NCERT Exemplar Class 12 Chemistry Solutions Chapter 4 Chemical Kinetics

Question 1: 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.

Answer:

The answer is the option (iii). Activation energy for the reaction is lowered by a catalyst.

Question 2: 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.

Answer:

The answer is the option (iii). Catalyst only eases the threshold for a reaction without participating directly in it. So, it has no effect on heat evolved or absorbed during reaction.

Question 3: 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 the probability of collision.
(iv) using catalyst.

Answer:

The answer is the option (ii). According to Arrhenius equation, activation energy can be calculated by measuring the rate constants at two different temperatures.
$log\left ( \frac{k_{2}}{k_{1}} \right )=\frac{E_{a}}{2.303R}(\frac{1}{T_{1}}-\frac{1}{T_{2}})$

Where, $E_{a}=\text {Activation energy}$
$T_{1}=\text {lower temperature}$
$T_{2}=\text {Higher temperature}$
$k_{1}=\text {rate constant at temperature }T_{1}$
$k_{2}=\text {rate constant at temperature }T_{2}$

Question 4: Consider Fig. 4.1 and mark the correct option.
(i) The activation energy of forward reaction is E₁ + E₂ and the product is less stable than reactant.
(ii) The activation energy of forward reaction is E₁ + E₂ and product is more stable than reactant.
(iii) The activation energy of both forward and backward reaction is E₁ + E₂ and reactant is more stable than the product.
(iv) The activation energy of the backward reaction is E₁ and the product is more stable than reactant.


Answer:

The answer is the option (i) $E_{a}\; \text {forward}=E_{1}+E_{2}$
As reactants have lower energy than the products, the products are less stable.

Question 5: Consider a first order gas phase decomposition reaction given below :
$A(g)\rightarrow B(g)+C(g)$
The initial pressure of the system before decomposition of A was p_i. After lapse of time ‘t’, total pressure of the system increased by x units and became ‘p_t'. The rate constant k for the reaction is given as _________.
$(i) k=\frac{2.303}{t} log\;\frac{P_{i}}{P_{i}-x}$

$(ii) k=\frac{2.303}{t} log\;\frac{P_{i}}{2P_{i}-P_{t}}$

$(iii) k=\frac{2.303}{t} log\;\frac{P_{i}}{2P_{i}+P_{t}}$

$(iv) k=\frac{2.303}{t} log\;\frac{P_{i}}{P_{i}+x}$

Answer:

The answer is the option (ii)

$P_{i}=P_{i}+x$
$x=P_{t}-P_{i}$
$P_{A}= \text {Pressure of A after time t}= \text {P}_{i}-\text {x}=\text {P}_{i}-(\text {P}_{i}-\text {P}_{i})$

$=2\text {P}_{i}-\text {P}_{t}$
$k=\frac{2.303}{t}log\frac{[A]_{0}}{[A]}$

$=\frac{2.303}{t}log\frac{P_{i}}{2P_{i}-P_{t}}$

Question 6: According to Arrhenius equation rate constant k is equal to $A \; e^{-E_{a} /RT}$. Which of the following options represents the graph of ln k vs $\frac{1}{T}$?

Answer:

The answer is the option (i) According to Arrhenius equation:
$k=Ae^{-\frac{E_{a}}{RT}}$
$ln (k)=ln\left ( Ae^{-\frac{E_{a}}{RT}} \right )$
$ln\; k =ln\; A-\frac{E_{a}}{RT}$

Question 7: Consider the Arrhenius equation given below and mark the correct option. $k = A \; e^{-E_{a} /RT}$
(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

Answer:

The answer is the option (iv). Increase in T leads to an increase in $-\frac{E_{a}}{RT}$ and thus, an increase in k. Similarly, a decrease in $E_{a}$ leads to an increase in $-\frac{E_{a}}{RT}$ and thus, an increase in k.

Question 8: 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.

(i) Average rate up to 40 seconds is $\frac{V_{3}-V_{2}}{40}$
(ii) Average rate up to 40 seconds is $\frac{V_{}3- V_{}2}{40-30}$
(ii) Average rate up to 40 seconds is $\frac{V_{}3}{40}$
(iv) Average rate up to 40 seconds is $\frac{V_{}3- V_{}1}{40-20}$

Answer:

The answer is the option (iii) Average rate of the reaction can be determined by change in concentration of $H_{2}$ upon time from $(0,0)$ to $(40, V_{3})$=$\frac{V_{3}-0}{40-0}=\frac{V_{3}}{40}$

Question 9: Which of the following statements is not correct about an 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 the molar concentration of the reactants in the rate law expression.

Answer:

The answer is the option (iii). The summation of powers of the reactant’s concentration is used for determining the order of reaction. The order of a reaction is always equal to the sum of the stoichiometric coefficients of reacting species in the balanced chemical equation.

Question 10: Consider the graph given in Fig. 4.2. Which of the following options does not show the instantaneous rate of reaction at 40^th second?

$(i)\; \frac{V_{5}-V_{2}}{50-30}$

$(ii)\; \frac{V_{4}-V_{2}}{50-30}$

$(iii)\; \frac{V_{3}-V_{2}}{40-30}$

$(iv)\; \frac{V_{3}-V_{1}}{40-20}$

Answer:

The answer is the option (ii). Instantaneous rate is the rate of a reaction at a particular moment of time. Option (ii) does not show instantaneous rate of reaction.

Question 11: Which of the following statements is correct?
(i) The rate of a reaction decreases with passage of time as the concentration of reactants decreases.
(ii) The rate of a reaction is the 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 the concentration of the reactant(s).

Answer:

The answer is the option (i). The rate of a reaction is defined as the change in concentration of the reactant per unit time. Therefore, it is dependent upon the concentration of reactants.

Question 12: Which of the following expressions is correct for the rate of reaction given below?
$5Br^{}-(aq)+BrO_{3}^{}-(aq)+6H^{}+(aq)\rightarrow 3Br_{2}(aq)+3H_{2}O(l)$

$(i) \frac{\Delta [Br^{-}]}{\Delta t}=5\frac{\Delta [H^{+}]}{\Delta t}$

$(ii) \frac{\Delta [Br^{-}]}{\Delta t}=\frac{6}{5}\frac{\Delta [H^{+}]}{\Delta t}$

$(iii) \frac{\Delta [Br^{-}]}{\Delta t}=\frac{5}{6}\frac{\Delta [H^{+}]}{\Delta t}$

$(iv) \frac{\Delta [Br^{-}]}{\Delta t}=6\frac{\Delta [H^{+}]}{\Delta t}$

Answer:

We can rewrite the rate law expression for the given equation
$5Br^{}-(aq)+BrO_{3}^{}-(aq)+6H^{}+(aq)\rightarrow 3Br_{2}(aq)+3H_{2}O(l)$

$-\frac{1}{5}\frac{\Delta [Br^{-}]}{\Delta t}=-\frac{\Delta [BrO_{3}^{-}]}{\Delta t}=-\frac{1}{6}\frac{\Delta [H^{+}]}{\Delta t}=\frac{1}{3}\frac{\Delta [Br_{2}]}{\Delta t}$

$-\frac{\Delta [Br^{-}]}{\Delta t}=-\frac{\Delta [BrO_{3}^{-}]}{\Delta t}=-\frac{5}{6}\frac{\Delta [H^{+}]}{\Delta t}$

$\frac{\Delta [Br^{-}]}{\Delta t}=\frac{5}{6}\frac{\Delta [H^{+}]}{\Delta t}$

$(iii) \frac{\Delta [Br^{-}]}{\Delta t}=\frac{5}{6}\frac{\Delta [H^{+}]}{\Delta t}$

Question 13: Which of the following graph represents the exothermic reaction?

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

Answer:

The answer is the option (i). In an exothermic reaction the reaction activation energy of product should be higher than that of reactant whereas the enthalpy of reactant should be greater than that of products.

Question 14: The rate law for the reaction $A+2B\rightarrow C$ is found to be
Rate $=k[A][B]$
The 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

Answer:

The answer is the option (i). Rate constant of a reaction is independent of the concentrations of the reactants.

Question 15: Which of the following statements is incorrect about the collision 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 determine 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.

Answer:

The answer is the option (iii) For a reaction to occur, collision theory mandates several conditions that are:
(i) Collision of molecules must take place with sufficient threshold energy and should be effective.
(ii) They should have proper orientation.

Question 16: A first-order reaction is 50% completed in $1.26 \times 10^{14} \; s$. How much time would it take for 100% completion?
$(i)\; 1.26\times 10^{15}\; s$
$(ii)\; 2.52\times 10^{14}\; s$
$(iii)\; 2.52\times 10^{28}\; s$
$(iv) \text {infinite}$

Answer:

The answer is the option (iv). Since whole of the substance never reacts, reaction is considered 100% complete only after an infinite time that cannot be calculated.

Question 17: Compounds ‘A’ and ‘B’ react according to the following chemical equation.
$A(g)+2B(g)\rightarrow 2C(g)$
The concentration of either ‘A’ or ‘B’ was 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.

(i) Rate = $k[A]^{2}[B]$
(ii) Rate =$k[A][B]^{2}$
(iii) Rate = $k[A][B]$
(iv) Rate = $k[A]^{2}[B]^{0}$

Answer:

The answer is the option (ii)
We know that Rate of a reaction is $=k[A]^{x}[B]^{y}$
On doubling the concentration of B, keeping the concentration of A constant, the rate of formation of C increases by a factor of four. Thus, the rate of reaction is directly proportional to the square of concentration of B. However, on doubling the concentration of A, the rate of formation of C becomes the double of the initial value. Thus the rate is directly proportional to the first power of concentration of A. Hence Rate = $=K[A][B]^{2}$

Question 18: 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 $\Delta G$ 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.

Answer:

The answer is the option (ii)
A catalyst increases the rate of both the forward and backward reaction to the same extent. Hence, option a is correct.
$\Delta G= \text {-RT ln Q where Q is the reaction quotient}$

Reaction quotient is the relation between the concentration of reactants and products. Thus the gibbs free energy is unaffected by the addition of catalyst. It does not affect the equilibrium of the reaction since equilibrium constant depends only on the concentration of reactants and products.

Question 19: The value of rate constant of a pseudo first order reaction ____________.
(i) depends on the concentration of reactants present in a 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.

Answer:

The answer is the option (ii). The value of rate constant reaction is dependent on the concentration of reactants present in excess in a pseudo first order reaction. This is because the excess reactants do not get altered much while during the reaction

Question 20: Consider the reaction A $\rightleftharpoons$ 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 ?

Answer:

The answer is the option (ii) A->B

With time, there is an exponential decrease in the concentration of reactants and an increase in the concentration of products.

Question 22: In the following questions two or more options may be correct.
Which of the following statements are applicable to a balanced chemical equation of an elementary reaction?
(i) Order is the same as molecularity.
(ii) Order is less than the molecularity.
(iii) Order is greater than the molecularity.
(iv) Molecularity can never be zero.

Answer:

The answer is the option (i, iv). The order and molecularity are same for an elementary or single step reaction with molecularity of a reaction never equal to zero.

Question 23: In the following questions two or more options may be correct.
In any unimolecular reaction ______________.
(i) only one reacting species is involved in the rate-determining step.
(ii) the order and the molecularity of the 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.

Answer:

The answer is the option (i, ii). Only one reactant is involved in rate determining step in an unimolecular reaction and the order and molecularity of rate determining or slowest step are equal to one.
$A\rightarrow B$
$Rate = k[A]$

Question 24: In the following questions two or more options may be correct.
For a complex reaction ______________.
(i) order of an overall reaction is the same as molecularity of the slowest step.
(ii) order of an overall reaction is less than the molecularity of the slowest step.
(iii) order of an overall reaction is greater than molecularity of the slowest step.
(iv) molecularity of the slowest step is never zero or non-integer.

Answer:

The answer is the option (i, iv).
The answer is the option (i) Order of overall reaction is same as the molecularity of slowest step for a complex reaction.
The rate of overall reaction is dependent on total number of molecules involved in slowest step of the reaction making molecularity of the slowest step equal to order of overall reaction.

Question 25: In the following questions two or more options may be correct.At high pressure, the following reaction is zero order.$2NH_{3}(g)\overset{1130 K \; \text {Platinum catalyst}}{\longrightarrow} \; N_{2}(g)+3H_{2}(g)$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.

Answer:

The answer is the option (i, iii, iv) The rate of reaction is not dependent on the concentration of ammonia in a zero-order reaction

Question 26: In the following questions two or more options may be 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

Answer:

The answer is the option (i, iv) Activated complex is an intermediate formed at the highest level of energy $(E_{a})$ of the system. Some energy is released with the decomposition of this unstable complex to give products. If the reaction is reversible, reactants might be formed.

Question 27: In the following questions two or more options may be correct.
According to Maxwell Boltzmann distribution 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.
(iii) most probable kinetic energy increases at higher temperatures.
(iv) most probable kinetic energy decreases at higher temperatures.

Answer:

The answer is the option (i, iii) Maxwell Boltzmann distribution of kinetic energy can be described by a graph plotted with fraction of molecules on y-axis versus kinetic energy on x-axis.

Most probable kinetic energy is the kinetic energy of maximum fraction of molecule. The peak of the graph shifts forward and downward with an increase in temperature.
This implies that the most probable kinetic energy increases and fraction of molecules with most probable K.E decreases with an increase in temperature.

Question 28: In the following questions two or more options may be correct.
In the graph showing Maxwell Boltzman distribution of energy, ___________.
(i) area under the curve must not change with an 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 the increase in the temperature, curve broadens and shifts to the right-hand side.

Answer:

The answer is the option (i, iv) Area under the curve should remain constant with an increase in temperature as the probability must be 1 for all the time. As the temperature rises, the curve broadens out and shifts to the right-hand higher energy value side.

Question 29: 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 an increase in temperature.
(iv) Rate of reaction decreases with decrease in activation energy.

Answer:

The answer is the option (i, iv). According to Arrhenius equation, the rate of reaction increases with increase in temperature and decrease in the activation energy. It gets double with every 10° change. As the rate of reaction increases, there is an exponential increase in the rate constant of the reaction.

Question 30: Mark the incorrect statements.
(i) Catalyst provides an alternative pathway to a reaction mechanism.
(ii) Catalyst raises the activation energy.
(iii) A catalyst lowers the activation energy.
(iv) Catalyst alters enthalpy change of the reaction.

Answer:

The answer is the option (ii, iv)
(i) On adding the catalyst to a reaction, medium rate of reaction increases with decreasing activation energy of molecule.
(ii) The enthapy change of reaction remains unchanged due to a catalyst. In both the catalysed and uncatalysed reaction, energy of reactants and products are alike.

Question 31: Which of the following graphs is correct for a zero-order reaction?

Answer:

The answer is the option (i, iv). On plotting [R] against t, a straight line is obtained with slope $=-k$ and intercept equal to $[R]_{0}$from the equation
$[R]=-kt+[R]_{0}$
The rate is directly proportional to the zero power of concentration in a Zero-order reaction.
Rate = $-\frac{d[R]}{dt}=k[R]^{0}$

Question 32: Which of the following graphs is correct for a first-order reaction?

Answer:

The answer is the option (i, iv) For a first order reaction
$t_{\frac{1}{2}}=\frac{0.693}{k}$ and

rate $=-\frac{d[R]}{dt}=k[R]$
$Since \; k=\frac{2.303}{t}\; log\; [\frac{R^{0}}{R}]$ the slope for a graph between log $log [\frac{R^{0}}{R}]$ versus t is $\frac{k}{2.303}$

Question 34: Write the rate equation for the reaction $2A+B\rightarrow C$ if the order of the reaction is zero.

Answer:

$2A+B \rightarrow C$
If the order of reaction is zero, the rate equation can be written as
Rate $= k[A]^{o} [B]^{o}$

Question 35: How can you determine the rate law of the following reaction?
$2NO (g) + O_{2} (g) \rightarrow 2NO_{2} (g)$

Answer:

The rate of this reaction is measured as a function of initial concentration for the determination of the rate law. This is done either by keeping the concentration of one of the reactants constant while changing the other or by changing the concentration of both the reactants. The rate law can be determined from the concentration dependence of rate.

Question 36: For which type of reactions, order and molecularity have the same value?

Answer:

In elementary reactions the value of order and molecularity is equal.

Question 37: 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?

Answer:

$\text {Rate}=[\text {A}]^{\alpha }$
$27*\text {Rate}=\text {k}[\text {3A}]^{\alpha }$
Dividing both the equations we get $\alpha =3$
Therefore the order of reaction is 3

Question 38: Derive an expression to calculate the time required for completion of the zero-order reaction.

Answer:

In case of a zero order reaction
$R=[R]_{0}-kt$
For the reaction to achieve completion $[R]=0$
$kt=[R]_{0}\; or\; t=\frac{[R]_{0}}{k}$

Question 39: For a reaction $A+B\rightarrow$ products, the rate law is — Rate $= k [A][B]^{3/2}$. Can the reaction be an elementary reaction? Explain.

Answer:

Given that the rate of reaction is $k[A][B]^{\frac{3}{2}}$
Order =$1+\frac{3}{2}=\frac{5}{2}$
The order of elementary reaction cannot be fractional.
Hence, This indicates that the reaction is not an elementary
reaction.

Question 40: For a certain reaction large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow. Why?

Answer:

For a certain reaction, large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow due to lack of proper orientation

Question 41: For a zero-order reaction will the molecularity be equal to zero? Explain.

Answer:

Molecularity of a reaction cannot be equal to zero in any case.

Question 42: For a general reaction $A\rightarrow B$, plot of concentration of A vs time is given in Fig. 4.3. Answer the following question based on this graph.
(i) What is the order of the reaction?
(ii) What is the slope of the curve?
(iii) What are the units of the rate constant?

Answer:

(i) The reaction is of zero order.
(ii) $[R]=[R]_{0}-kt$
Thus, the slope of the curve is -k
(iii) $\frac{molL^{-1}}{s}=k[A]^{0}\; or\; k=molL^{-1}s^{-1}$

Question 43: The reaction between $H_{2}(g)$ and $O_{2}(g)$ 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.

Answer:

$2H_{2}(g)+O_{2}(g)\rightarrow 2H_{2}O(l)$
The reaction between H₂ and O₂ to form water does not take place at room temperature because the activation energy of the reaction is very high.

Question 44: Why does the rate of a reaction increase with a rise in temperature?

Answer:

The rate of a reaction increases with a rise in temperature as larger fraction of colliding particles can cross the energy barrier (i.e. the activation energy).

Question 45: Oxygen is available in plenty in the air yet fuels do not burn by themselves at room temperature. Explain.

Answer:

Oxygen does not burn by themselves in spite of being available in plenty in air as the activation energy or $E_{a}$ for combustion reactions of fuels is extremely high at room temperature.

Question 46: Why is the probability of reaction with molecularity higher than three very rare?

Answer:

The low possibility of more than three molecules colliding simultaneously makes the probability of reaction with molecularity higher than three very rare.

Question 47: Why does the rate of any reaction generally decrease during the reaction?

Answer:

We know that the rate of a reaction is dependent on the concentration of reactants. During the course of the reaction, the concentration of reactants keeps decreasing because the reactants keep getting converted to products.

Question 48: Thermodynamic feasibility of the reaction alone cannot decide the rate of the reaction. Explain with the help of one example.

Answer:

Gibbs free energy determines the thermodynamic feasibility of a reaction. For spontaneous reaction, $\Delta G$ should be negative whereas
Activation energy determines the kinetic feasibility of the reaction.
$\text {Diamond}\rightarrow \text {Graphite}\; \text {where}\;\Delta G=-ve$

Question 49: Why in the redox titration of $KMnO_{4}$ vs oxalic acid, we heat oxalic acid solution before starting the titration?

Answer:

In the redox titration of $KMnO_{4}$ and oxalic acid we heat the oxalic acid solution before starting the titration to increase the rate of reaction as it is very slow.

Question 50: Why can’t molecularity of any reaction be equal to zero?

Answer:

Molecularity can be defined as the number of molecules involved in the process. It cannot be less than one.

Question 51: Why molecularity is applicable only for elementary reactions and order is applicable for elementary as well as complex reactions?

Answer:

A complex reaction involves many elementary reactions. Numbers of molecules in each elementary reaction can vary leading to different molecularity at each step. Therefore, the molecularity is not applicable in case of overall complex reaction. Whereas the slowest step is responsible for the experimental determination of the order of a complex and is therefore, applicable even in the case of complex reactions.

Question 52: Why can we not determine the order of a reaction by taking into consideration the balanced chemical equation?

Answer:

As reactions can be multi-stepped, the rate of reaction is determined by slowest step
$KClO_3+6FeSO_4+3H_2 SO_4\rightarrow KCl+3H_2 O+3Fe_2 (SO_4 )_3$
Given the above equation, it may seem like a reaction with an order of ten, but in reality this is a second order reaction. Order is determined via experiments and expresses the dependence of the rate of reaction on the concentration of reactants.

Question 54: Match the statements given in column I and Column II.

Answer:

(i) → (c) (ii) → (a) (iii) → (d) (iv) → (f) (v) → (b) (vi) → (e)
(i) Catalyst can change the rate of reaction by decreasing the $(E_{a})$ or activation energy.
(ii) Molecularity cannot be fraction or zero. No reaction is possible if molecularity is zero.
(iii) The first and second half-life of first order reaction are same since half-life is temperature dependent. ‘
(iv) In Arrhenius equation, $e-\frac{E_{a}}{RT}$ denotes the fraction of molecules with K.E equal to or greater than activation energy.
(v) The activation energy and proper orientation of the molecules together determine the criteria for effective collision in collision theory.
(vi) Since total probability of molecules must be equal to one at all times, area under the Maxwell-Boltzmann curve remains constant.

Question 55: Match the items of Column I and Column II.

Answer:

(i $\longrightarrow$b), (ii$\longrightarrow$a), (iii$\longrightarrow$c)
(i) The conversion of diamond to graphite does not take place under ordinary condition.
(ii) Instantaneous rate of reaction is calculated for a very short time interval.
(iii) Average rate of reaction is calculated for a long duration of time.

Question 56: Match the items of Column I and Column II.

Answer:

(i $\longrightarrow$ b), (ii $\longrightarrow$ a), (iii $\longrightarrow$ d), (iv$\longrightarrow$ c)
(i) Mathematical expression for rate of reaction is termed as rate law.
(ii) Rate for zero order reaction is equal to rate constant $r=k[A]$
(iii) Unit of rate constant for zero order reaction is same as that of rate of reaction.
(iv)The slowest step or the rate determining step helps in finding the order of a complex reaction.

Question 58: In the following question, a statement of Assertion The answer is the option (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion The answer is the option (i): 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.
(i) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(ii) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
(v) Assertion is incorrect but Reason is correct.

Answer:

The answer is the option (v). While balanced stoichiometric equation is used to calculate molecularity order can only be measured through experiments.

Question 59: In the following question, a statement of Assertion The answer is the option (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion The answer is the option (i): 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.
(i) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(ii) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
(v) Assertion is incorrect but Reason is correct.

Answer:

The answer is the option (i) $\Delta H$ = Activation Energy of forward reaction – Activation Energy of reverse reaction.
The heat of reaction remains unaltered by the catalysts because it does not involve directly in the reaction and only affects the activation energy reactions. The enthalpy of reaction remains unchanged due to the addition of catalyst.

Question 60: In the following question, a statement of Assertion The answer is the option (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion The answer is the option (i): 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.
(i) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(ii) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
(v) Assertion is incorrect but Reason is correct.

Answer:

The answer is the option (v). Only the molecular collisions with sufficient kinetic energy and proper orientation bring out the formation of product.

Question:61 In the following question, a statement of Assertion The answer is the option (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion The answer is the option (i): 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.
(i) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(ii) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
(v) Assertion is incorrect but Reason is correct.

Answer:

The answer is the option (iii). The criteria for an effective collision and rate of chemical reaction is determined together by activation energy and proper orientation of molecules in the collision theory.

Question 63: What happens to most probable kinetic energy and the energy of activation with increase in temperature?

Answer:

On increasing the temperature, the curve flattens out as there are more molecules with higher kinetic energies. Because of this shift, the most probable kinetic energy (the modal value) also increases slightly.

Question 64: Describe how does the enthalpy of reaction remain unchanged when a catalyst is used in the reaction?

Answer:

Catalyst doesn’t participate in the reaction but increases the speed of reaction. The “Intermediate complex formation theory” claims that the reactants combine with the catalyst to form highly active radical complex. These complex compounds then decompose to create the product and the catalyst.
The intermediate formed with catalyst possesses much lower potential energy than the intermediate formed in the normal reaction, lowering the activation energy required. The following diagram shows the impact of using the catalyst on the potential energy and thus, the activation energy.

Question 65: Explain the difference between instantaneous rate of a reaction and average rate of a reaction.

Answer:

The change in concentration of reactants or products per unit time is the average rate of reaction.
$\text {Average rate}=-\frac{\Delta [R]}{\Delta T}=\frac{\Delta [P]}{\Delta T}$
It is calculated over a large time frame and, in most scenarios, will not give the rate of reaction for future. Instantaneous rate is the actual rate of reaction at a given instance and can be represented as follows:

Instantaneous rate $=\lim _{\Delta T \rightarrow 0}-\frac{\Delta[R]}{\Delta T}=\lim _{\Delta T \rightarrow 0} \frac{\Delta[P]}{\Delta T}=-\frac{d[R]}{d T}=\frac{d[P]}{d T}$

$=\lim_{\Delta T\rightarrow 0}\frac{\Delta [P]}{\Delta T}$

$=-\frac{d[R]}{dT}=\frac{d[P]}{dT}$
While average rate can be measured for multi-step and elementary reaction, instantaneous rate can’t be measured.

Question 66: With the help of an example explain what is meant by pseudo first order reaction?

Answer:

Pseudo-first order reaction is a reaction where one reactant has a very high concentration and during the course of the reaction, it doesn’t change significantly. The reaction proceeds as a first order reaction in such a case. Take for instance the hydrolysis of ethyl acetate and inversion of cane sugar
Hydrolysis of ethyl acetate:
$$
\begin{array}{lccll}
& \mathrm{CH}_3 \mathrm{COOC}_2 \mathrm{H}_5 & +\mathrm{H}_2 \mathrm{O} & \xrightarrow{\mathrm{H}^{+}} & \mathrm{CH}_3 \mathrm{COOH} \\
t=0 & 0.01 \mathrm{~mol} & 10 \mathrm{~mol} & & +\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \\
t=t & 0 \mathrm{~mol} & 9.99 \mathrm{~mol} & & 0 \mathrm{~mol}
\end{array}
$$
$\text {Rate of reaction}=k[CH_{3}COOC_{2}H_{5}]$

Where, $k=k'[H_{2}O]$

Inversion of cane sugar:

$C_{12}H_{22}O_{11}\; \; \; \; \; \; \; +\; \; \; \; \; H_{2}O\; \; \; \; \; \; \overset{H^{+}}{\rightarrow}\; \; \; \; \; \; \; C_{6}H_{12}O_{6}\; \; \; \; \; \; +\; \; \; \; \; \; C_{6}H_{12}O_{6}$
$\text {Rate of reaction}=k[C_{12}H_{22}O_{11}]$
Where, $k=k'[H_{2}O]$