equal to the rate of reaction when molar concentration of reactant is unity. Its units depends upon the order of reaction.
Question. Define ‘order of a reaction’.
Answer : It is defined as “the sum of the powers or exponents to which the concentration terms are raised in the rate law expression.”
If rate = k[A]m [B]n, then order = m + n.
Question. Express the rate of the following reaction in terms of disappearance of hydrogen in the reaction :
3H2(g) + N2(g) → 2NH3(g)
Answer : Rate = −1/3 d[H2]/dt
Question. Define the half-life period of reaction (t½).
Answer : The time taken for half of the reaction to complete, i.e., the time in which the concentration of a reactant is reduced to half of its original value Is called half-life period of the reaction.
t = t R1/2 when [R] = [R0]/2
Question. For the hydrolysis of methyl acetate in aqueous solution, the following results were obtained :
(i) Show that it follows pseudo first order reaction, as the concentration of water remains constant.
(ii) Calculate the average rate of reaction between the time interval 30 to 60 seconds.
(Given : log 2 = 0.3010, log 4 = 0.6021)
Answer : (i) For a first order reaction,
Question. Th decomposition of NH3 on platinum surface :
2NH3(g) →pt N2(g) + 3H2(g)
is a zero order reaction with k = 2.5 × 10–4 mol L–1 s–1 What are the rates of production of N2 and H2?
Answer : The reaction is
Question. With the help of a labelled diagram explain the role of activated complex in a reaction.
Answer : When the colliding molecules possess the kinetic energy equal to Ea, the atomic configuration of species formed at this stage is different from the reactant as well as the products. This stage is called the activated state and the specific configuration bearing state is called activated complex.
For example, in the reaction between H2(g) and I2(g), activated complex has configuration in which H—H and I—I bonds are breaking and H—I bonds, are forming as shown below.
Question. For the reaction, Cl2(g) + 2NO(g) → 2NOCl(g) the rate law is expressed as
Rate = k[Cl2] [NO]2
What is the overall order of this reaction?
Answer : Rate = k[Cl2] [NO]2
Hence order of the reaction = 1 + 2 = 3
Question. If half-life period of a first order reaction is x and 3/4th life period of the same reaction is y, how are x and y related to each other?
Answer :
Question. Define the following :
Energy of activation of reaction.
Answer : The minimum extra energy which must be supplied to the reactants to enable them to cross over the potential energy barrier between reactants and products is called activation energy. It is denoted by Ea.
Ea = ET – ER
Question. Define the following :
Energy of activation of reaction.
Answer : The minimum extra energy which must be supplied to the reactants to enable them to cross over the potential energy barrier between reactants and products is called activation energy. It is denoted by Ea.
Ea = ET – ER
Question. In some cases it is found that a large number of colliding molecules have energy more than threshold energy, yet the reaction is slow, Why?
Answer : It is due to improper orientation. Energy more than threshold energy and proper orientation are the two main factors which are responsible for a reaction to occur.
Short Answer Questions :
Question. Th following data were obtained during the first order thermal decomposition of SO2Cl2 at a constant volume :
SO2Cl2(g) → SO2(g) + Cl2(g)
Calculate the rate constant.
(Given : log 4 = 0.6021, log 2 = 0.3010)
Answer : The given reaction is
Question. Th half-life for a first order reaction is 5 × 104 s. What percentage of the initial reactant will react in 20 hours ?
Answer :
Question. A first order decomposition reaction takes 40 minutes for 30% decomposition. Calculate its t1/2 value.
Answer : Here t = 40 min, t1/2 = ?
Question. Rate constant ‘k’ of a reaction varies with temperature ‘T’ according to the equation :
log k = log A − Ea/2.303 {1/T}
where Ea is the activation energy. When a graph is plotted for log k vs 1/T, a straight line with a slope of – 4250 K is obtained. Calculate ‘Ea’ for the reaction.
(R = 8.314 J K–1 mol–1).
Answer :
Question. Th first order reaction take 100 minutes for completion of 60% of the reaction. Find the time when 90% of the reaction will be completed.
Answer : For the first order reaction,
Question. For the first order thermal decomposition reaction, the following data were obtained :
C2H5Cl(g) → C2H4(g) + HCl(g)
(Given : log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021)
Answer : The given reaction is
Question. A first order reaction is 15% completed in 20 minutes. How long will it take to complete 60% of the reaction?
Answer :
Question. A reactant has a half-life of 10 minutes.
(i) Calculate the rate constant for the first order reaction.
(ii) What fraction of the reactant will be left after an hour of the reaction has occurred?
Answer :
Question. A first order reaction takes 30 minutes for 50% completion. Calculate the time required for 90% completion of this reaction. (log 2 = 0.3010)
Answer : t50% or t1/2 = 30 minutes
Question. Distinguish between order and molecularity of a reaction. When could order and molecularity of a reaction be the same?
Answer : Distinction between order and molecularity of a reaction :
Generally, in a complex reaction the order of reaction is equal to the molecularity of the slowest step.
Question. A reaction is second order in A and first order in B.
(i) Write the differential rate equation.
(ii) How is the rate affected on increasing the concentration of A three times?
(iii) How is the rate affected when the concentration of both A and B are doubled?
Answer : Since the reaction is second order in A and rst order in B
(i) Differential rate equation is
Rate = k[A]2[B]
(ii) The new concentration of A = [3A]
∴ New rate = k[3A]2[B] = 9k[A]2[B]
∴ New rate = 9 times of the original rate
(iii) New concentration of A = [2A]
New concentration of B = [2B]
∴ New rate = k[2A]2[2B] = 8k[A]2[B]
∴ New rate = 8 times of the original rate.
Question. Nitrogen pentoxide decomposes according to equation :
2N2O5(g) → 4NO2(g) + O2(g)
(i) Calculate the rate constant. Include units with answer.
(ii) What will be the concentration of N2O5 after 100 minutes?
(iii) Calculate the initial rate of reaction.
Answer :
Question. A first order reaction has a rate constant value of 0.00510 min–1. If we begin with 0.10 M concentration of the reactant, how much of the reactant will remain after 3.0 hours?
Answer :
Question. For a chemical reaction R P, the variation in the concentration, ln [R] vs. time (s) plot is given as
(i) Predict the order of the reaction.
(ii) What is the slope of the curve?
(iii) Write the unit of the rate constant for this reaction.
Answer : (i) The reaction is of 1st order.
(ii) For first order reaction
ln[R] = –kt + ln [R]0
comparing eqn. y = m × x + c
we get a straight line with slope = –k and intercept equal to ln[R]0.
(iii) Unit of rate constant for first order reaction
mol L−1/s X 1/(molL−1)1 = s−1
Question. Th following results have been obtained during the kinetic studies of the reaction :
2A + B → C + D
Calculate the rate of formation of D when [A] = 0.5 mol L–1 and [B] = 0.2 mol L–1.
Answer : Suppose order with respect to A is m and with respect to B is n. Then the rate law will be
Question. Explain the difference between the average rate and instantaneous rate of chemical reaction.
Answer : Change in concentration i.e., either (decrease in concentration of reactant or increase in concentration of product) per unit time is called rate of reaction.
Rate of reaction = C2 − C1/t2 − t1 = ΔC/Δt
The ratio of change of concentration of reactants to the time consumed in that change is called average rate of reaction.
rav = Δx/Δt = − C2 − C1/t2 − t1
The rate of reaction at a particular instant (time) is called instantaneous rate of reaction.
rins dt = dx/dt
dx = small change in concentration
dt = small time interval
Question. Th rate constant for a first order reaction is 60 s–1. How much time will it take to reduce the initial concentration of the reactant to its 1/10th value?
Answer :
Question. (i) A reaction is first order in A and second order in B.
(a) Write differential rate equation.
(b) How is rate affected when concentration of B is tripled?
(c) How is rate affected when concentration of both A and B is doubled?
(ii) What is molecularity of a reaction?
Answer : (i) (a) Differential rate equation of reaction is
dx/dt = k[A]1[B]2 = k[A][B]2
When conc. of B is tripled, it means conc. of B becomes [3 × B]
∴ New rate of reaction, dx′/dt = k[A][3B]2
= 9k[A][B]2 = 9 (dx/dt)
i.e., the rate of reaction will become 9 times.
(i) When conc. of A is doubled and that of B is also doubled, then conc. of A becomes [2A] and that of B becomes [2B].
∴ Now rate of reaction, dx′/dt = k[2A][2B]2
= 8k [A][B]2
i.e., the rate of reaction will become 8 times.
(ii) Molecularity of a reaction is the number of reacting particles which collide simultaneously to bring about the chemical change. It is a theoretical concept.
