Tag: order of reactions

Questions Related to order of reactions

Consider the reaction represented by the equation represented by the equation
$CH _3Cl(g)\, +\, H _2O(g)\, \rightarrow\, CH _3OH(g)\, +\, HCl(g)$
These kinetic data were obtained for the given reaction concentrations

Initial conc (M) Initial conc (M) Initial rate of disappearance of
$[CH _3Cl]$ $[H _2O]$ $CH _3Cl\, (M\, s^{-1}$
0.2 0.2 1
0.4 0.2 2
0.4 0.4 8

If $H _2O$ is taken in large excess, the order of the reaction will be:

pseudo first order reaction order of reactions chemical kinetics electrochemistry and chemical kinetics chemistry

  1. 1

  2. 2

  3. 3

  4. None of these

Show answer
Correct Option: A
Explanation:

Comparing the rate law, the order w.r.t. $H _2O\, =\, 2$.
w.r.t. $[CH _3Cl]\, =\, 1$
$\therefore \, r\, =\, [CH _3Cl]\, [H _2O]^2$
Overall order = 1 + 2 = 3
If $H _2O$ is in excess, the order of the reaction will be 1.

Rate constant in case of first order reaction is :

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

  1. Inversely proportional to the concentration units

  2. Independent of concentration units

  3. Directly proportional to concentration units

  4. Inversely proportional to the square of concentration units

Show answer
Correct Option: B
Explanation:

For First order of reaction,
$Rate= k [A]$,


$k = \cfrac {mol/L}{sec\times {mol/L}}=sec^{-1}$

Option B is correct.

Fill in the blanks by choosing the correct option;
Order of the reaction is the $X$ of the powers to which concentration terms are raised in experimentally determined rate equation. The unit of first order rate constant is $Y$. The unit of first order rate constant when concentration is measured in terms of pressure and time in minutes is $Z$.

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

  1. $X\rightarrow product, Y\rightarrow mol\ L^{-1} time^{-1}, Z\rightarrow atm\ min^{-1}$

  2. $X\rightarrow sum, Y\rightarrow L\ mol^{-1}time^{-1}, Z\rightarrow atm\ min^{-1}$

  3. $X\rightarrow product, Y\rightarrow L\ mol^{-1}, Z\rightarrow atm\ min^{-1}$

  4. $X\rightarrow sum, Y\rightarrow time^{-1}, Z \rightarrow min^{-1}$

Show answer
Correct Option: D
Explanation:

The order of a chemical reaction is defined as the sum of the powers of the concentration of the reactants in the rate equation of that particular chemical reaction.


General formula for the unit of rate constant = mole$^{(1-n)}$ L$^{(n-1)}$ min$^{-1}$.

For 1$^{st}$ order reaction, 
n = 1
Therefore, unit of rate constant $=$ min$^{-1}$

The unit of rate constant in terms of pressure and time:
  mol $^{( 1-n)}$  L $^{ (n-1)}$ min$^{ -1 }$ or  atm$^{( 1-n) }$ min $^{ -1 }$


For 1$^{st}$ order reaction, 
n = 1
Therefore, the unit is min$^{-1}.$

Hence, the correct answer is option $\text{D}$.

Match the rate law given in column I with the dimensions of rate constant given in column II and mark the appropriate choice.

Column I Column II
(A) $Rate = k[NH _{3}]^{0}$ (i) $mol\ L^{-1} s^{-1}$
(B) $Rate = k[H _{2}O _{2}][I^{-}]$ (ii) $L\ mol^{-1} s^{-1}$
(C) $Rate = k[CH _{3}CHO]^{3/2}$ (iii) $s^{-1}$
(D) $Rate = k[C _{2}H _{5}Cl]$ (iv) $L^{1/2} mol^{-1/2} s^{-1}$
chemistry chemical kinetics dependence of reaction rate on concentration of reactants order of reactions factors influencing rate of a reaction

  1. $(A)\rightarrow (iv), (B) \rightarrow (iii), (C)\rightarrow (ii), (D) \rightarrow (i)$

  2. $(A)\rightarrow (i), (B) \rightarrow (ii), (C)\rightarrow (iii), (D) \rightarrow (iv)$

  3. $(A)\rightarrow (ii), (B) \rightarrow (i), (C)\rightarrow (iv), (D) \rightarrow (iii)$

  4. $(A)\rightarrow (i), (B) \rightarrow (ii), (C)\rightarrow (iv), (D) \rightarrow (iii)$

Show answer
Correct Option: D
Explanation:
(A) $Rate= k[NH _3]^0$
It is zero order reaction.
$\therefore$ Units of rate constant are same as rate.
i.e $ mol\ l^{-1}s^{-1}$

(B) $Rate=k[H _2O _2]^1[I^-]^1$
It is second order reaction as order $=1+1=2$
$\therefore$ Units of Rate constant are $L \ mol^{-1}s^{-1}$

(C) $Rate=k[CH _3CHO]^{3/2}$
It is fractional order reaction with order $= \cfrac 32$
$\therefore$ Units of rate constant are $L^{\cfrac 12}mol^{-\cfrac 12}s^{-1}$

(D) $Rate=k[C _2H _5Cl]^{-1}$
It is first order reaction. So units of rate constant are $s^{-1}$

For the second order reaction, concentration $(x)$ of the product at time $t$ starting with initial concentration $[A] _0$ is:

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

  1. $\dfrac{kt[A _0]^2}{1 + kt[A _0]}$

  2. $\dfrac{k + [A _0]^2}{1 + kt}$

  3. $\dfrac{1 + kt[A _0]^2}{k + [A _0]^2}$

  4. none of these

Show answer
Correct Option: A
Explanation:

A reaction said to be a second order when the overall order is $2$. The rate of second order reaction may be proportional to one concentration squared.

$R=K[A]^2$
For rate proportional to single concentration squared, the time dependance of concentration is given by
$\cfrac{1}{[A]}=\cfrac{1}{A _0}+Kt
Therefore, concentration of product after time $t=\cfrac{kt[A_0]^2}{1+kt[A_0]}$.

Units of rate constant of a first order reaction is :

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

  1. $mole.lit^{-1}$

  2. $lit. mole$

  3. $mole. sec^{-1}$

  4. $sec^{-1}$

Show answer
Correct Option: D
Explanation:

A $\rightarrow$ product

For first order reaction, rate is dependent on single reactant A for example, rate = k[A]
$k=\frac{rate}{[A]}$

$=\frac{mole}{liter}sec\times \frac{liter}{mole}$
$=sec^{-1}$

A gaseous reaction, $A _{2}\left ( g \right )\rightarrow B\left ( g \right )+\frac{1}{2}\left ( g \right )$ 
Show increase in pressure from 40 mm to 120 mm in 5 minutes. the rate of disappearance of$A _{2}$ is ?

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

  1. 4 mm $min ^{-1}$

  2. 8mm $min^{-1}$

  3. 16 mm $min^{-1}$

  4. 2 mm $min^{-1}$

Show answer
Correct Option: B

Consider the reaction $2A+B$ $\rightarrow$products,when the concentration of a alone was doubled, the half-life of the  reaction did not change.When the concentration of B alone was double,the rate was not altered.The unit of rate constant for this reaction is

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

  1. $S^{-1}$

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

  3. $mol\ L^{-1}\ s^{-1}$

  4. $mol^{-2}\ L^{5}\ S^{-1}$

Show answer
Correct Option: B

For the reaction $A\rightarrow C+D$, the initial concentration of $A$ is $1000 M$. After $10^{2} sec$ concentration of $A$ is $100\ M$. The rate constant of the reaction has the numerical value of $9.0$. What is the unit of the reaction rate constant? 

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

  1. $M^{-1}s^{-1}$

  2. $Ms^{-1}$

  3. $s^{-1}$

  4. $M^{-1.5}s^{-1}$

Show answer
Correct Option: B

The second order rate constant is usually expressed as :

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

  1. $Mol\ L\ s^{-1}$

  2. $Mol^{-1}\, L^{-1}\, s^{-1}$

  3. $Mol\, L^{-1}\, s^{-1}$

  4. $Mol^{-1}\, L\, s^{-1}$

Show answer
Correct Option: D
Explanation:

As we know,
for second order reaction:
$r = k[A]^2$
so unit of rate constant is $Mol^{-1}\, L\, s^{-1}$.