CBSE 12th Standard Chemistry Subject Chemical Kinetics Chapter Case Study Questions 2021
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CBSE 12th Standard Chemistry Subject Chemical Kinetics Case Study Questions 2021
12th Standard CBSE
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Reg.No. :
Chemistry
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Read the passage given below and answer the following questions :
The half-life of a reaction is the time required for the concentration of reactant to decrease by half, i.e.,
\([A]_{t}=\frac{1}{2}[A]\)
For first order reaction,
\(t_{1 / 2}=\frac{0.693}{k}\) this means t1/2 is independent of initial concentration. Figure shows that typical variation of concentration of reactant exhibiting first order kinetics. It may be noted that though the major portion of the first order kinetics may be over in a finite time, but the reaction will never cease as the concentration of reactant will be zero only at infinite time
The following questions are multiple choice questions. Choose the most appropriate answer:
(i) A first order reaction has a rate constant k = 3.01 x 10-3 is. How long it will take to decompose half of the reactant?(a) 2.303 s (b) 23.03 s (c) 230.3 s (d) 2303 s (ii) The rate constant for a first order reaction is 7.0x 10-4 s-1. If initial concentration of reactant is 0.080 M, what is the half life of reaction?
(a) 990 s (b) 79.2 s (c) 12375 s (d) 10.10 x 10-4 s (iii) For the half-life period of a first order reaction, which one of the following statements is generally false?
(a) It is independent of initial concentration. (b) It is independent of temperature. (c) It decreases with the introduction of a catalyst (d) None of these (iv) The rate of a first order reaction is 0.04 mol L-1 s-1 at 10 minutes and 0.03 mol L-1 s-1 at 20 minutes after initiation. The half-life of the reaction is
(a) 4.408 min (b) 44.086 min (c) 24.086 min (d) 2.408 min -
Read the passage given below and answer the following questions :
The following reaction, \(A_{(g)} \stackrel{\Delta}{\longrightarrow} P_{(g)}+Q_{(g)}+R_{(g)}\) follows first order kinetics. The half-life period of this reaction is 69.3 s at 500°C. The gas A is enclosed in a container at 500°C and at a pressure of 0.4 atm
The following questions are multiple choice questions. Choose the most appropriate answer:
(i) The rat constant for the reaction is(a) 0.4 s-1 (b) 0.02 s-1 (c) 0.01 s-1 (d) 0.3 s-1 (ii) The total pressure of the system after 230 s will be
(a) 2.15 atm (b) 1.12 atm (c) 0.4 atm (d) 3.08 atm (iii) The plot of ln[A] vs t will be
(a) linear with slope = k (b) linear with intercept = In[A]o (c) linear with slope = In[A]o (d) linear with intercept = [A]0 (iv) Which of the following is not an example of first order reaction?
(a) \(\mathrm{C}_{2} \mathrm{H}_{4(g)}+\mathrm{H}_{2(\mathrm{~g})} \rightarrow \mathrm{C}_{2} \mathrm{H}_{6(\mathrm{~g})}\) (b) \(2 \mathrm{~N}_{2} \mathrm{O}_{5(g)} \rightarrow 4 \mathrm{NO}_{2(g)}+\mathrm{O}_{2(g)}\) (c) \(2 \mathrm{NH}_{3(g)} \frac{\mathrm{Pt}}{\Delta} \mathrm{N}_{2(g)}+3 \mathrm{H}_{2(g)}\) (d) \(2 \mathrm{~N}_{2} \mathrm{O}_{(g)} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{~N}_{2(g)}+\mathrm{O}_{2(g)}\) -
Read the passage given below and answer the following questions:
A reaction in which rate of reaction is independent of concentration of the reactants is called zero order reaction. Photochemical combination of hydrogen and chlorine to give hydrogen chloride is an example of zero order reaction. The rate constant of a zero order reaction is equal to the rate of reaction. The half life period of a zero order reaction is directly proportional to initial concentration of the reactant. For a zero order reaction, \(k=\frac{1}{t}\left\{[A]_{0}-[A]\right\}\)
In these questions (i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.(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 (i) Assertion : For a zero order reaction, plot of rate vs concentration will be a straight line parallel to concentration axis.
Reason : For a zero order reaction, rate is independent of concentration.
(ii) Assertion : Photochemical combination of hydrogen and chlorine to give hydrogen chloride is an example of zero order reaction.
Reason : The rate of reaction depends on the concentration of hydrogen and independent of concentration of chlorine.
(iii) Assertion : If in a zero order reaction, the concentration of the reactant is doubled, the half-life period is also doubled.
Reason : For a zero order reaction, the rate of reaction is independent of initial concentration
(iv) Assertion : In a reaction A -7 products, the concentration of the reactant is reduced to zero after a finite time.
Reason : The order of reaction is zero. -
Read the passage given below and answer the following questions :
Decrease in concentration of reactant or increase in concentration of product per unit time is called rate of reaction. It is of two types:
(i) Instantaneous rate of reaction : Rate of change of concentration of reactant or product at a particular time is called instantaneous rate of reaction.
\(r_{\text {inst. }}=\frac{d C}{d t}\)
where, dC = infinitely small change in concentration
dt = infinitely small change in time.
(ii) Average rate of reaction: Ratio of change in concentration and time required for the change is average rate of reaction.
\(r_{a v}=\frac{\Delta x}{\Delta t}=\frac{\text { Change in concentration }}{\text { Time required for the change }}\)
For a reaction of the type, \(m_{1} A+m_{2} B \rightarrow n_{1} C+n_{2} D\)
Rate of reaction is given as
\(-\frac{1}{m_{1}} \frac{d[A]}{d t}=-\frac{1}{m_{2}} \frac{d[B]}{d t}=+\frac{1}{n_{1}} \frac{d[C]}{d t}=+\frac{1}{n_{2}} \frac{d[D]}{d t}\)
In these questions (i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.(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. (i) Assertion : The kinetics of the reaction,\(m A+n B+p C \rightarrow m^{\prime} X+n^{\prime} Y+p^{\prime} Z\) obey the rate expression as \(\frac{d x}{d t}=k[A]^{m}[B]^{n}\) .
Reason : The rate of the reaction does not depend upon the concentration of C.
(ii) Assertion: Instantaneous rate of reaction is equal to dx/ dt.
Reason : It is the rate of reaction at any particular instant of time.
(iii) Assertion : For the reaction,\(R \mathrm{Cl}+\mathrm{NaOH} \rightarrow \mathrm{ROH}+\mathrm{NaCl}\) the rate of reaction is reduced to half on reducing the concentration of RCl to half.
Reason : The sate of reaction is represented by k[RCl].
(iv) Assertion : In rate law, unlike in the expression for equilibrium constants, the exponents for concentrations do not necessarily match the stoichiometric coefficients.
Reason: It is the mechanism and not the balanced chemical equation for the overall change that governs the reaction rate. -
Read the passage given below and answer the following questions :
For a first order reaction \(A \rightarrow \text { Products, } k=\frac{2.303}{t} \log \frac{a}{a-x}\) were a is the initial concentration of A and (a-x) is the concentration of A after time t. k is rate constant. Its value is constant at constant temperature for a reaction. The time in which half of the reactant is consumed is called half-life period. Half-life period of a first order reaction is constant. Its value is independent of initial concentration or any other external conditions.
In these questions (i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(i) Assertion : Rate of reaction doubles when concentration of reactant is doubled if it is a first order reaction.
Reason : Rate constant also doubles,(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. (ii) Assertion : Hydrolysis of ethyl acetate in presence of acid is a reaction of first order whereas in presence of alkali, it is a reaction of second order.
Reason : Acid only acts as a catalyst whereas alkali acts as one of the reactants.(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. (iii) Assertion : For a first -order reaction, the concentration of the reactant decreases exponentially with time.
Reason : Rate of reaction at any time depends upon the concentration of the reactant at that time.(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. (iv) Assertion : Half-life period for a first order reaction is independent of initial concentration of the reactant.
Reason : For a first order reaction, \(t_{1 / 2}=\frac{0.693}{k}\), where k is rate constant.(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.
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CBSE 12th Standard Chemistry Subject Chemical Kinetics Case Study Questions 2021 Answer Keys
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(i) (c) : For a first order reaction :
\(t_{1 / 2}=\frac{0.693}{k}, k=3.01 \times 10^{-3} \mathrm{~s}^{-1}\)
\(\therefore \quad t_{1 / 2}=\frac{0.693}{3.01 \times 10^{-3}}=230.3 \mathrm{~s}\)
(ii) (a) : Half life (1/2) of a first order reaction is given as :
\(t_{1 / 2}=\frac{0.693}{k}=\frac{0.693}{7.0 \times 10^{-4}}=990 \mathrm{~s}\)
(iii) (b) : For a first order reaction \(t_{1 / 2}=\frac{0.693}{k}\) therefore t1/2 depends upon k and hence depends on temperature because rate constant k is a function of temperature.
(iv) (c) : Let the concentrations of the reactant after
10 min and 20 min be C1 and C2 respectively.
Rate after 10 min = k,C2
= 0.03 x 60 mol L-1 min-1
\(\therefore \frac{C_{1}}{C_{2}}=\frac{4}{3}\)
Let the reaction starts after 10 minutes.
\(k=\frac{2.303}{10} \log \frac{C_{1}}{C_{2}}=\frac{2.303}{10} \log \frac{4}{3}=0.02878\)
\(\therefore \quad t_{1 / 2}=\frac{0.6932}{k}=\frac{0.6932}{0.02878}=24.086 \mathrm{~min}\) -
(i) (c) : t1/2 = 69.3 s
For first order reaction,
\(k=\frac{0.693}{t_{1 / 2}}=\frac{0.693}{69.3}=0.01 \mathrm{~s}^{-1}\)
(ii) (b) : For the given reaction,
\(\begin{array}{lccc} & A_{(g)} \stackrel{\Delta}{\longrightarrow} P_{(g)} & +Q_{(g)}+R_{(g)} \\ \text { Initial pressure } & 0.4 & 0 & 0 & 0 \\ \text { Final pressure } & 0.4-0.36 & 0.36 & 0.36 & 0.36 \end{array}\)
Total pressure = (0.4 - 0.36) + (3 x 0.36) = 1.12 atm
(iii) (b) : Expression that relates concentration of reactant and time for first order reaction is
In [A]= -kt +In [A]o
So, the plot of ln [A]0 vs t will be linear with slope = -k and intercept = In [A]0
(iv) (c) : Decomposition of ammonia on a hot platinum surface at high pressure is a zero order reaction. -
(i) (a) :
(ii) (c) : The reaction proceeds with a constant rate which is independent of concentration of hydrogen and chlorine. That is why, this reaction is a zero order reaction.
(iii) (b) : For a zero order reaction t1/2 = [a]/2k. .
(iv) (a) : -
(i) (a) : Rate expression \(\frac{d x}{d t}=k[A]^{m}[B]^{n}\)
shows that the total order of reactions is m + n + 0 = m + n, as the rate of reaction is independent of concentration of C, i.e., the order with respect to C is zero. This is the reason that C does not figure in the rate expression.
(ii) (b) : Instantaneous rate of a reaction is equal to small change in concentration (dx) during a small interval of time (dt) at that particular instant of time divided by the time interval.
(iii) (a) : For the given reaction, rate of reaction (r) = k[RCl] (where k is rate constant). Therefore if the concentration of [RCl] is reduced to half, then new rate \(\left(r^{\prime}\right)=\frac{k}{2}[R \mathrm{Cl}]\)
(iv) (a) -
(i) (c) : For first order .reaction, Rate. = k[A1 ]
According to question,
[A2] = [2A1]
\(\therefore\) Rate2 = k[2A1]
\(\Rightarrow \) Rate2, = 2 Rate1.
For a given reaction, rate constant is constant and independent of the concentration of reactant.
(ii) (a) : \(\begin{aligned} \mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5}+\mathrm{H}_{2} \mathrm{O} \stackrel{\mathrm{H}^{+}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{COOH} &+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} \end{aligned}\)
\(\text { Rate } \propto\left[\mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5}\right]\)
\(\mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5}+\mathrm{NaOH} \rightarrow \mathrm{CH}_{3} \mathrm{COONa}+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)
\(\text { Rate } \propto\left[\mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5}\right][\mathrm{NaOH}]\)
(iii) (b) : For a first order reaction, \(\lfloor A]=[A]_{0} e^{-k t}\)
or \(\log [A]=-\frac{k t}{2.303}+\log [A]_{0}\)
(iv) (a) : For a first order reaction, t1/2 is inversely proportional to k, it does not depend on the initial concentration of the reactant.
