Tag: order of reactions

Questions Related to order of reactions

In a first order reaction $A \rightarrow B, 60\%$ of the given sample of compounds decomposes in $45$ minutes. The half life of the reaction is ______________.

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

  1. $12 min$

  2. $34 min$

  3. $40 min$

  4. $43 min$

Show answer
Correct Option: C

The acid hydroylsis of ester is:

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

  1. first order reaction

  2. bimolecular reaction

  3. pseudo unimolecular reaction

  4. none of the above

Show answer
Correct Option: A,B,C
Explanation:

The acid hydrolysis of ester is a first-order reaction, bimolecular reaction and pseudo unimolecular reaction.


When an ester (such as ethyl acetate) is mixed with water it is converted into alcohol and acid according to the following equation: 

$CH _3COOC _2H _5 + H _2O \rightarrow CH _3COOH + C _2H _5OH$

When the amount of water is relatively large, the reaction goes practically to completion (the equilibrium shifts to the right) and the rate is first order with respect to the ester. The hydrolysis takes place slowly with pure water and is catalyzed by acids.

Hence, options A, B and C are correct.

Inversion of cane-sugar in dilute acid is a:

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

  1. biomolecuar reaction

  2. pseudo-unimolecular reaction

  3. unimolecular reaction

  4. trimolecular reaction

Show answer
Correct Option: B

The rate of reaction, $A+B\longrightarrow $ Products, is given by the equation, $r=k\left[ A \right] \left[ B \right] $. If $B$ is taken in large excess, the order of reaction would be:

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

  1. $2$

  2. $1$

  3. $0$

  4. Unpredictable

Show answer
Correct Option: B
Explanation:

If $B$ is in excess then $A$ is limiting reagent thus

$ \vartheta =k[A]$
$ \therefore$ order$=1.$

In pseudo-unimolecular reactions:

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

  1. one of the reactants is present in large excess

  2. both the reactants have same concentration

  3. both the reactants are present in low concentration

  4. one of the reactants is less reactive

Show answer
Correct Option: A
Explanation:

The reactions having molecularity but order of reaction is one are called pseudo unimolecular reactions. In pseudo unimolecular reaction one recatent is always in excess amount. Due to its excess amount there is no significant changes in its comcetatration that's why rate of reaction only depend on second recatent.

Ex. Hydrolysis of ester in water. In this water is in excess.
So answer will be A

Which of the following are pseudo unimolecular reactions?

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

  1. $C{ H } _{ 3 }COO{ C } _{ 2 }{ H } _{ 5 }+HOH\xrightarrow [ ]{ \quad { H }^{ + }\quad } C{ H } _{ 3 }COOH+{ C } _{ 2 }{ H } _{ 5 }OH$

  2. $C{ H } _{ 3 }COO{ C } _{ 2 }{ H } _{ 5 }+HOH\xrightarrow [ ]{ \quad O{ H }^{ - }\quad } C{ H } _{ 3 }COOH+{ C } _{ 2 }{ H } _{ 5 }OH$

  3. $\begin{matrix} { C } _{ 12 }{ H } _{ 22 }{ O } _{ 11 } \ \end{matrix}\begin{matrix} +HOH \ \end{matrix}\begin{matrix} \xrightarrow [ ]{ \quad { H }^{ + }\quad } \ \end{matrix}\begin{matrix} { C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 } \ glucose \end{matrix}\begin{matrix} + \ \end{matrix}\begin{matrix} { C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 } \ fructose \end{matrix}$

  4. $S{ O } _{ 2 }{ Cl } _{ 2 }\left( g \right) \longrightarrow S{ O } _{ 2 }\left( g \right) +{ Cl } _{ 2 }\left( g \right) $

Show answer
Correct Option: A,C
Explanation:

In a chemical reaction if two or more reactants are involved but the rate of reaction depends only upon the concentration of one of the reactant and independent of other reactants

$C{ H } _{ 3 }COO{ C } _{ 2 }{ H } _{ 5 }+HOH\xrightarrow [ ]{ \quad { H }^{ + }\quad } C{ H } _{ 3 }COOH+{ C } _{ 2 }{ H } _{ 5 }OH$

$\begin{matrix} { C } _{ 12 }{ H } _{ 22 }{ O } _{ 11 } \ \end{matrix}\begin{matrix} +HOH \ \end{matrix}\begin{matrix} \xrightarrow [ ]{ \quad { H }^{ + }\quad } \ \end{matrix}\begin{matrix} { C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 } \ glucose \end{matrix}\begin{matrix} + \ \end{matrix}\begin{matrix} { C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 } \ fructose \end{matrix}$ 
is unimolecular reaction 

The rate of a first order reaction is $0.04$ mole ${ \ell  }^{ -1 }{ S }^{ -1 }$ at $10$ seconds and $0.03$ mol ${ \ell  }^{ -1 }{ S }^{ -1 }$ at $20$ seconds after initiation of the reaction. The half-life period of the reaction is:

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

  1. $48.1$ s

  2. $24.1$ s

  3. $34.1$ s

  4. $44.1$ s

Show answer
Correct Option: A
Explanation:
$\vartheta =kA\rightarrow $first order

$\vartheta =kA _{0}e^{-kt}$

$\vartheta =\vartheta _{0}e^{-kt}\rightarrow $first order wrt $\vartheta$
so,

$\vartheta _{10}=0.04=\vartheta _{0}e^{-10k}\rightarrow (1)$

$\vartheta _{30}=0.03=\vartheta _{0}e^{-30k}\rightarrow (2)$

now using (1)/(2)
$+l _{n}(1.33)=20k$

$k=0.0144$

now,$t _{\frac{1}{2}}=\frac{0.693}{k}$

$=48.125$

$\cong 50$ seconds

In a pseudo first order acid catalysed hydrolysis of ester in water the following results were obtained:
t/s             0      30       60        90
[ester]/M    0.55 0.31   0.17       0.085
Which of the following is/are correct for the given reaction?

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

  1. The average rate of reaction between time interval 30 to 60 seconds is $4.67\times 10^{-3} mol L^{-1} s^{-1}$

  2. Order of reaction is 2.

  3. Pseudo first order rate constant for the acid catalysed hydrolysis of ester is $1.92\times 10^{-2}s^{-1}$

  4. All are correct.

Show answer
Correct Option: A,C
Explanation:

$rate _{avg}=0.14/30=4.67\times 10^{-3}$
$\therefore k=1.92\times 10^{-2}$

An optically active compound A upon acid catalysed hydrolysis yield two optically active compound B and C by pseudo first order kinetics. The observed rotation of the mixture after 20 min was $ 20^{\circ}$ while after completion of the reaction it was $-20 ^{\circ} .$ If optical rotation per mole of A,B &C are 60$ ^{\circ} ,40$ $^{\circ}$& $-80 ^{\circ} . $Calculate half life of the reaction.

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

  1. 20 min

  2. 24 min

  3. 28 min

  4. 32 min

Show answer
Correct Option: A
Explanation:

As after completion of the reaction rotation is $-20 ^{\circ} $
So according to first order reaction
$k = 2.303log(r _0 - r _{\infty })/(r _t - r _{\infty })/20 = 2.303log(80/40)/20$
$t _{1/2} = 0.693/k = 20 min$

Which of the following is/are examples of pseudo unimolecular reactions ?

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

  1. $\displaystyle CH _3CO _2C _2H _5\, +\, H _2O\, \overset{H^{\oplus}}{\rightarrow}\, CH _3CO _2H\, +\, C _2H _5OH$

  2. $\displaystyle C _{12}H _{22}O _{11}\, +\, H _2O\, \overset{H^{\oplus}}{\rightarrow}\, \begin{matrix} & C _6H _{12}O _6\ & (Glucose)\end{matrix}\, =\, \begin{matrix} & C _6H _{12}O _6\ & (Fructose)\end{matrix}$

  3. $CH _3COCl\, +\, H _2O\, \rightarrow\, CH _3CO _2H\, +\, HCl$

  4. $\displaystyle CH _3CO _2C _2H _5\, +\, H _2O\, \overset{OH^{\oplus}}{\rightarrow}\, CH _3CO _2H\, +\, C _2H _5OH$

Show answer
Correct Option: A,B,C
Explanation:

The reactions of the options A, B and C represents pseudo unimolecular reactions as water is present in large excess. Hence, the concentration of water remains practically unchanged and does not affect the rate of the reaction.  

Hydrolysis of ester in acidic medium is pseudo unimolecular and in alkaline medium is bimolecular. Inversion of cane sugar and the hydrolysis of acetyl chloride is also pseudo unimolecular.