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Pseudo first order reaction - class-XII

Description: pseudo first order reaction
Number of Questions: 21
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Tags: chemical kinetics chemistry electrochemistry and chemical kinetics
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In pseudo unimolecule reactions:

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

  1. both the reactants are present in low concentration

  2. Both the reactants are present in same concentration

  3. One of the reactant is present in excess

  4. One of the reactant is non-reactive

Show answer
Correct Option: C
Explanation:

Pseudo Unimolecular Reaction. The reactions having molecularity two but the order of reaction is one is called pseudo unimolecular reactions,in such type of reactions, one reagent is take in the excess amount so that the disappearance of that reagent is negligible and it does not determine the rate of reaction.

Example:
  • Hydrolysis of ester$CH _3COOC _2H _5$,
  • Hydrolysis of sucrose.

The value of rate of a pseudo first order reaction depends upon:

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

  1. the concentration of both the reactants present in the reaction

  2. the concentration of the reactant present in small amount

  3. the concentration of the reactant present in excess

  4. the value of $\Delta H$ of the reaction

Show answer
Correct Option: B
Explanation:

$CH _3COOC _2H _5 + H _2O \rightarrow CH _3COOH + CH _3OH$
          0.01                    10                   0                      0

            0                      9.99              0.01               0.01 

The above reaction is a Pseudo first order reaction. As it can be seen from the above reaction that the concentration of water is not changing much. Thus the rate of the reaction is not much affacted by the change in concentration of  water. However rate of the reaction get significantly affacted by the concentration gradient of ethyl acetate. Hence, the rate of Pseudo first order reaction depends upon the concentration of the reactant present in small amount.

Option B is correct.

         

The value of rate constant of a pseudo first order reaction ________.

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

  1. depends on the concentration of reactants present in small amount

  2. depends on the concentration of reactants present in excess

  3. is independent of the concentration of reactants

  4. depends only on temperature

Show answer
Correct Option: B
Explanation:

Pseudo first-order reaction is not the first-order reaction by nature. It is made the first order by adding something in excess.so, the rate constant depends on the concentration of reactants present in excess.

Which of the following is/are correct?

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

  1. Acid hydrolysis of an ester follows pseudo first order kinetics

  2. Inversion of cane sugar follows pseudo first order kinetics

  3. Decomposition of varous gases on the surface of metallic catalysts, such as decomposition of $HI$ on gold surface also exhibits zero roder kinetics

  4. Radioactivity follows first order kinetics

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

 Pseudo first order reaction is a reaction in which concentration of one of the species dominates over the other. Hence the reaction follows the first order kinetics.

Options A, B and C all are the examples of pseudo first order reaction.
Radioactive decay follows first order reaction.

$\mathbf {Hence\ all\ the\ options\ are\ correct.}$

Nitrosy ligand binds to d-metal atoms in liner and bent fashion and behaves, respectively, as _______________.

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

  1. $NO^{+}$ and $NO^{+}$

  2. $NO^{+}$ and $NO^{-}$

  3. $NO^{-}$ and $NO^{-}$

  4. $NO^{-}$ and $NO^{+}$

Show answer
Correct Option: B

Triple point temperature for water is nearly:

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

  1. $273.16K$

  2. $373.16K$

  3. ${100}^{o}C$

  4. ${444.6}^{o}C$

Show answer
Correct Option: A
Explanation:

Triple point temperature for water is nearly $0.01^{o} \ C$, which is $273.16 \ K$.

Hence, option A is correct.

Which of the following is pseudo-unimolecular reaction?

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

  1. $2{ H } _{ 2 }{ O } _{ 2 }\longrightarrow 2{ H } _{ 2 }O+{ O } _{ 2 }$

  2. ${ C } _{ 6 }{ H } _{ 5 }{ N } _{ 2 }Cl+HOH\longrightarrow { C } _{ 6 }{ H } _{ 5 }OH+{ N } _{ 2 }+HCl$

  3. $C{ H } _{ 3 }COO{ C } _{ 2 }{ H } _{ 5 }+NaOH\longrightarrow C{ H } _{ 3 }COONa+{ C } _{ 2 }{ H } _{ 5 }OH$

  4. $2{ O } _{ 3 }\longrightarrow 3{ O } _{ 2 }$

Show answer
Correct Option: B
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 then it is said to be a pseudo unimolecular reaction.

so the option B shows the pseudotype of reaction 

Rate law for the following reaction; $Ester+H^+\rightleftharpoons Acids+ Alcohol$; is $\frac {dx}{dt}=k [ester]^1 [H^+]^0$. What would be the effect on the rate if concentration of $H^+$ ion is doubled?

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

  1. Same

  2. Doubled

  3. Half

  4. Data insufficient

Show answer
Correct Option: A
Explanation:

As reaction does not depend on the concentration of $H^+$, therefore, rate will be the same.

A first-order reaction which is $30\%$ complete in $30$ minutes has a half-life period of:

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

  1. $24.2\ min$

  2. $58.2\ min$

  3. $102.2\ min$

  4. $120.2\ min$

Show answer
Correct Option: B
Explanation:

It is a 1st order reaction


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

but $k=\dfrac{0.693}{t _{\frac{1}{2}}}$

$\dfrac{0.693}{{t} _{1/2}} = \dfrac{2.303}{30} log \dfrac{100}{70}$

${t} _{1/2} = 58.2 min$

Hence, the correct option is $B$

The acid hydrolysis of an ester is___________.

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

  1. Pseudo first order reaction

  2. Bimolecular reaction

  3. Pseudo unimolecular reactions

  4. None of the above

Show answer
Correct Option: A,C

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.

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.

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