Chapter 4 – Chemical Kinetics

Class 12 Chemistry · 65 questions · 65 with answers

Questions

Q1Multiple choice

The role of a catalyst is to change ______________.

(i)gibbs energy of reaction.
(ii)enthalpy of reaction.
(iii)activation energy of reaction.
(iv)equilibrium constant.

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(iii) activation energy of reaction.

Q2Multiple choice

In the presence of a catalyst, the heat evolved or absorbed during the reaction ___________.

(i)increases.
(ii)decreases.
(iii)remains unchanged.
(iv)may increase or decrease.

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(iii) remains unchanged.

Q3Multiple choice

Activation energy of a chemical reaction can be determined by _____________.

(i)determining the rate constant at standard temperature.
(ii)determining the rate constants at two temperatures.
(iii)determining probability of collision.
(iv)using catalyst.

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(ii) determining the rate constants at two temperatures.

Q4Multiple choice

Consider Fig. 4.1 and mark the correct option.

This question refers to a figure in the original PDF.

(i)Activation energy of forward reaction is E1 + E2 and product is less stable than reactant.
(ii)Activation energy of forward reaction is Fig. 4.1 E1+E2 and product is more stable than reactant.
(iii)Activation energy of both forward and backward reaction is E1+E2 and reactant is more stable than product.
(iv)Activation energy of backward reaction is E1 and product is more stable than reactant.

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(i) Activation energy of forward reaction is E1 + E2 and product is less stable than reactant.

Q5Multiple choice

Consider a first order gas phase decomposition reaction given below : A(g) → B(g) + C(g) The initial pressure of the system before decomposition of A was pi. After lapse of time ‘t’, total pressure of the system increased by x units and became ‘pt ’ The rate constant k for the reaction is given as _________. 2.303 pi

(i)k= log t pi − x 2.303 pi
(ii)k= log t 2 pi − pt 2.303 pi
(iii)k= log t 2 pi + pt 2.303 pi
(iv)k= log t pi + x

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(ii) k= log t 2 pi − pt 2.303 pi

Q6Multiple choice

According to Arrhenius equation rate constant k is equal to A e –Ea / RT . Which of the following options represents the graph of ln k vs ?

(i)
(ii)
(iii)
(iv)

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(i)

Q7Multiple choice

Consider the Arrhenius equation given below and mark the correct option. k = A e –Ea / RT

(i)Rate constant increases exponentially with increasing activation energy and decreasing temperature.
(ii)Rate constant decreases exponentially with increasing activation energy and decreasing temperature.
(iii)Rate constant increases exponentially with decreasing activation energy and decreasing temperature.
(iv)Rate constant increases exponentially with decreasing activation energy and increasing temperature.

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(iv) Rate constant increases exponentially with decreasing activation energy and increasing temperature.

Q8Multiple choice

A graph of volume of hydrogen released vs time for the reaction between zinc and dil.HCl is given in Fig. 4.2. On the basis of this mark the correct option. V3 − V2

This question refers to a figure in the original PDF.

(i)Average rate upto 40s is V3 − V2
(ii)Average rate upto 40 seconds is 40 − 30 V3
(iii)Average rate upto 40 seconds is V3 − V1
(iv)Average rate upto 40 seconds is 40 − 20 Fig. 4.2

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(iii) Average rate upto 40 seconds is V3 − V1

Q9Multiple choice

Which of the following statements is not correct about order of a reaction.

(i)The order of a reaction can be a fractional number.
(ii)Order of a reaction is experimentally determined quantity.
(iii)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.
(iv)The order of a reaction is the sum of the powers of molar concentration of the reactants in the rate law expression.

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(iii) 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.

Q10Multiple choice

Consider the graph given in Fig. 4.2. Which of the following options does not show instantaneous rate of reaction at 40th second? V5 − V2

This question refers to a figure in the original PDF.

(i)50 − 30 V 4 − V2
(ii)50 − 30 V3 − V2
(iii)40 − 30 V3 − V1
(iv)40 − 20 49 Chemical Kinetics

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(ii) 50 − 30 V3 − V2

Q11Multiple choice

Which of the following statements is correct?

(i)The rate of a reaction decreases with passage of time as the concentration of reactants dereases.
(ii)The rate of a reaction is same at any time during the reaction.
(iii)The rate of a reaction is independent of temperature change.
(iv)The rate of a reaction decreases with increase in concentration of reactant(s).

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(i) The rate of a reaction decreases with passage of time as the concentration of reactants dereases.

Q12Multiple choice

Which of the following expressions is correct for the rate of reaction given below? – – + 5Br (aq) + BrO3 (aq) + 6H (aq) → 3Br2(aq) + 3H2O(l) − + ∆[Br ] ∆[H ]

(i)=5 ∆t ∆t − + ∆[Br ] 6 ∆[H ]
(ii)= ∆t 5 ∆t − + ∆[Br ] 5 ∆[H ]
(iii)= ∆t 6 ∆t − + ∆[Br ] ∆[H ]
(iv)=6 ∆t ∆t

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(iii) = ∆t 6 ∆t − + ∆[Br ] ∆[H ]

Q13Multiple choice

Which of the following graphs represents exothermic reaction? (a) (b) (c)

(i)(a) only
(ii)(b) only
(iii)(c) only
(iv)(a) and (b)

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(i) (a) only

Q14Multiple choice

Rate law for the reaction A + 2B → C is found to be Rate = k [A][B] Concentration of reactant ‘B’ is doubled, keeping the concentration of ‘A’ constant, the value of rate constant will be______.

(i)the same
(ii)doubled
(iii)quadrupled
(iv)halved

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(ii) doubled

Q15Multiple choice

Which of the following statements is incorrect about the collison theory of chemical reaction?

(i)It considers reacting molecules or atoms to be hard spheres and ignores their structural features.
(ii)Number of effective collisions determines the rate of reaction.
(iii)Collision of atoms or molecules possessing sufficient threshold energy results into the product formation.
(iv)Molecules should collide with sufficient threshold energy and proper orientation for the collision to be effective.

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(iii) Collision of atoms or molecules possessing sufficient threshold energy results into the product formation.

Q16Multiple choice

A first order reaction is 50% completed in 1.26 × 1014 s. How much time would it take for 100% completion?

(i)1.26 × 1015 s
(ii)2.52 × 1014 s
(iii)2.52 × 1028 s
(iv)infinite

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(iv) infinite

Q17Multiple choice

Compounds ‘A’ and ‘B’ react according to the following chemical equation. A (g) + 2 B (g) → 2C (g) 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. Following results were obtained. Choose the correct option for the rate equations for this reaction. Experiment Initial Initial Initial rate of concentration concentration formation of –1 –1 –1 –1 of [A]/mol L of [B]/mol L [C]/mol L s 1. 0.30 0.30 0.10 2. 0.30 0.60 0.40 3. 0.60 0.30 0.20

(i)Rate = k [A]2 [B]
(ii)Rate = k [A] [B]2
(iii)Rate = k [A] [B]
(iv)Rate = k [A]2 [B]0 51 Chemical Kinetics

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(ii) Rate = k [A] [B]2

Q18Multiple choice

Which of the following statement is not correct for the catalyst?

(i)It catalyses the forward and backward reaction to the same extent.
(ii)It alters ∆G of the reaction.
(iii)It is a substance that does not change the equilibrium constant of a reaction.
(iv)It provides an alternate mechanism by reducing activation energy between reactants and products.

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(ii) It alters ∆G of the reaction.

Q19Multiple choice

The value of rate constant of a pseudo first order reaction ____________.

(i)depends on the concentration of reactants present in small amount.
(ii)depends on the concentration of reactants present in excess.
(iii)is independent of the concentration of reactants.
(iv)depends only on temperature.

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(ii) depends on the concentration of reactants present in excess.

Q20Multiple choice

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?

(i)
(ii)
(iii)
(iv)

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(ii)

Q21Multiple correct

Rate law cannot be determined from balanced chemical equation if _______.

(i)reverse reaction is involved.
(ii)it is an elementary reaction.
(iii)it is a sequence of elementary reactions.
(iv)any of the reactants is in excess.

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(i) reverse reaction is involved.

(iii) it is a sequence of elementary reactions.

(iv) any of the reactants is in excess.

Q22Multiple correct

Which of the following statements are applicable to a balanced chemical equation of an elementary reaction?

(i)Order is same as molecularity.
(ii)Order is less than the molecularity.
(iii)Order is greater than the molecularity.
(iv)Molecularity can never be zero.

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(i) Order is same as molecularity.

(iv) Molecularity can never be zero.

Q23Multiple correct

In any unimolecular reaction ______________.

(i)only one reacting species is involved in the rate determining step.
(ii)the order and the molecularity of slowest step are equal to one.
(iii)the molecularity of the reaction is one and order is zero.
(iv)both molecularity and order of the reaction are one.

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(i) only one reacting species is involved in the rate determining step.

(ii) the order and the molecularity of slowest step are equal to one.

Q24Multiple correct

For a complex reaction ______________.

(i)order of overall reaction is same as molecularity of the slowest step.
(ii)order of overall reaction is less than the molecularity of the slowest step.
(iii)order of overall reaction is greater than molecularity of the slowest step.
(iv)molecularity of the slowest step is never zero or non interger.

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(i) order of overall reaction is same as molecularity of the slowest step.

(iv) molecularity of the slowest step is never zero or non interger.

Q25Multiple correct

At high pressure the following reaction is zero order. 1130 K 2NH3(g)  Platinum catalyst → N (g) + 3H (g) 2 2 Which of the following options are correct for this reaction?

(i)Rate of reaction = Rate constant
(ii)Rate of the reaction depends on concentration of ammonia.
(iii)Rate of decomposition of ammonia will remain constant until ammonia disappears completely.
(iv)Further increase in pressure will change the rate of reaction.

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(i) Rate of reaction = Rate constant

(iii) Rate of decomposition of ammonia will remain constant until ammonia disappears completely.

(iv) Further increase in pressure will change the rate of reaction.

Q26Multiple correct

During decomposition of an activated complex

(i)energy is always released
(ii)energy is always absorbed
(iii)energy does not change
(iv)reactants may be formed

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(i) energy is always released

(iv) reactants may be formed

Q28Multiple correct

According to Maxwell Boltzmann distributon of energy, __________.

(i)the fraction of molecules with most probable kinetic energy decreases at higher temperatures.
(ii)the fraction of molecules with most probable kinetic energy increases at higher temperatures. 53 Chemical Kinetics
(iii)most probable kinetic energy increases at higher temperatures.
(iv)most probable kinetic energy decreases at higher temperatures. 28. In the graph showing Maxwell Boltzman distribution of energy, ___________. (i) area under the curve must not change with increase in temperature. (ii) area under the curve increases with increase in temperature. (iii) area under the curve decreases with increase in temperature. (iv) with increase in temperature curve broadens and shifts to the right hand side.

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(i) the fraction of molecules with most probable kinetic energy decreases at higher temperatures.

(iv) most probable kinetic energy decreases at higher temperatures. 28. In the graph showing Maxwell Boltzman distribution of energy, ___________. (i) area under the curve must not change with increase in temperature. (ii) area under the curve increases with increase in temperature. (iii) area under the curve decreases with increase in temperature. (iv) with increase in temperature curve broadens and shifts to the right hand side.

Q29Multiple correct

Which of the following statements are in accordance with the Arrhenius equation?

(i)Rate of a reaction increases with increase in temperature.
(ii)Rate of a reaction increases with decrease in activation energy.
(iii)Rate constant decreases exponentially with increase in temperature.
(iv)Rate of reaction decreases with decrease in activation energy.

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(i) Rate of a reaction increases with increase in temperature.

(ii) Rate of a reaction increases with decrease in activation energy.

Q30Multiple correct

Mark the incorrect statements.

(i)Catalyst provides an alternative pathway to reaction mechanism.
(ii)Catalyst raises the activation energy.
(iii)Catalyst lowers the activation energy.
(iv)Catalyst alters enthalpy change of the reaction.

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(ii) Catalyst raises the activation energy.

(iv) Catalyst alters enthalpy change of the reaction.

Q31Multiple correct

Which of the following graphs is correct for a zero order reaction?

(i)
(ii)
(iii)
(iv)

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(i)

(iv)

Q32Multiple correct

Which of the following graphs is correct for a first order reaction?

(i)
(ii)
(iii)
(iv)

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(i)

(iv)

Q33Short answer

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

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Bimolecular reaction becomes kinetically first order when one of the reactants is in excess.

Q34Short answer

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

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Rate = k [A]0[B]0 or Rate = k

Q35Short answer

How can you determine the rate law of the following reaction? 2NO (g) + O2 (g) → 2NO2 (g)

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See page no. 99 of NCERT textbook for Class XII.

Q36Short answer

For which type of reactions, order and molecularity have the same value?

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If the reaction is an elementary reaction, order is same as molecularity.

Q37Short answer

In a reaction if the concentration of reactant A is tripled, the rate of reaction becomes twenty seven times. What is the order of the reaction?

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Three, because rate = k [A]3

Q38Short answer

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

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[R] = [R]0 – kt for completion [R] = 0 [R ]0 ∴ t=

Q39Short answer

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

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During an elementary reaction, the number of atoms or ions colliding to react is referred to as molecularity. Had this been an elementary reaction the order of reaction with respect to B would have been 1, but in the given rate law it is . This indicates that the reaction is not an elementary reaction.

Q40Short answer

For a certain reaction large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow. Why? 55 Chemical Kinetics

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Apart from the energy considerations, the colliding molecules should also have proper orientation for effective collision. This condition might not be getting fulfilled in the reaction.

Q41Short answer

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

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No, the molecularity can never be zero or a fractional number. –1

Q42Multiple choice

For a general reaction A → B, plot of concentration of A vs time is given in Fig. 4.3. Answer the following question on the basis of this graph.

This question refers to a figure in the original PDF.

(i)What is the order of the reaction?
(ii)What is the slope of the curve?
(iii)What are the units of rate constant? Fig. 4.3

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(i) What is the order of the reaction?

(ii) What is the slope of the curve?

(iii) What are the units of rate constant? Fig. 4.3

Q43Short answer

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.

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This is because activation energy for the reaction is very high at room temperature.

Q44Short answer

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

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At higher temperatures, larger fraction of colliding particles can cross the energy barrier (i.e. the activation energy), which leads to faster rate.

Q45Short answer

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

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The activation energy for combustion reactions of fuels is very high at room temperature therefore they do not burn by themselves.

Q46Short answer

Why is the probability of reaction with molecularity higher than three very rare?

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The probability of more than three molecules colliding simultaneously is very small. Hence possibility of molecularity being three is very low.

Q47Short answer

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

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The rate of a reaction depends on the concentration of reactants. As the reaction progresses, reactants start getting converted to products so the concentration of reactants decreases hence the rate decreases.

Q48Short answer

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

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Thermodynamically the conversion of diamond to graphite is highly feasible but this reaction is very slow because its activation energy is high.

Q49Short answer

Why in the redox titration of KMnO4 vs oxalic acid, we heat oxalic acid solution before starting the titration?

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The reaction between KMnO4 and oxalic acid is very slow. By raising the temperature we can enhance the rate of reaction.

Q50Short answer

Why can’t molecularity of any reaction be equal to zero?

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Molecularity is the number of molecules taking part in an elementary step. For this we require at least a single molecule leading to the value of minimum molecularity of one.

Q51Short answer

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

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A complex reaction proceeds through several elementary reactions. Numbers of molecules involved in each elementary reaction may be different i.e., the molecularity of each step may be different. Therefore, discussion of molecularity of overall complex reaction is meaningless. On the other hand, order of a complex reaction is determined by the slowest step in its mechanism and is not meaningless even in the case of complex reactions.

Q52Short answer

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

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Balanced chemical equation often leads to incorrect order or rate law. For example the following reaction seems to be a tenth order reaction. KClO3 + 6FeSO4 + 3H2SO4 → KCl + 3H2O + 3Fe2 (SO4)3 This is actually a second order reaction. Actually the reaction is complex and occurs in several steps. The order of such reaction is determined by the slowest step in the reaction mechanism. Order is determined experimentally and is confined to the dependence of observed rate of reaction on the concentration of reactants. IV. Matching Type

Q53Multiple choice

Match the graph given in Column I with the order of reaction given in Column II. More than one item in Column I may link to the same item of Column II. Column I Column II

(i)
(ii)(a) Ist order
(iii)(b) Zero order
(iv)

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(i)

(ii) (a) Ist order

(iii) (b) Zero order

(iv)

Q54Multiple choice

Match the statements given in Column I and Column II Column I Column II (i) Catalyst alters the rate of reaction

(a)cannot be fraction or zero (ii) Molecularity
(b)proper orientation is not there always 57 Chemical Kinetics (iii) Second half life of first order reaction
(c)by lowering the activation energy – E /R T (iv) e a
(d)is same as the first (v) Energetically favourable reactions (e) total probability is one are sometimes slow (vi) Area under the Maxwell (f) refers to the fraction of Boltzman curve is constant molecules with energy equal to or greater than activation energy

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(a) cannot be fraction or zero (ii) Molecularity

(b) proper orientation is not there always 57 Chemical Kinetics (iii) Second half life of first order reaction

(d) is same as the first (v) Energetically favourable reactions (e) total probability is one are sometimes slow (vi) Area under the Maxwell (f) refers to the fraction of Boltzman curve is constant molecules with energy equal to or greater than activation energy

Q55Multiple choice

Match the items of Column I and Column II. Column I Column II (i) Diamond

(a)short interval of time (ii) Instantaneous rate
(b)ordinarily rate of conversion is imperceptible (iii) Average rate
(c)long duration of time

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(a) short interval of time (ii) Instantaneous rate

(b) ordinarily rate of conversion is imperceptible (iii) Average rate

Q56Multiple choice

Match the items of Column I and Column II. Column I Column II (i) Mathematical expression for

(a)rate constant rate of reaction (ii) Rate of reaction for zero order
(b)rate law reaction is equal to (iii) Units of rate constant for zero order
(c)order of reaction is same as that of slowest step (iv) Order of a complex reaction is
(d)rate of a reaction determined by

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(a) rate constant rate of reaction (ii) Rate of reaction for zero order

(b) rate law reaction is equal to (iii) Units of rate constant for zero order

(d) rate of a reaction determined by

Q57Assertion & reason

Assertion (A): Order of the reaction can be zero or fractional.

Reason (R): We cannot determine order from balanced chemical equation.

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Q58Assertion & reason

Assertion (A): Order and molecularity are same.

Reason (R): Order is determined experimentally and molecularity is the sum of the stoichiometric coefficient of rate determining elementary step.

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(v)

Q59Assertion & reason

Assertion (A): The enthalpy 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.

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Q60Assertion & reason

Assertion (A): All collision of reactant molecules lead to product formation.

Reason (R): Only those collisions in which molecules have correct orientation and sufficient kinetic energy lead to compound formation.

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(v)

Q61Assertion & reason

Assertion (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.

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Q62Long answer

All energetically effective collisions do not result in a chemical change. Explain with the help of an example.

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Hint: Proper orientation of molecule should be explained in detail.

Q63Long answer

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

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Hint : • Flattening of curve and shifting of maxima towards high energy value. • Area under the curve beyond the activation energy increases.

Q64Long answer

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

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Hint : • Enthalpy is a state function. • Difference in energy between reactants and product is constant.

Q65Long answer

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

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See NCERT textbook for Class XII.

Q66Long answer

With the help of an example explain what is meant by pseudo first order reaction. 59 Chemical Kinetics

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See NCERT textbook for Class XII.