Tag: order of reactions

Questions Related to order of reactions

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:

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

  1. first order

  2. second order

  3. third order

  4. zero order

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Correct Option: B
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.

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:

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

  1. 1

  2. 2

  3. 1/2

  4. 5/4

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Correct Option: D
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 $= 1/2+1/2+1/4 = 5/4$

The rate constant of nth order has units:

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

  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. none of the above

Show answer
Correct Option: C
Explanation:

For nth order; unit of rate constant may be derived by
$rate  = k[a]^n$
$K=\frac {rate}{[reactant]^n}$.

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:

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

  1. mol $L^{-1}\, min^{-1}$

  2. $L^{2}\, mol^{-2}\, min^{-1}$

  3. $L\, mol^{-1}\, min^{-1}$

  4. $min^{-1}$

Show answer
Correct Option: B
Explanation:

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

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

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

  1. First order reaction

  2. Second order reaction

  3. Third order reaction

  4. Zero order reaction

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Correct Option: D
Explanation:

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

What is the order of reaction which has a rate expression as follows:
rate = k$[A]^{3/2}[B]^{-1}$

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

  1. $3/2$

  2. $1/2$

  3. Zero

  4. None of these

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Correct Option: B
Explanation:

rate = k$[A]^{3/2}[B]^{-1}$
OR = $\displaystyle \frac{3}{2}\, -\, 1\, =\, \frac{1}{2}$

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

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

  1. $atm$

  2. $atm$ $s$

  3. $atm$ $s^{-1}$

  4. $atm^2s^2$

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Correct Option: C
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).

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:

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

  1. $1$

  2. $\displaystyle \frac{1}{2}$

  3. $2$

  4. $\displaystyle \frac{13}{12}$

Show answer
Correct Option: D
Explanation:

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

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?

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

  1. $mol\, L^{-1}\, time^{-1}$

  2. $time^{-1}$

  3. $mol^{-1}\, L^{2}\, time^{-1}$

  4. $mol^{-2} L^{2} time^{-1}$

Show answer
Correct Option: D
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 -

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

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 $Mol^{-1}\, L\, s^{-1}$.