Chemistry

Chemical Kinetics

276 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

Reaction $A+B\longrightarrow C+D$ follows rate law, $r=k{ \left[ A \right]  }^{ 1/2 }{ \left[ B \right]  }^{ 1/2 }$ starting with $1M$ of $A$ and $B$ each. What is the time taken for concentration of $A$ become $0.1M$?
[Given $2.303\times { 10 }^{ -2 }sec^{ -1 }$].

  1. $10sec$
  2. $100sec$
  3. $1000sec$
  4. $434sec$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The rate constant is $2.303\times { 10 }^{ -2 }sec^{ -1 }$. The overall reaction order is 1.


$t = \dfrac {2.303}{k}log \dfrac {a}{a-x}$

$t = \dfrac {2.303}{2.303 \times 10^{-2}}log \dfrac {1}{0.1}=100$

Hence, the time taken to consume 90% of concentration is 100 sec.

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 for a given reaction is $M^{1-n}L^{n-1}t^{-1}$ where n is order of reaction.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The statement is true. The unit of rate constant for a given reaction is  $M^{1-n}L^{n-1}t^{-1}$ where $n$ is order of reaction.It is generally form for expressing rate constant. Rates are usually given in concentrations units over time units. The units of the rate constants depend upon the order of the reaction. The concentration is molarity and time is in seconds.

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

A 22.4 litre flask contains 0.76 mm of ozone at $25^oC$. Calculate:
(i) the concentration of oxygen atom needed so that the reaction, $O+O _3\rightarrow 2O _2$ having rate constant equal to $1.5\times 10^7$ litre $mol^{-1} sec^{-1}$ can proceed with a rate of 0.15 mol $litre^{-1} sec^{-1}$
(ii) the rate of formation of oxygen under this condition.

  1. (i) $2.45\times 10^{-4}$, (ii) $0.30 mol L^{-1} t^{-1}$
  2. (i) $4.9\times 10^{-4}$, (ii) $0.60 mol L^{-1} t^{-1}$
  3. (i) $2.45\times 10^{-5}$, (ii) $0.30 mol L^{-1} t^{-1}$

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

$\displaystyle [O _3] = \frac {P}{RT} = \frac {0.76}{760 \times 0.0821 \times 298} = 4.08 \times 10^{-5} $
$\displaystyle [O] = \frac {rate}{k \times [O _3]} = \frac {0.15}{1.5 \times 10^{7} \times 4.08 \times 10^{-5} = 2.45 \times 10^{-4}  }$
The rate of formation of oxygen under this condition is equal to twice the rate of the reaction.
It is equal to $\displaystyle 2 \times 0.15 : mol litre^{-1} sec^{-1} = 0.30 : mol litre^{-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 and value of rate constant and that of rate of reaction are same for:

  1. first order

  2. zero order

  3. second order

  4. all are wrong
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

As we know,
For zero order reaction, $\frac {dx}{dt}=K$.
so, unit and value of rate constant and that of rate of reaction are same for zero order.

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 $n^{th}$ order has units:

  1. $Litre^{-1} mol^{1-n} sec^{-1}$
  2. $Mol^{1-n} litre^{1-n} sec^{-1}$
  3. $Mol^{1-n} litre^{n-1} sec^{-1}$
  4. $Mol^{1-n^{2}} litre^{n^{2}} sec^{-1}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

As we know,
For nth order; unit of rate constant may be derived by
$K=\frac {rate}{[reactant]^n}$
$=Mol^{1-n} litre^{n-1} sec^{-1}$

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

A reaction proceeds in three stages. The first stage is slow and involves two molecules of reactants. The second and third stage are fast. The overall order of the reaction is:

  1. first order

  2. second order

  3. third order

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

The overall order of the reaction is second order.
Reactions are proceeded in many steps, it does not means that no. of step will be equal to order of reaction. But rate is determined only by slow step of mechanism.

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

The rate of the reaction, $A+B+C\rightarrow P$; is given by; $r=-\frac {d[A]}{dt}=K[A]^{1/2}[B]^{1/2}[C]^{1/4}$. The order of the reaction is:

  1. 1

  2. 2

  3. 1/2

  4. 5/4
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

As we know,

For a reaction, $A+B\rightarrow Product$, if rate $=K[A]^m[B]^n$, the order of reaction $=m+n$.

So here for the given reaction, order of reaction $= 1/2+1/2+1/4 = 5/4$

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

If the concentration is measured in mol $L^{-1}$ and time in minutes, the unit for the rate constant of a third order reaction is:

  1. mol $L^{-1}\, min^{-1}$
  2. $L^{2}\, mol^{-2}\, min^{-1}$
  3. $L\, mol^{-1}\, min^{-1}$
  4. $min^{-1}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

As we know,
$k\, =\, [conc]^{1-n}\, min^{-1}$
For third order reaction = $[mol\, L^{-1}]^{1-3}\, min^{-1}$
$L^2\, mol^{-2}\, min^{-2}$

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

For which of the following reactions, the units of rate constant and rate of reaction are same ?

  1. First order reaction

  2. Second order reaction

  3. Third order reaction

  4. Zero order reaction
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

For zero order,
R = k[A]
Unit of rate = mol $L^{-1}\, t^{-1}$
Unit of zero order = mol $L^{-1}\, t^{-1}$

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

If a reaction involves gaseous reactants and products, the units of its rate are:

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

If a reaction involves gaseous reactants and products, the unit of its rate is $atm\ s^{-1}$. It represents the change in pressure (in atm units) in unit time ($1$ second).

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

The rate of certain hypothetical reaction A + B + C $\rightarrow$ Products, is given by $\displaystyle r\, =\, - \frac{dA}{dt}\, = k[A]^{1/2}[B]^{1/3}[C]^{1/4}$ The order of a reaction is given by:

  1. $1$
  2. $\displaystyle \frac{1}{2}$
  3. $2$
  4. $\displaystyle \frac{13}{12}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$A + B + C$ $\rightarrow$ Products
$\displaystyle r\, =\, - \frac{dA}{dt}\, = k[A]^{1/2}[B]^{1/3}[C]^{1/4}$
so order is
$\displaystyle \frac{1}{2}\, +\, \frac{1}{3}\, +\, \frac{1}{4}\, =\,\frac{13}{12}$

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

Consider the following elementary reaction,
    $2A + B + C \rightarrow Products$.
All reactant are present in the gaseous state and reactant C is taken in excess.

What is the unit of rate constant of the reaction?

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

Since C is taken in excess, so its concentration does not change. So it is not taken in rate expression of the reaction.


and rate law is given by,

$Rate = k[A]^{2}[B]$

Order of the reaction $= 3$

 For the third-order reaction, the unit of the rate constant is given by -

      $= [conc.]^{1 - n} {time}^{-1}= [conc.]^{1 - 3} {time}^{-1}=\, mol^{-2} L^{2} time^{-1}$