Chemistry

Chemical Kinetics

256 Questions

Chemical kinetics involves the study of chemical reaction rates and the factors affecting them, such as temperature and concentration. This topic covers rate laws, half-life, and zero, first, and second order reactions. It is a crucial part of the chemistry syllabus for various competitive examinations.

Reaction rate parametersFirst order kineticsZero and second orderHalf-life of reactionRate constant units

Chemical Kinetics Questions

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

Units of rate constants for first and zero order reactions in terms of molarity $M$ unit are respectively:

  1. ${ sec }^{ -1 },M\ { sec }^{ -1 }$
  2. ${ sec }^{ -1 },M$
  3. $M\ { sec }^{ -1 },{ sec }^{ -1 }$
  4. $M,{ sec }^{ -1 }$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$K=\frac{dx}{dt[A]} $ for Ist order = $sec^{−1}$
$ K = \frac{dx}{dt}$ for zero order = $mol \space litre^{−1}sec^{−1}$

Hence A is the correct answer.

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

Consider the reaction, $2A+B\longrightarrow $ Products. When the concentration of $B$ alone was doubled, the half-life did not change. When the concentration of $A$ alone was doubled, the rate increased by two times. The unit of the rate constant for this reaction is:

  1. ${ s }^{ -1 }$
  2. $L$ ${ mol }^{ -1 }{ s }^{ -1 }$

  3. Unitless
  4. $mol$ ${ L }^{ -1 }{ s }^{ -1 }$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Concentration change in '$B$' does not change half life, it means the reaction is first order with respect to $B$. When concentration of only '$A$' is doubled, the rate of reaction becomes double, thus order with respect to $A$ will also be one.
Overall order of reaction $=2$
Unit of rate constant $=L$ ${ mol }^{ -1 }{ s }^{ -1 }$

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

Consider following two reactions:
$A\longrightarrow $ Product,         $-\dfrac { d\left[ A \right]  }{ dt } ={ k } _{ 1 }{ \left[ A \right]  }^{ 0 }$
$B\longrightarrow $ Product,         $-\dfrac { d\left[ B \right]  }{ dt } ={ k } _{ 2 }{ \left[ B \right]  }^{ 1 }$
${ k } _{ 1 }$ and ${ k } _{ 2 }$ are expressed in terms of molarity $\left( mol\ { L }^{ -1 } \right) $ and time $\left( { s }\right) $ as:

  1. ${ s }^{ -1 },M{ s }^{ -1 }{ L }^{ -1 }$
  2. $M{ s }^{ -1 },M{ s }^{ -1 }$
  3. ${ s }^{ -1 },{ M }^{ -1 }{ s }^{ -1 }$
  4. $M{ s }^{ -1 },{ s }^{ -1 }$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The units of rate of reaction is $Ms^{-1}$.

For zero order reaction, unit of K, will be $Ms^{-1}$
For 1st order reaction, unit of $K _2$ will be $s^{-1}$

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

Rate law expression of a reaction is:
            Rate $=k{ \left[ A \right]  }^{ { 2 }/{ 3 } }\left[ B \right] $
Which of the following are correct about the corresponding reaction?

  1. Order of reaction $=\dfrac { 2 }{ 3 } +1=\dfrac { 5 }{ 3 } $
  2. Unit of rate constant $={ L }^{ { 2 }/{ 3 } }{ mol }^{ { -2 }/{ 3 } }{ sec }^{ -1 }$
  3. Unit of rate constant $={ atm }^{ { -2 }/{ 3 } }{ sec }^{ -1 }$
  4. Unit of rate constant $=mol$ ${ L }^{ -1 }{ sec }^{ -1 }$
Reveal answer Fill a bubble to check yourself
A,B,C Correct answer
Explanation

Overall order=$\frac{2}{3}+1=\frac{5}{3}$

Unit of rate constant=$mol^{1-n}L^{n-1}sec^{-1}$
where n=order of reaction
For given reaction, unit of rate constant=$mol^{1-5/3}L^{5/3-1}sec^{-1}$
=$L^{2/3}mol^{-2/3}sec^{-1}$
For gaseous reaction, concentration term= pressure in atm
unit of rate constant=$atm^{-2/3}sec^{-1}$

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

Mechanism of a hypothetical reaction $X _2+Y _2\rightarrow 2XY$ is given below;
(i) $ X _2\rightarrow X+X$ (fast)
(ii) $X+Y _2\rightleftharpoons XY+Y$ (slow)
(iii) $ X+Y \rightarrow XY$ (fast)
The overall order of the reaction will be: 

  1. $2$
  2. $0$
  3. $3$
  4. $1$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Order of Reaction is given by the slowest step of Reaction. In slowest step we 

have 2 reactants so, $2^{nd}$ order reaction. 

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

A reaction, which is second-order, has a rate constant of $0.002  L\, mol^{-1}\, s^{-1}$. If the initial conc. of the reactant is 0.2 M, how long will it take for the concentration to become 0.0400 M?

  1. 1000 sec

  2. 400 sec

  3. 200 sec

  4. 10,000 sec
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
$\dfrac{1}{a}=\dfrac{1}{a _{0}}+kt$
$\dfrac{1}{0.04}=\dfrac{1}{0.2}+0.002t$
$t=10000sec$
Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

For the reaction $CO(g)+2{ H } { 2 }(g)\rightleftharpoons { CH } _{ 3 }OH(g)$. If active mass of $CO$ is kept constant and active mass of ${H} _{2}$ is tripled, the rate of forward reaction will become _____ of its initial value.

  1. three times

  2. six times

  3. eight times

  4. nine times
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

For the following reaction, Rate is defined as


$rate = k[CO][{{H} {2}}]^{2}$
Now, the active mass of CO is kept constant and active mass of {H}{2} is tripled. Now the rate is,

${rate}^{'} =k[CO][{3 \times {H} _{2}}]^{2}$
${rate}^{'} = 9 \times rate$
 
So, the rate of forward reaction will become nine times of its initial value.

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

The reaction $2{NO} _{(g)}+{H} _{2(g)}\longrightarrow {N} _{2}{O} _{(g)}+{H} _{2}{O} _{(g)}$ follows the rate law $\cfrac { d{ P } _{ \left( { N } _{ 2 }O \right)  } }{ dt } =k{ \left( { P } _{ NO } \right)  }^{ 2 }{ p } _{ { H } _{ 2 } }$. If the reaction is initiated with ${P} _{NO}=1000mm$ $Hg$ and ${ p } _{ { H } _{ 2 } }=10mm$ $Hg$, then the reaction will follow:

  1. third order kinetics

  2. second order kinetics

  3. first order kinetics

  4. zero order kinetics
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The rate law is given as Rate = k[P_NO]^2 [P_H2]. Since the initial concentration of NO (1000 mm Hg) is much larger than the concentration of H2 (10 mm Hg), the concentration of NO remains effectively constant throughout the reaction. Thus, the reaction behaves as pseudo-first order with respect to H2.

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

For a reaction $r=K{[CH _3COCH _3]}^{3/2}$. The unit of rate of reaction and rate constant respectively is:

  1. $mol \displaystyle L^{-1}s^{-1},\quad mol^{-\frac{1}{2}}L^{\frac{1}{2}}s^{-1}$
  2. $\displaystyle mol^{-1}L^{-1}s^{-1},\quad mol^{-\frac{1}{2}}L^{-\frac{1}{2}}s^{-1}$
  3. $\displaystyle mol L^{-1}s^{-1},\quad mol^{\frac{1}{2}}L^{\frac{1}{2}}s^{-1}$
  4. $mol Ls,\quad \displaystyle mol^{\frac{1}{2}}L^{\frac{1}{2}}s$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For $1.5$ order rate law the units are $molL^{-1}s^{-1}$ for the rate while the [rate constant]$=\cfrac{molL^{-1}s^{-1}}{mol^{3/2}L^{-3/2}}$

$=mol^{-1/2}L^{1/2}s^{-1}$

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

Which of the following corresponds to the units of rate constant for n$^{th}$ order reaction ?

  1. $mole^{n-1} l^{1-n} s^{-1}$
  2. $mole^{n-1} l^{n-1} s^{-1}$
  3. $mole^{1-n} l^{n-1} s^{-1}$
  4. $mole^{n-1} l^{n} s^{-1}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$ r= K\left [ A \right ]^{n}$

$K = \dfrac{r}{\left [ A \right ]^{n}}= \dfrac{mole \ l^{-1} \ sec^{-1}}{mole^{n} \ l ^{-n}}$ $= mole^{1-n} 1^{n-1} sec^{-1}$

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

The unit of rate of a first order reaction is:

  1. $mol\ lit^{-1}$
  2. $l\ mol^{-1} \ s^{-1}$
  3. $s^{-1}$
  4. $l^2 \ mol^{-2} \ s^{-1}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

For a first order reaction; rate law can be wriiten as; $r = k[A]^{1}$
Therefore k = $\dfrac{r}{[A]} = \dfrac{mol \times l^{-1} \times  s^{-1}}{mol \times l^{-1}}$ = $s^{-1}$ where concentration of $A =$ moles per litre and rate of reaction; r = change in concentration of $A$ with time.

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

Compound $A$ and $B$ react to form $C$ and $D$ in a reaction that was found to be second-order over all and second-order in $A$. The rate constant -at ${ 30 }^{ 0 }C$ is $0.622$ L ${ mol }^{ -1 }{ min }^{ -1 }$. What is the half-life of A when $4.10\times { 10 }^{ -2 }$ M of A is mixed with excess $B$?

  1. $40$ min
  2. $39.21$ min
  3. $28.59$ min

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$A+B\longrightarrow C+D$


 rate$=k{ [A] }^{ 2 }$ (given)

$ =0.622{ [4.10\times { 10 }^{ -2 }] }^{ 2 }$

$ =0.001$  is the rate of reaction initially

 Half-life$={ t } _{ 1/2 }=\cfrac { 1 }{ K[A] } =\cfrac { 1 }{ 0.622\times [4.1\times { 10 }^{ -2 }] } \\ =0.3921\times { 10 }^{ 2 }\\ =39.21\quad minutes.$

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

The decomposition of dimethyl ether leads to the formation of $CH _4, H _2$ and CO and the reaction rate is given by $Rate=k[CH _3OCH _3]^{\frac {3}{2}}$
The rate of reaction is followed by increase in pressure in a closed vessel, so the rate can also be expressed in terms of the partial pressure of dimethyl ether, i.e., $Rate=k(P _{CH _3OCH _3})^{\frac {3}{2}}$
If the pressure is measured in bar and time in minutes, then the unit of rate constants is:

  1. $bar^{\frac {1}{2}} min$
  2. $bar^{\frac {3}{2}} min^{-1}$
  3. $bar^{-\frac {1}{2}} min^{-1}$
  4. $bar min^{-1}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

As $Rate=k(P _{CH _3OCH _3})^{\frac {3}{2}}$
$bar/min=k(bar)^{\frac {3}{2}}$
$\therefore$ unit of k$=bar^{-\frac {1}{2}}min^{-1}$

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

Taking the reaction, $A + 2B\rightarrow Products$, to be of the second order, which of the following may be the correct rate law expressions?

  1. $\frac {dx}{dt}=k[A][B]$
  2. $\frac {dx}{dt}=k[A][B]^2$
  3. $\frac {dx}{dt}=k[A]^2$
  4. $\frac {dx}{dt}=k _1[A]+k _2[B]^2$
Reveal answer Fill a bubble to check yourself
A,C Correct answer
Explanation

option A and C are correct as the sum of their exponents equals to 2

Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

The rate constant of a second order reaction is $10^{-2} lit.mole ^{-1}.sec^{-1}$. The rate constant when expressed as $cc. \ molecule^{-1} .\ min^{-1}$ is:

  1. $9.96\times 10^{-22}$
  2. $9.96\times 10^{-23}$
  3. $9.96\times 10^{-21}$
  4. $9.96\times 10^{-24}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The rate constant of a second order reaction is $10^{-2} lit.mole ^{-1}.sec^{-1}$.
$1L=1000cc$


$1 mole = 6.023\times 10^{23}$molecules
$1min=60sec$
Hence, rate constant $=10^{-2} lit.mole ^{-1} sec^{-1}\times \dfrac {1000cc}{1L} \times \dfrac {1mole} {6.023\times 10^{23}molecules} \times \dfrac {60 sec} {1 min}$
$=9.96\times 10^{-22}cc\ molecule^{-1} \ min^{-1}$