Chemistry

Chemical Kinetics

Question:

Compounds ‘A' and ‘B' react according to the following chemical equation.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-18
Concentration of either ‘A' or ‘B' were changed keeping the concentrations of one of the reactants constant and rates were measured as a function of initial concentration. The following results were obtained. Choose the correct option for the rate equations for this reaction.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-19

Answer:

(b)
Rate=K[A]x[B]y  
When concentration of B is doubled keeping the concentration of A constant, the rate of formation of C increases by a factor of four. This indicates that the rate of reactions depends upon the square of concentration of B. When concentration of A is doubled, the rate of formation of C also doubles from the initial value. This shows that the rate depends on first power of concentration of A. Hence Rate=  K[A]x[B]y.

previuos
next

Chemical Kinetics

Q 1.

Oxygen is available in plenty in air yet fuels do not bum by themselves at room temperature. Explain.

Q 2.

Compounds ‘A' and ‘B' react according to the following chemical equation.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-18
Concentration of either ‘A' or ‘B' were changed keeping the concentrations of one of the reactants constant and rates were measured as a function of initial concentration. The following results were obtained. Choose the correct option for the rate equations for this reaction.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-19

Q 3.

For a certain reaction, large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow. Why?

Q 4.

The role of a catalyst is to change
(a) Gibbs energy of reaction (b) enthapy of reaction
(c) activation energy of reaction (d) equilibrium constant

Q 5.

How can you determine the rate law of the following reaction?
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-34

Q 6.

Why can we not determine the order of reaction by taking into consideration the balanced chemical equation?

Q 7.

Match the items of Column I and Column II.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-45

Q 8.

Which of the following expressions is correct for the rate of reaction given below?
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-13

Q 9.

Match the items of Column I and Column II.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-46

Q 10.

In the presence of a catalyst, heat evolved or absorbed during reaction
(a) increases (b) decreases
(c) remains unchanged .(d) may increase or decrease

Q 11.

Explain the difference between instantaneous rate of a reaction and average rate of a reaction.

Q 12.

According to Maxwell Boltzmann distribution of energy
(a) the fraction of molecules with most probable kinetic energy decreases at higher temperatures
(b) the fraction of molecules with most probable kinetic energy increases at higher temperatures
(c) most probable kinetic energy increases at higher temperatures
(d) most probable kinetic energy decreases at higher temperatures

Q 13.

Why cannot molecularity be applicable only for elementary reactions and order applicable for elementary as well as complex reaction?

Q 14.

Write the rate equation for the reaction 2A + B–>C if the order of the
reaction is zero.

Q 15.

What happens to most probable kinetic energy and the energy of activation with increase in temperature?

Q 16.

Rate law for the reaction ‘B' is doubled, keeping the concentration of ‘A'
constant, the value of rate constant will be
(a) the same (b) doubled (c) quadrupled (d) halved

Q 17.

Consider the graph given in Q. 8. Which of the following option does not show instantaneous rate of reaction at 40th second?
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-111

Q 18.

Which of the following statement is not correct about order of a reaction?
(a) The order of a reaction can be a fractional number.
(b) Order of a reaction is experimentally determined quantity.
(c) The order of a reaction is always equal to the sum of the stoichiometric coefficients of reactants in the balanced chemical equation for a reaction.
(d) The order of a reaction is the sum of the powers of molar concentration of the reactants in the rate law expression.

Q 19.

Describe how does the enthalpy of reaction remain unchanged when a catalyst is used in the reaction?

Q 20.

A graph of volume of hydrogen released vs time for the reaction between zinc and dil. HCl is shown below. On the basis of this, mark the correct option.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-9

Q 21.

State a condition under which a biomolecular reaction is kinetically first order reaction.

Q 22.

Which of the following statements are applicable to a balanced chemical equation of an elementary reaction?
(a) Order is same as molecularity
(b) Order is less than the molecularity
(c) Order is greater than the molecularity
(d) Molecularity can never be zero

Q 23.

Derive an expression to calculate time required for completion of zero order reaction. ‘

Q 24.

Which of the following statement is incorrect about the collision theory of chemical reaction?
(a) It considers reacting molecules or atoms to be hard spheres and ignores their structural features.
(b) Number of effective collisions determines the rate of a reaction.
(c) Collision of atoms or molecules possessing sufficient threshold energy results into the product formation.
(d) Molecules should collide with sufficient threshold energy and proper orientation for the collision to be effective.

Q 25.

The reaction between H2(g) and O2(g) is highly feasible yet allowing the gases to stand at room temperature in the same vessel does not lead to the formation of water. Explain.

Q 26.

The value of rate constant of a pseudo first order reaction
(a) depends on the concentration of reactants present in small amount
(b) depends on the concentration of reactants present in excess
(c) is independent of the concentration of reactants
(d) depends only on temperature

Q 27.

In the following questions, a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
(a) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(b) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(c) Assertion is correct but Reason is incorrect.
(d) Both Assertion and Reason are incorrect.
(e) Assertion is incorrect but Reason is correct.

The enthapy of reaction remains constant in the presence of a catalyst.
Reason (R): A catalyst participating in the reaction, forms different activated complex and lowers down the activation energy but the difference in energy of reactant and product remains the same.

Q 28.

Consider the reaction A —> B. The concentration of both the reactants and the products varies exponentially with time. Which of the following figures correctly describes the change in concentration of reactants and products with time?
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-21

Q 29.

Which of the following graphs are correct for a first order reaction?
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-31
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-32

Q 30.

For a zero order reaction will the molecularity be equal to zero? Explain.

Q 31.

Why, in the redox titration of KMnO4versus oxalic acid we heat oxalic acid solution before starting the titration.

Q 32.

At high pressure the following reaction is zero order.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-24
Which of the following options are correct for this reaction?
(a) Rate of reaction = Rate constant
(b) Rate of the reaction depends on concentration of ammonia.
(c) Rate of decomposition of ammonia will remain constant until ammonia disappears completely
(d) Further increase in pressure will change the rate of reaction.

Q 33.

In any uni molecular reaction,
(a) only one reacting species is involved in the rate determining step
(b) the order and the molecularity of slowest step are equal to one
(c) the molecularity of the reaction is one and order is zero
(d) both molecularity and order of the reaction

Q 34.

For a reaction A + B —> Products, the rate law is — Rate = k[A][B]3/2. Can – reaction be an elementary reaction? Explain.

Q 35.

Why does the rate of any reaction generally decreases during the course of the reaction?

Q 36.

Consider the following figure and mark the correct option.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-2
(a) Activation energy of forward reaction is E1 + E2   and product is less stable than reactant.
(b) Activation energy of forward reaction is E1 + E2 and product is more stable than reactant.
(c) Activation energy of both forward and backward reactions is E1 + E2 and reactant is more stable than product.
(d) Activation energy of backward reaction is E, and product is more stable than reactant.

Q 37.

During decomposition of an activated complex
(a) energy is always released (b) energy is always absorbed
(c) energy does not change (d) reactions may be formed

Q 38.

In the following questions, a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
(a) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(b) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(c) Assertion is correct but Reason is incorrect.
(d) Both Assertion and Reason are incorrect.
(e) Assertion is incorrect but Reason is correct.

(A): Rate constants determined from Arrhenius equation are fairly accurate for simple as well as complex molecules.
Reason (R): Reactant molecules undergo chemical change irrespective of their orientation during collision.

Q 39.

Rate law cannot be determined from balanced chemical equation if
(a) reverse reaction is involved
(b) it is an elementary reaction
(c) it is a sequence of elementary reactions
(d) any of the reactants is in excess

Q 40.

In the following questions, a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
(a) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(b) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(c) Assertion is correct but Reason is incorrect.
(d) Both Assertion and Reason are incorrect.
(e) Assertion is incorrect but Reason is correct.

Assertion (A): Order of the reaction can be zero or fractional.
Reason (R): We cannot determine order from balanced chemical equation.

Q 41.

Why does the rate of a reaction increase with rise in temperature?

Q 42.

For a general reaction A –> B, plot of concentration of A versus time is
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-38
given in figure. Answer the following questions on the basis of this graph.
(i) What is the order of the reaction?
(ii) What is the slope of the curve?
(iii) What are the units of rate constant?

Q 43.

Which of the following graph represents exothermic reaction?
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-16

Q 44.

Why is molecularity applicable only for elementary reactions and order applicable for elementary as well as complex reaction?

Q 45.

Thermodynamic feasibility of the reaction alone cannot decide the rate of the reaction. Explain with the help of one example.

Q 46.

A first order reaction is 50% completed in 1.26×1014 s. How much time would it take for 100% completion?
(a) 1 26 x 1015 s (b) 2.52×1014 s  
(c) 2.52 x 1028 s (d) infinite

Q 47.

Match the statements given in Column I and Column II.
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-44

Q 48.

With the help of an example explain what is meant by pseudo first order reaction?

Q 49.

According to Arrhenius equation rate constant k is equal to  Ae-Ea/RT. Which of the following options represents the graph of In k versus ?
ncert-exemplar-problems-class-12-chemistry-chemical-kinetics-6

Q 50.

Which of the following statement is not correct for the catalyst?
(a) It catalyses the forward and backward reactions to the same extent.
(b) It alters AG of the reaction.
(c) It is a substance that does not change the equilibrium constant of a reaction.
(d) It provides an alternate mechanism by reducing activation energy between reactants and products.