jee main chemistry important questions | Chemical Kinetics jee main questions FREE

Q 1 :

${NO}_{2}$ required for a reaction is produced by decomposition of $N_{2} O_{5}$ in ${CCl}_{4}$ as by equation

$2 N_{2} O_{5 (g)} \to 4 {NO}_{2 (g)} + O_{2 (g)}$

The initial concentration of $N_{2} O_{5}$ is 3 mol $L^{- 1}$ and it is 2.75 mol $L^{- 1}$ after 30 minutes.

The rate of formation of ${NO}_{2}$ is $x \times 10^{- 3}$ mol $L^{- 1}$ $\min^{- 1}$ , Value of $x$ is ________ . (nearest integer) [2024]

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(17)

${Rate of dissociation of N}_{2} O_{5} :$ $- \frac{Δ [N_{2} O_{5}]}{t} = - \frac{(2.75 - 3) {mol L}^{- 1}}{30 min} = \frac{1}{120} \frac{mol}{L \min}$

$By stoichiometry of the reaction, 2 N_{2} O_{5} (g) \to 4 {NO}_{2} (g) + O_{2} (g)$

$Rate of reaction = - \frac{1}{2} \frac{Δ [N_{2} O_{5}]}{t} = \frac{1}{4} \frac{Δ [{NO}_{2}]}{t}$

$\Rightarrow \frac{Δ [{NO}_{2}]}{t} = 2 \times \frac{Δ [N_{2} O_{5}]}{t}$

$\frac{Δ [{NO}_{2}]}{t} = 2 \times \frac{1}{120} \frac{mol}{L \min} = 16.67 \times 10^{- 3} \frac{mol}{L \min} \approx 17 \times 10^{- 3} \frac{mol}{L \min}$

Q 2 :

Consider the following single step reaction in gas phase at constant temperature.

$2 A_{(g)} + B_{(g)} \to C_{(g)} $

The initial rate of the reaction is recorded as $r_{1}$ when the reaction starts with 1.5 atm pressure of A and 0.7 atm pressure of B. After some time, the rate $r_{2}$ is recorded when the pressure of C becomes 0.5 atm. The ratio $r_{1} : r_{2}$ is _________ $\times 10^{- 1} .$ (Nearest Integer) [2024]

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(315)

$2 A(g)$ $+$ $B(g)$ $\to$ $C(g)$

t = 0 1.5 atm 0.7 atm

(press)

t = t (1.5 - 2p) atm (0.7 - p) atm p atm

(press)

Since reaction is single step reaction, its rate law can be written from stoichiometry of the reaction as:

$r = k {(P_{A})}^{2} (P_{B})$

Initial rate $r_{1}$ is thus $r_{1}$ = $k {(1.5)}^{2} (0.7)$

As given in question p = 0.5 atm

Thus pressure of A after sometime

$= (1.5 - 2 p) atm = (1.5 - 2 \times 0.5) atm = 0.5 atm$

Pressure of B after sometime

$= (0.7 - p) atm = (0.7 - 0.5) atm = 0.2 atm$

Rate after sometime, $r_{2} = k {(0.5)}^{2} (0.2)$

$\frac{r_{1}}{r_{2}} = \frac{k {(1.5)}^{2} (0.7)}{k {(0.5)}^{2} (0.2)} = 315 \times 10^{- 1}$

Q 3 :

Consider the following first order gas phase reaction at constant temperature

$A (g) \to 2 B (g) + C (g)$

If the total pressure of the gases is found to be 200 torr after 23 sec. and 300 torr upon the complete decomposition of A after a very long time, then the rate constant of the given reaction is ______ $\times 10^{- 2} s^{- 1}$ (nearest integer).

[Given: $\log_{10} (2) = 0.301$ ] [2024]

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(3)

$A(g)$ $\to$ $2 B(g)$ $+$ $C(g)$

t = 0 $P^{o}$ 0 0

(press.)

t = 23 s $P^{o} - P$ $2 P$ $P$ $Total = P^{o} + 2 P = 200$ ...(i)

(press.)

t = $\infty$ 0 $2 P^{o}$ $P^{o}$ $Total = 3 P^{o} = 300$ ...(ii)

(press.)

From (i) and (ii)

$P^{o} = 100$ torr, $P = 50$ torr

For first order reaction, rate constant k is given as:

$k = \frac{2.303}{t} \log_{10} \frac{P^{o}}{P^{o} - P}$

$k = \frac{2.303}{23} \log_{10} \frac{100}{100 - 50}$

$k = \frac{2.303}{23} \log_{10} 2 s^{- 1} = \frac{2.303}{23} \times 0.301 s^{- 1} = 0.0301 s^{- 1} \approx 3 \times 10^{- 2} s^{- 1}$

Q 4 :

Consider an elementary reaction A(g) + B(g) → C(g) + D(g)

If the volume of reaction mixture is suddenly reduced to $\frac{1}{3}$ of its initial volume, the reaction rate will become ‘ $x$ ’ times of the original reaction rate. The value of $x$ is: [2025]

$\frac{1}{3}$
$\frac{1}{9}$
3
9

1

2

3

4

(4)

A(g) + B(g) → C(g) + D(g)

Since it is an elementary reaction, r = k[A][B]. i.e. it is a second order reaction. Reducing volume three times increases concentration of both A and B three times. So rate of reaction becomes 3 × 3 = 9 times.

Q 5 :

Rate law for a reaction between A and B is given by $R = k {[A]}^{n} {[B]}^{m}$

If concentration of A is doubled and concentration of B is halved from their initial value, the ratio of new rate of reaction to the initial rate of reaction $(\frac{r_{2}}{r_{1}})$ is [2025]

$2^{(n - m)}$
$(n - m)$
$(m + n)$
$\frac{1}{2^{m + n}}$

1

2

3

4

(1)

$r_{1} = k {[A]}^{n} {[B]}^{m}$

$r_{2} = k {[2 A]}^{n} {[\frac{B}{2}]}^{m}$

$\frac{r_{2}}{r_{1}} = \frac{k {[2 A]}^{n} {[\frac{B}{2}]}^{m}}{k {[A]}^{n} {[B]}^{m}}$

$\frac{r_{2}}{r_{1}} = 2^{n - m}$

Q 6 :

$A (g) \to B (g) + C (g)$ is a first order reaction. [2025]

Time T $\infty$

$P_{system}$ $P_{t}$ $P_{\infty}$

The reaction was started with reactant A only.

Which of the following expression is correct for rate constant $k$ ?

$k = \frac{1}{t} \ln \frac{2 (P_{\infty} - P_{t})}{P_{t}}$
$k = \frac{1}{t} \ln \frac{P_{\infty}}{P_{t}}$
$k = \frac{1}{t} \ln \frac{P_{\infty}}{2 (P_{\infty} - P_{t})}$
$k = \frac{1}{t} \ln \frac{P_{\infty}}{(P_{\infty} - P_{t})}$

1

2

3

4

(3)

$\begin{matrix} A(g) & \to & B(g) & + & C(g) \\ t =0 (press) & P_{o} & 0 & 0 \\ t = t (press) & P_{o} - p & p & p & Total = P_{o} + p = P_{t} - I \\ t =∞ (press) & 0 & P_{o} & P_{o} & Total = 2 P_{o} = P_{\infty} - I I \end{matrix}$

From I and II:

$P_{o} = \frac{P_{\infty}}{2}, p = P_{t} - \frac{P_{\infty}}{2}$

For first order reaction, rate constant $k$ is:

$k = \frac{2.303}{t} \log \frac{P_{o}}{P_{o} - p}$

$= \frac{2.303}{t} \log \frac{\frac{P_{\infty}}{2}}{\frac{P_{\infty}}{2} - (P_{t} - \frac{P_{\infty}}{2})}$

$= \frac{2.303}{t} \log \frac{P_{\infty}}{2 (P_{\infty} - P_{t})}$

Q 7 :

For certain chemical reaction $X \to Y$ , the rate of formation of product is plotted against the time as shown in the figure. The number of correct statement/s from the following is _______ [2023]

[IMAGE 148]

(A) Over all order of this reaction is one
(B) Order of this reaction can’t be determined
(C) In region I and III, the reaction is of first and zero order respectively
(D) In region-II, the reaction is of first order
(E) In region-II, the order of reaction is in the range of 0.1 × 0.9.

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(2)

(B) Order of this reaction can’t be determined.

(C) In region I and III, the reaction is of first and zero order respectively.

Q 8 :

For conversion of compound $A \to B$ , the rate constant of the reaction was found to be $4.6 \times 10^{- 5}$ L ${mol}^{- 1} s^{- 1}$ . The order of the reaction is _______. [2023]

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(2)

For $n^{th}$ order reaction

Units of $k = \frac{Rate}{{(conc)}^{n}} = {mol}^{1 - n} L^{n - 1} s^{- 1}$

For a given reaction, unit of $k = {mol}^{- 1} {L sec}^{- 1}$

$∴$ $Order of the reaction (n) = 2$

Q 9 :

${KClO}_{3} + 6 {FeSO}_{4} + 3 H_{2} {SO}_{4} \to KCl + 3 {Fe}_{2} {({SO}_{4})}_{3} + 3 H_{2} O$

The above reaction was studied at 300 K by monitoring the concentration of ${FeSO}_{4}$ in which initial concentration was 10 M and after half an hour became 8.8 M. The rate of production of ${Fe}_{2} {({SO}_{4})}_{3}$ is ______ $\times 10^{- 6}$ ${mol L}^{- 1} s^{- 1} .$ (Nearest Integer) [2023]

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(333)

${KClO}_{3} + 6 {FeSO}_{4} + 3 H_{2} {SO}_{4} \to KCl + 3 {Fe}_{2} {({SO}_{4})}_{3} + 3 H_{2} O$

$ROR = - \frac{Δ [{KClO}_{3}]}{Δ t} = \frac{- 1}{6} \frac{Δ [{FeSO}_{4}]}{Δ t}$

$= \frac{+ 1}{3} \frac{Δ [{Fe}_{2} {({SO}_{4})}_{3}]}{Δ t}$

$\frac{Δ [{Fe}_{2} {({SO}_{4})}_{3}]}{Δ t} = \frac{1}{2} \frac{- Δ [{FeSO}_{4}]}{Δ t}$

$= \frac{1}{2} \frac{(10 - 8.8)}{30 \times 60}$

$= 0.333 \times 10^{- 3}$

$= 333 \times 10^{- 6} {mol litre}^{- 1} {sec}^{- 1}$

Topic Question Set

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Q 4 :

Consider an elementary reaction A(g) + B(g) → C(g) + D(g)

If the volume of reaction mixture is suddenly reduced to $\frac{1}{3}$ of its initial volume, the reaction rate will become ‘ $x$ ’ times of the original reaction rate. The value of $x$ is: [2025]

Q 5 :

Rate law for a reaction between A and B is given by $R = k {[A]}^{n} {[B]}^{m}$

If concentration of A is doubled and concentration of B is halved from their initial value, the ratio of new rate of reaction to the initial rate of reaction $(\frac{r_{2}}{r_{1}})$ is [2025]

Q 6 :

$A (g) \to B (g) + C (g)$ is a first order reaction. [2025]

Time T $\infty$

$P_{system}$ $P_{t}$ $P_{\infty}$

The reaction was started with reactant A only.

Which of the following expression is correct for rate constant $k$ ?

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Q 8 :

For conversion of compound $A \to B$ , the rate constant of the reaction was found to be $4.6 \times 10^{- 5}$ L ${mol}^{- 1} s^{- 1}$ . The order of the reaction is _______. [2023]

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