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 how far? how fast? collision theory collision theory of chemical reactions rate of chemical reaction

Among the following which will decrease the rate of the reaction?
i. Using highly concentrated reactants
ii. Decreasing the temperature by $25\ K$
iii. Stirring the reactants

  1. i only

  2. ii only

  3. i and iii only

  4. ii and iii only

  5. i, ii, and iii
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\bullet $  Using highly concentrated reactants will increase the rate of the reaction as rate is directly proportional to the 

    concentration of reactants.
$\bullet $  Decreasing the temperature by 25 K will decrease the rate constant and hence the rate of reaction will decrease.
$\bullet $  Stirring the reactants increases the rate of interaction between the reactants and hence the rate of reaction increases.
$\therefore $ The correct answer is (ii) only.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

For a chemical reaction, $A \rightarrow products$, the rate of reaction doubles when the concentration of A is increased by a factor of 4, the order of reaction is :

  1. 2

  2. 0.5

  3. 4

  4. 1
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
let x be order of reaction and K be rate of constant.
* A$\rightarrow $ products.
$ \cfrac { K\cfrac { { \left( A \right)  }^{ x } }{ time }  }{ K\cfrac { { \left( 4A \right)  }^{ x } }{ time }  } or,\quad \cfrac { 1 }{ 2 } = { \left( \cfrac { A }{ 4A }  \right)  }^{ x }\\ or,\quad \cfrac { 1 }{ 2 } ={ \left( \cfrac { 1 }{ 4 }  \right)  }^{ x }$      (reciprocating)
$\\ or,\quad 2={ \left( 4 \right)  }^{ x }\\ or,\quad 2={ { 2 }^{ 2 } }^{ x }\\ or,\quad 2x=1\\ or,\quad x=\cfrac { 1 }{ 2 } \\ x=0.5$
Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

The term $-\dfrac{dc}{dt}$ in a rate equation refers to:

  1. the concentration of a reactant

  2. the decrease in concentration of the reactant with time

  3. the velocity constant of reaction

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

The term $-\dfrac{dc}{dt}$ in a rate expression indicates the decrease in concentration of the reactant with time. It is the minus sign which is used to show the decrease in concentration of the reactant.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

For a first order reaction, A$\rightarrow$ products, the concentration of A changes from $0.1$M to $0.025$ M $80$ minutes. The rate of reaction when the concentration of A is $0.01$M, is:

  1. $1.73\times 10^{-5}$M/min
  2. $3.47\times 10^{-4}$M/min
  3. $3.47\times 10^{-5}$M/min
  4. $1.73\times 10^{-4}$M/min
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

K$= \frac { 2.303 }{ t } \log { \left[ \dfrac { { A } _{ 0 } }{ A }  \right]  } $


$\quad =\dfrac { 2.303 }{ 80 } \log [{ \dfrac { 0.1 }{ 0.025 }  }]$

$k= 0.0173\quad { min }^{ -1 }$

$\therefore $ Rate$= K\left[ A \right]$

$\quad \quad \quad =0.0173\left[ 0.01 \right]$

$\quad \quad \quad = 1 .73 \times { 10 }^{ -4 } $

$\quad \quad \quad =1.73\times{ 10 }^{ -4 }M/min$

Hence, the correct option is $(D)$

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

Which of the following is (are) true for first order reaction?

  1. Rate of reaction is fastest at the beginning of reaction.

  2. Rate of reaction is fastest when (reactants)=(products)

  3. Rate of reaction increases with temperature

  4. Ea decreases considerably as temperature as temperature increases, hence the reaction becomes faster
Reveal answer Fill a bubble to check yourself
A,C Correct answer
Explanation

A] True. for first order reactions, products are formed at the fastest rate during the  beginning of the reaction.

B] False. When reactants>Products, the rate of reaction is fast.
C] True. Increasing the temperature increases the rate of reaction.
D] False. Activating energy does not decrease, but increasing the temperature allows crossing of energy barrier.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

For the reaction $2A + B + C \rightarrow 2D$. The observed rate law is Rate=$K[A]{ [B] }^{ 2 }$. Correct statements are
a) An increase of cone .of C does not affect the rate
b)Doubling the conc of A doubles the rate
c)Tripling the conc of B increases the rate by 9 times
d)Doubling the conc of C, doubling the rate

  1. a,b,c

  2. b,c,d

  3. d

  4. c,d
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Given  rate is $k[A]{ [B] }^{ 2 }$

 It is overall ${ 3 }^{ rd }$ order  reaction with a following  first order  and B following second order.
 (a) True  C is not affecting the rate of reaction.
 (b) True $rate _{ 1 }\quad =k[A]{ [B] }^{ 2 }\ rate _{ 2 }\quad =k[2A]{ [B] }^{ 2 }=2k[A]{ [B] }^{ 2 }\ =2\times rate _{ 1 }$
 (c) True $rate _{ 1 }\quad =k[A]{ [B] }^{ 2 }\ rate _{ 3 }=k[A]{ [3B] }^{ 2 }=9Kk[A]{ [B] }^{ 2 }\ =9\times rate _{ 1 }$
(d)False C does not affected  the rate in any manner.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

${ SO } _{ 2 }$ react with ${ O } _{ 2 }$ as follows :


 ${ 2SO } _{ 2 }+{ O } _{ 2 }\rightarrow { 2SO } _{ 3}$ 


The rate of disappearance of ${ SO } _{ 2 }$ is $2.4\times { 10 }^{ -4 }$ mol ${ lit }^{ -1 }{ min}^{ -1 }$, then :

  1. Rate of reaction is $1.2\times { 10 }^{ -4 }\quad mole{ lit }^{ -1\quad }{ min }^{ -1 }$
  2. Rate of appearance of ${ SO } _{ 3 }$ is $2.4\times { 10 }^{ -4 } { mole\quad lit }^{ -1 }min^{ -1 }$
  3. Rate of disappearance of ${ O } _{ 2 }$ is $1.2\times { 10 }^{ -4 } { mole\quad lit }^{ -1 }min^{ -1 }$
  4. Rate of reaction is twice the rate of disappearance of ${ SO } _{ 2 }$
Reveal answer Fill a bubble to check yourself
B,C,D Correct answer
Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

Rate of formation of $SO _{3}$ according to the reaction $2SO _{2}+O _{2} \rightarrow 2SO _{3}$ is $1.6 \times 10^{-3}\ kg\ min^{-1}$ Hence rate at which $SO _{2}$ reacts is :-

  1. $1.6 \times 10^{-3}\ kg\ min^{-1}$
  2. $8.0 \times 10^{-4}\ kg\ min^{-1}$
  3. $3.2 \times 10^{-3}\ kg\ min^{-1}$
  4. $1.28\times 10^{-3}\ kg\ min^{-1}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

$C _{4}H _{8}\rightarrow 2C _{2}H _{4}$; rate constant $=2.303\times 10^{4}\sec^{-1}$, After what time the molar ratio of $\dfrac{C _{2}H _{4}}{C _{4}H _{8}}$ attain the value $1$

  1. $176\ sec$
  2. $3522\ sec$
  3. $1661\ sec$
  4. $1761\ sec$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For the reaction C4H8 -> 2C2H4, if the ratio [C2H4]/[C4H8] = 1, then [C2H4] = [C4H8]. Using the integrated rate law for a first-order reaction, we solve for t.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

On increasing the pressure three fold, the rate of reaction of ${ 2H } _{ 2 }{ S }$ + ${ O } _{ 2 }$ $\rightarrow $ products would increase

  1. 3 times

  2. 9 times

  3. 12 times

  4. 27 times
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Rate$=$ ${K _{{p^2}}}{H _2}S \times P{O _2} = x$ $($rate$)$


On increasing the pressure three fold :


Rate$=$ $K{\left( {3{P _{{H _2}S}}} \right)^2}\left( {3P{O _2}} \right)$

$=$ $K \times 9{P^2} _{{H _2}S} \times 3P{O _2}$

$=$ $K \times 27 \times {P _{{H _2}S}} \times P{O _2} = 27$

the rate will increases $27$ times 

Hence, option $(D)$ is correct answer.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

Which of the following is not a valid way to describe the rate of the following reaction?
$A + B + C \rightarrow D + E$

  1. $\dfrac {-\triangle [A]}{\triangle t}$
  2. $\dfrac {-\triangle [B]}{\triangle t}$
  3. $\dfrac {-\triangle [C]}{\triangle t}$
  4. $\dfrac {-\triangle [D]}{\triangle t}$
  5. $\dfrac {-\triangle [E]}{\triangle t}$
Reveal answer Fill a bubble to check yourself
D,E Correct answer
Explanation

$A+B+C\longrightarrow D+E$

 Rate of reaction is defined as the change in concentration of reactant or product to time.
 Rate $ (R)=\cfrac { -d[A] }{ dt } =\cfrac { -d[B] }{ dt } =\cfrac { -d[C] }{ dt } =\cfrac { d[D] }{ dt } =\cfrac { d[E] }{ dt } $
Therefore, (D)  &  (E) i.e. $ \cfrac { -d[D] }{ dt } & \quad \cfrac { -d[E] }{ dt } $ respectively are not valid ways of describing the rate of the following reaction.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

The rate constant of the relation $ A \rightarrow B $ is $ 0.6 \times 10^{-3} $ mole per second. If the concentration of $B$ after $20$ minutes is :

  1. $0.36$ M
  2. $0.72$ M
  3. $1.08$ M
  4. $3.60$ M
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

By unit of rate constant it is clear,that the $reaction$ is zero order. 

$\therefore \left [ B \right ]=k\times t$
         $=0.72M$

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

The rate law for a reaction between the substances $A$ and $B$ is given by rate$=k{ \left[ A \right]  }^{ n }{ \left[ B \right]  }^{ m }$. On doubling the concentration of $A$ and having the concentration of $B$ halved, the ratio of the new rate to the earlier rate of the reaction will be as:

  1. $\cfrac { 1 }{ { 2 }^{ m+n } } $
  2. $(m+n)$
  3. $(n-m)$
  4. ${2}^{(n-m)}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Given that

                    $R=K[A]^n[B]^m$
after doubling the concentration of $A$ and concentration of $B$ is halfed 
$R^1=K[2A]^n[\dfrac{B}{2}]^m$
$R^1 =(2)^{n-m} R$
$ \dfrac{R^1}{R}= \dfrac{2^{n-m}}{1}$

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

The rate equation for the reaction $2A+B \rightarrow C$ is found to be rate = $k[A] [B]$. The correct statement in relation to this reaction is that the :

  1. units of $k$ must be$\ mol^{-1} L$ $s^{-1}$.
  2. $t _{1/2}$ is constant

  3. rate of formation of C is twice the rate of disappearance of A
  4. value of $k$ is independent of the initial concentration of A and B
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The given reaction is $2A+B\longrightarrow C$

The given rate equation is:-
$rate=K[A] [B]$

The unit of rate is $mol L^{-1} s^{-1}$
Unit of $[A]= mol L^{-1}$
Unit of $[B]= mol L^{-1}$

Unit of $K$=$\cfrac {mol L^{-1} s^{-1}}{mol L^{-1} mol L^{-1}}$
$=mol^{-1} L$ $s^{-1}$.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

The reaction $A(g)+2B(g)\rightarrow C(g)+D(g)$ is an elementary process. In an experiment in volving this reaction. The initial pressure of A and B are $P _A=0.6$ atm $P _B=0.8$atm respectively when $P _C=0.2$ atm, the rate of reaction relative to the initial rate is:

  1. $\displaystyle\frac{1}{6}$
  2. $\displaystyle\frac{1}{12}$
  3. $\displaystyle\frac{1}{36}$
  4. $\displaystyle\frac{1}{18}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
        $A(g) + 2(B) \rightarrow C(g) + D(g)$
 t = 0  0.6         0.8              0     0
 at t     0.6-x      0.8-x          x       x
since this the elementary reaction
rate,r = $K[B]^2 [A]$
now $r _i = k (0.6)(0.8)^2 = 0.38K$
when $P _i = x - 0.2$ atm
when $P _A= 0.6-x =0.4$ atm
when $P _B= 0.8 - 2x =0.4$ atm
$r _f = K(0.4) (0.4)^2 = 0.064K$
$r _1/r _2 = 0.064/0.384 = 1/6$