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

The unit of rate constant obeying the rate expression, $r=k{ \left[ A \right]  }{ \left[ B \right]  }^{ { 2 }/{ 3 } }$ is:

  1. ${ mol }^{ { -2 }/{ 3 } }\ { litre }^{ { 2 }/{ 3 } }\ { time }^{ -1 }$
  2. ${ mol }^{ { 2 }/{ 3 } }\ { litre }^{ { -2 }/{ 3 } }\ { time }^{ -1 }$
  3. ${ mol }^{ { -5 }/{ 3 } }\ { litre }^{ { 5 }/{ 3 } }\ { time }^{ -1 }$

  4. none of the above
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The unit of rate constant is ${ mol }^{ { -2 }/{ 3 } }\ { litre }^{ { 2 }/{ 3 } }\ { time }^{ 2/3 }$ and it does not match with any of the A, B, C options. So option D is correct

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

For the second order reaction, if the concentration of reactant changes from $0.08M$ to $0.04M$ in 10 minutes. Calculate the time at which concentration of reactant becomes $0.01M$.

  1. $20min$
  2. $30min$
  3. $50min$
  4. $70min$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

For a second order reaction we have 

$ \cfrac { 1 }{ { [A] } _{ t } } -\cfrac { 1 }{ [A] _{ o } } =kt$
 In $10$ minutes,  concentration changes from $0.08M $ to $0.04M$
$ \left( \cfrac { 1 }{ 0.04 } -\cfrac { 1 }{ 0.08 }  \right) \cfrac { 1 }{ 10 } =k{ \quad min }^{ -1 }\ 1.25{ min }^{ -1 }=k$ 
Substituting value of $k$, we get it become $ \left( \cfrac { 1 }{ 0.01 } -\cfrac { 1 }{ 0.08 }  \right) \cfrac { 1 }{ 1.25 } =t\ 70min=t$
 Hence answer is $[D]$

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

For a second order reaction rate at a particular time is $x$. Ifthe initial concentration is trapled, the rate will becomes?

  1. $3x$
  2. $9x^{2}$
  3. $9x$
  4. $27x$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Its a second Order reaction.


The rate at a particular time is x. 

$x = k{[A]}^{2}$

If the initial concentration is tripled, the rate becomes

$rate = k[{3[A]}^{2}]$ = $9x$

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 for forward and backward reaction of hydrolysis of ester are $1.1\times 10^{-2}$ and $1.5\times 10^{-3}$ per minute respectively.  


Equilibrium constant for the reaction is :


$CH _3COOC _2H _5 + H _2O\    \rightleftharpoons\ CH _3COOH+C _2H _5OH$ 

  1. 4.33

  2. 5.33

  3. 6.33

  4. 7.33
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Rate constant of forward reaction $(K _f)=1.1\times 10^{-2}\ min^{-1}$

Rate constant of backward reaction $(K _b)=1.5\times 10^{-3}\ min^{-1}$ 
Equilibrium constant $(K _c)$$=\dfrac {K _f}{K _b}$

$=\dfrac {1.1\times 10^{-2}}{1.5\times 10^{-3}}$

$=7.33$
Hence, option $(D)$ is correct.

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

The dimensions of rate constant of a second order reaction involves:

  1. neither time nor concentration

  2. only time

  3. time and concentration

  4. time square and concentration
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

second order
$2A \rightarrow product$
$rate=K[A]^{2}$
$k=\frac{rate}{[A]^{2}}$
$=\frac{concentration}{time (concentration )^{2}}$
$=\frac{1}{time . concentration}$
So it involves both time & concentration.

Except for first ofder reactions, the unit of rate constant depends on the dimensions
($mol^{1-n}.lit^{-1}.sec^{-1}  $) concentration and time.
Where, n= order of the reaction.


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

Which is not true for a second order reaction ?

  1. It can have rate constant $1 \times 10^{-2} $ $L mol^{-1} s^{-1} $

  2. Its half - life is inversely propotional to its initial concentration
  3. Time to compelete $75 % $ reaction is twice of half - life
  4. $ T _{50} $ = $ \frac { 1 }{ K\quad \times \quad Initial\quad conc. } $
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Unit of Rate Constant for a second order reaction is ${mol}^{-1}L{s}^{-1}$.


${t} _{1/2}\space \alpha\space {a}^{1-n}$. Here n =2 

${t} _{1/2}\space \alpha\space {a}^{-1}$. So Its Inversely Proportional to Half Life.

${t} _{1/2}$ = $\dfrac{1}{ak}$ where a is initial concentration

TIme to complete 75% reaction is twice to half life is false. Its is true for 1st Order Reaction

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

For a second order reaction rate at a particular time is $X$. If the initial concentration is tripled, the rate will become:

  1. $3X$
  2. $9X^2$
  3. $9X$
  4. $27X$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Its a second Order Reaction. Rate is given as X

$\therefore rate=[x]^2$
When the concentration is tripled then,
$rate=[3x]^2=9x^2$

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

Units of the rate constant of 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

For first order, $M/sec=k[M].$
$\therefore k=sec^{-1}$
For zero order, $M/sec = k[M]^0.$
$\therefore k=M sec^{-1}.$

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 constant obeying the rate expression $r=K[A]^{1}[B]^{2/3}$ is:

  1. $mole^{-2/3}lit^{2/3}time^{-1}$
  2. $mole^{2/3}lit^{-2/3}time^{-1}$
  3. $mole^{-5/3}lit^{2/3}time^{-1}$

  4. none of the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$r=K[A][B]^{2/3}$


$K=\dfrac{r}{[A][B]^{2/3}}$

$K=\dfrac{mol}{liter.sec}$$\left ( \dfrac{liter}{mole} \right )\left ( \dfrac{liter}{mole} \right )^{2/3}$

$K=mole^{-2/3} liter^{2/3} sec^{-1}$

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

The reaction, $2A+ B \rightarrow$ Products, follows the mechanism:


$2A \leftrightharpoons A _2$ (at equilibrium)
$A _2 + B \rightarrow$ Products (slow) 

The order of the reaction is:

  1. $2$
  2. $1$
  3. $3$
  4. $1\dfrac{1}{2}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

From the slow step, $rate = k [A _2][B]$ ...... (1)
From the equilibrium reaction, equilibrium  constant  $K = \dfrac {[A _2]} {[A]^2}$
$[A _2]=K [A]^2$......(2)


Substitute equation (2) in equation (1).
$rate = k K [A]^2[B]$

Thus the overall order of the reaction is $2+1=3$.

Hence, option C is correct.

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

For the elementary reaction 2A $\rightarrow $ C ,the concentration of A after 30 minutes was found to be 0.01 mole/lit. If the rate constant of the reaction is $2.5 \times 10^{-2}$ lit mole$^{-1}$ sec$^{-1}$. The rate of the reaction at 30 minutes is:

  1. $2.5 \times 10^{-4}$ lit mole$^{-1}$ sec$^{-1}$
  2. $2.5 \times 10^{-6}$ lit mole$^{-1}$ sec$^{-1}$
  3. $2.5 \times 10^{-2}$ lit mole$^{-1}$ sec$^{-1}$
  4. $2.5 \times 10^{-8}$ lit mole$^{-1}$ sec$^{-1}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Rate of Reaction = Rate Constant  x ${ [A] }^{ 2 }$

 $= 2.5 \times { 10 }^{ -2 } \times   { [0.01] }^{ 2 }$

$ =  2.5  \times   { 10 }^{ -2 }  \times   { 10 }^{ -4 }$

$=  2.5  \times   { 10 }^{ -6 }$  lit   mol $^{ -1 }$ sec$^{ -1 }$

Hence, the correct option is $\text{B}$
Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

The specific rate of a reaction is $1.51 \times10^{-4}$ lit mole$^{-1}$ sec$^{-1}$. If the reaction is commenced with 0.2 mole lit$^{-1}$ of the reactant, the initial rate of the reaction in mole lit$^{-1}$ sec$^{-1}$ is:

  1. $1.5 \times 10^{-4}$
  2. $3 \times 10^{-5}$
  3. $6 \times 10^{-6}$
  4. $6 \times 10^{-5}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

For a general reaction of order $n$, the units of rate constant is given as $(mol/litre)^{1-n}sec^{-1}$.
So for our question, the unit of rate constant is $(mol/litre)^{-1}sec^{-1}$ so $ 1-n = -1$.


So, the value of $n =2$, so the reaction is of second order.

The rate is given as $r = k[A]^{2}$. Specific rate of equation is the rate for concentration of 1 $mol/litre$. 

From this, the value of k is $1.51 \times1 0^{-4}$.
$r = 1.51 \times 10^{-4}[A]^{2}$.
[A] =  $0.2 mol/litre$. 

Initial rate is $6 \times1 0^{-6}$.

Hence, option C is correct.

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

Read the following table and chose the appropriate option


Rate equation Units of K
I) rate $=$ k[A] a) mol lit$^{-1}$ sec $^{-1}$
II) rate $=$ k[A][B] b) mol$^{-2}$ lit$^{2}$ sec $^{-1}$
III) rate $=$ k[A][B]$^2$ c) sec $^{-1}$
IV) rate $=$ k d) lit mol$^{-1}$ sec $^{-1}$

  1. I - d, II - c, III - a, IV - b

  2. I - c, II - d, III - b, IV - a

  3. I - a, II - b, III - c, IV - d

  4. I - b, II - a, III - d, IV - c
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Units of k $= mole^{1-n} lit^{n-1} sec^{-1}$ , where n is the order of reaction.


For option A, its first order reaction. Rate constant $=sec^{-1}$

For option B, its second order reaction. Rate constant $=mole^{-1} lit sec^{-1}$


For option C, its third order reaction. Rate constant $=mole^{-2} lit^{2} sec^{-1}$

For option D, its zero order reaction. rate constant $=mole/lit sec^{-1}$

Hence, the correct option is $\text{B}$
Multiple choice chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

Identify the reaction order from each of the following rate constants.
(i) $k=2.3 \times 10^{-5} L \quad mol^{-1} \quad s^{-1}$
(ii) $k=3 \times 10^{-4} \quad s^{-1}$

  1. (i) First order (ii) Second order

  2. (i)Second order (ii) First order

  3. (i) Zero order (ii) First order

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

(i) Rate of the reaction is given by,

$Rate=k\times [A]^x$, where x is the order of reaction
$(\dfrac{mol}{L})=2.3\times 10^{-5}L(mol^{-1})(s)^{-1}[A]^x$
$({\dfrac{mol}{L}})^2=constant(s^{-1})[\dfrac{mol}{L}]^x$
Therefore, $x=2$ and the reaction is second order
(ii)$Rate=k\times [A]^x$, where x is the order of reaction
$(\dfrac{mol}{L})=3\times 10^{-4}(s)^{-1}[A]^x$
$({\dfrac{mol}{L}})=constant(s^{-1})[\dfrac{mol}{L}]^x$
Therefore, $x=1$ and reaction is first order