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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

Which of the following statements is incorrect?

  1. A second order reaction must be a bimolecular elementary reaction

  2. A bimolecular elementary reaction must be a second order reaction

  3. Zero order reaction must be a complex reaction

  4. First order reaction may be complex or elementary reaction.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A bimolecular elementary reaction may or may not be a Second Order Reaction whereas a Second Order reaction must be a bimolecular elementary reaction.

Ex: Decomposition of ${N} _{2}O _{5}$ is a 1st Order Reaction though its a bimolecular reaction.

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 solvolysis of tert-butyl bromide will be maximum in which of the following solvents?

  1. ${ Et } _{ 2 }O$
  2. ${CCl} _{4}$
  3. $EtOH$
  4. ${H} _{2}O$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The rate of solvolysis of Tert - butyl bromide will be maximum in Water.

tert-butyl bromide follows $SN1$ mechanism thus it prefers weak nucleophiles like water.

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

The inversion of cane sugar to produce glucose and fructose is represented by the reaction
     ${ C } _{ 12 }{ H } _{ 22 }{ O } _{ 11 }+{ H } _{ 2 }O\quad \xrightarrow { { H }^{ + } } { C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 }+{ C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 }$
The reaction is:

  1. second order

  2. unimolecular

  3. pseudo uni-molecular

  4. bi molecular
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Inversion of Cane Sugar to produce glucose and fructose is a Pseudo uni-molecular Reaction.

Here the rate of reaction is made first order by changing the concentration of sucrose ($C _{12}H _{22}O _{11}$).

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.