CBSE Class 12 Chemistry Chemical Kinetics VBQs

Question. For a reaction P + Q → 2 R + S , the incorrect statement is
(a) Rate of disappearance of P = Rate of appearance of S
(b) Rate of disappearance of Q = 2 x Rate of appearance of R
(c) Rate of disappearance of Q = Rate of disappearance of P
(d) Rate of disappearance of Q = ½ x Rate of appearance of R
Answer. B

Question. The reaction NO₂ + CO → NO + CO₂ takes place in two steps. 2NO₂ → NO + NO₃ (k₁) – slow NO₃ + CO → CO₂ + NO₂ (k₂) – fast The rate law is
a) R = k1 [NO₂]³
b) R = k2 [NO₃] [CO]
c) R = k1 [NO₂]²
d) R = k1 [NO₂ ]
Answer. C

Question. In a reaction, 2X → Y, the concentration of X decreases from 0.50 M to 0.38 M in 10 min. What is the rate of reaction in Ms^-1 during this interval ?
(a) 2 × 10^-4
(b) 4 × 10^-2
(c) 2 × 10^-2
(d) 1 × 10^-2
Answer. A

Question. A first order reaction takes 40 min for 30% decomposition. t^1/2 will be
(a) 77.7 min
(b) 52.5 min
(c) 46.2 min
(d) 22.7 min
Answer. A

Question. If the initial concentration of reactant is doubled, t ^1/2 is also doubled, the order of reaction is
a) 1/2
b) 1
c) 2
d) 0
Answer. D

Question. In the reaction 2 A + B → A2B , if the concentration of A is doubled and that of B is halved , the rate of it reaction will
(a) Increase by 4 times
(b) Increase by 2 times
(c) Decrease by 2 times
(d) Remains the same
Answer. B

Question. On increasing the temperature by 10 K , the rate of reaction becomes almost double. The most appropriate reason is
(a) Increase in the number of collisions
(b) Decrease in activation energy
(c) Increase in the energy of collisions
(d) Almost double the number of molecules crossing the energy barrier
Answer. D

Question. A first order reaction has a rate constant 1.15 × 10^-3 s^-1. Time taken for 5 g of this reactant to reduce to 3 g is
(a) 444 s
(b) 400 s
(c) 528 s
(d) 669 s
Answer. C

CASE BASED QUESTIONS

1. Read the passage given below and answer the following questions:
The rate of a reaction, which may also be called its velocity or speed, can be defined with relation to the concentration of any of the reacting substances, or to that of any product of the reaction. If the species chosen is a reactant which has a concentration c at time t the rate is -dc/dt, while the rate with reference to a product having a concentration x at time t is dx/dt. Any concentration units may be used for expressing the rate; thus, if moles per litre are employed for concentration and seconds for the time, the units for the rate are moles litre-1 sec-1. For gas reactions pressure units are sometimes used in place of concentrations, so that legitimate units for the rate would be (mm. Hg) sec-1 and atm. sec-1 The order of a reaction concerns the dependence of the rate upon the concentrations of reacting substances; thus, if the rate is found experimentally to be proportional to the αth power of the concentration of one of the reactants A, to the βth power of the concentration of a second reactant B, and so forth, via., rate = k [A] ^α [B] ^β (1)the over- all order of the reaction is simn = α + β + (2) Such a reaction is said to be of the αth order with respect to the substance A, the β th order with respect to B and so on... (Laidler, K. J., & Glasstone, S. (1948). Rate, order and molecularity in chemical kinetics. Journal of Chemical Education, 25(7), 383.)
In the following questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage.
A. Assertion and reason both are correct statements and reason is correct explanation for assertion.
B. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
C. Assertion is correct statement but reason is wrong statement.
D. Assertion is wrong statement but reason is correct statement.

Question. Assertion: Rate of reaction is a measure of decrease in concentration of reactant with respect to time.
Reason: Rate of reaction is a measure of increase in concentration of product with respect to time.
Answer. B

Question. Assertion: For a reaction: P +2Q → Products, Rate = k [P]^1/2[Q]¹ so the order of reaction is 1.5
Reason: Order of reaction is the sum of stoichiometric coefficients of the reactants.
Answer. C

Question. Assertion: The unit of k is independent of order of reaction.
Reason: The unit of k is moles L^-1 s ^-1 for a zero order reaction
Answer. D

Question. Assertion: Reactions can occur at different speeds.
Reason: Rate of reaction is also called speed of reaction.
Answer. B

Question. Assertion : The rate of a chemical reaction is expressed in terms of Instantaneous rate rather than average rate.
Reason : The concentration of reactants continuously changes with time
Answer. A

II . Observe the table given showing volume of CO₂ obtained by reaction of CaCO₃ and dilute HCl after every minute. Answer the questions that follow:
Table showing volume of CO₂ at one minute interval by reaction of CaCO₃ with dilute HCl.
Time/mm    Volume of CO₂/cm
0                 0
1                24 cm³
2                34 cm³
3                38 cm
4                40 cm
5                40 cm
6                40 cm

Question. What happens to rate of reaction with time?
Answer. Rate first decreases with time and then becomes constant.

Question. Why does CaCO₃ powder react faster than marble chips?
Answer. CaCO₃ has greater surface area.

Question. What happens to rate of reaction if concentrated HCl is used?
Answer. Increases with increase in concentration

Question. In manufacture of NH₃, N2(g) + 3H₂(g)→ 2NH₃+ heat, what is effect of catalyst on the rate of reaction?
Answer. Equilibrium is attained faster.

Question. Why does rate of reaction becomes almost double for energy 10° rise in temperature?
Answer. No. of effective collisions become double

SHORT ANSWER QUESTIONS

Question. Define ‘rate of a reaction’. 
Answer. Rate of a reaction: Either, The change in the concentration of any one of the reactants or products per unit time is called rate of a reaction. Or, The rate of a chemical reaction is the change in the molar concentration of the species taking part in a reaction per unit time.

Question. Define ‘order of a reaction’. 
Answer. The sum of powers of the concentration of the reactants in the rate law expression is called the order of reaction.

Question.Define ‘activation energy’ of a reaction. 
Answer. The minimum extra amount of energy absorbed by the reactant molecules to form the activated complex is called activation energy. The activation energy of the reaction decreases by the use of catalyst.

Question. If the rate constant of a reaction is k = 3 × 10^-4 s^-1, then identify the order of the reaction. 
Answer. S^-1 is the unit for rate constant of first order reaction.

Question. Write the unit of rate constant for a zero order reaction.
Answer. Mol L^-1 S^-1 is unit of rate constant for a zero order reaction.

Question. Define rate of reaction. 
Answer. The change in concentration of reactant or product per unit time is called rate of reaction.

Question. Define rate constant (K).
Answer. Rate constant. It is defined as the rate of reaction when the concentration of reaction is taken as unity.

Question. For a reaction R → P, half-life (t_1/2) is observed to be independent of the initial concentration of reactants. What is the order of reaction? 
Answer. The t_1/2 of a first order reaction is independent of initial concentration of reactants.

Question. For the zero order reaction A → 2B, the rate constant is 2 x 10^-6M min^-1. The reaction is started with 10M of A.
i) What will be the concentration of A after 2 days?
ii) What is the initial half-life of the reaction?
iii) In what time, the reaction will complete?
Answer. i) kt = [R₀] –[R] , [R] = [R₀] – kt
[R] = 10 - 2 x 10^-6 x 2 x 24 x 60 = 9.994M
ii) t_1/2 = [R₀]/2k = 10/ 2 x 2 x 10^-6 = 2.5 x 10⁶ min.
iv) When reaction is complete, [R] = 0
t = [R₀] –[R] / k = [R0]/k
t = 10/ 2 x 10^-6 = 5 x 10⁶ min.

Question. A catalyst is a substance that enhances the rate of a reaction. There are two types of catalysis i.e., homogeneous and heterogeneous. The heterogeneous catalysis is an important application in automobile catalytic converters.
Answer the following questions:
(i) As a student of chemistry, what chemical process do you think occurs in the heterogeneous catalysis in the converters?
(ii) Why the use of such catalytic converters in the vehicles is important?
(iii) What environmental values are associated with the use of such converters?
Answer:
(i) The use of heterogeneous catalysis in automobile exhausts helps to reduce polluting gases, NO and CO which are responsible for photochemical smog.
(ii) The catalytic converter, which is a part of automobile exhaust system, oxidises CO to CO2 and NO to NO2 gas.
(iii) Value: Awareness of environmental pollution caused by highly toxic gases like CO and NO.

Question. On one winter morning Sudha’s mother received a phone call from a close relative stating that three of them would be coming over for lunch. Sudha’s mother noticed that she did not have enough curd to serve the guests. Sudha, a class XII student suggested that her mother could warm the milk from the refrigerator or keep it under direct sunlight and curdle it.
a) Will it be helpful if her mother follows Sudha’s suggestion? How
b) Why is curdling of milk slower during winter?
c) What values are associated with Sudha’s suggestion?
Answer:
(a) Yes, because a rise in temperature (to an optimum value only) increases the rate of curdling.
(b) As per collision theory, a reaction takes place by collision of reacting particles. At lower temp collisions between the particles are less in number 
(c)Ability to take quick decisions at right moment, helping mentality, awareness of scientific principles lying behind domestic activities, presence of mind.

Question. Teacher asked two Students ‘A’ and ‘B’ to demonstrate the reaction between Aluminium and dilute Sulphuric acid to the class. She provided the following chemicals for the experiment: Al foil, Al powder & dilute Sulphuric acid.Student A chose Al foil & dilute Sulphuric acid. Student B chose Al powder & dilute Sulphuric acid.
a) Who according to you has made the correct choice of chemicals?
b) Give a reason to justify the correct choice.
c) What value do you associate with the above? 
Answer:
(a) Student B has made the correct choice.
(b) Al powder has larger surface area than the Al foil and hence it can react faster.
(c) Scientific awareness about factors affecting rate of reactions, ability to apply the gained knowledge in a different situations.

Question. In the lesson ‘Chemical Kinetics’, Robin learnt about need of activation energy and energy barrier for every chemical reaction.
(i) What is the significance of energy barriers in chemical reactions in our life?
(ii) How do these barriers help us to become more efficient in our lives?
(iii) What value is conveyed here in this concept?
Answer:
(i) Everybody on this earth has to face energy barriers in their lives in the form of various difficulties and unfortunate incidents. These energy barriers are a part of everybody’s life.
(ii) These barriers teach us how to tackle them intelligently and wisely. They teach us to be positive in our attitudes even in hardships.
(iii) We should not give up or surrender ourselves in times of difficulties.

Question. Anuj and Sukhbeer visited a car manufacturing company. They saw different sections and found that different parts of cars are made in very large numbers but still the rate of manufacturing of car is very less.
(i) Does this concept help us to understand the concept of order of a reaction?
(ii) What values Anuj and Sukhbeer might have got from this company in respect of people living in our society.
Answer: 
(i) Yes, this helps in understanding that the rate determining factor is the part that is being manufactured at slower rate supporting the fact that slow steps are rate determining steps. 
(ii) Anuj and Sukhbeer might have learnt that weaker sections of society are as important as others and that they too be streamlined at par with others.

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