Chemistry

Chemical Kinetics

276 Questions

Chemical kinetics involves the study of chemical reaction rates and the factors affecting them, such as temperature and concentration. This topic covers rate laws, half-life, and zero, first, and second order reactions. It is a crucial part of the chemistry syllabus for various competitive examinations.

Reaction rate parametersFirst order kineticsZero and second orderHalf-life of reactionRate constant units

Chemical Kinetics Questions

Multiple choice chemistry reaction kinetics determining rates graphically calculating rate of reaction graphically rate of a chemical reaction

A reaction is represented as $2A + B \mapsto  2C + 3D$. The concentration of C at 10 s is 4 moles $l^{-1}$. The concentration of C at 20 seconds is 5.2 moles $l^{-1}$. The rate of reaction of B in the same time interval could be :

  1. $-0.12$ mole $l^{-1} S^{-1}$
  2. $-0.6$ mole $l^{-1} S^{-1}$
  3. $-0.06$ mole $l^{-1} S^{-1}$
  4. $-1.2$ mole $l^{-1} S^{-1}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$r _{B} = \dfrac{1}{2}\dfrac{\Delta C}{\Delta t}= \dfrac{-1}{2}\dfrac{(1.2)}{10}$


$(\Delta C= 5.2 -4 = 1.2, \Delta t= 10 sec.)$

$= -0.06$ mole l$^{-1}$ sec$^{-1}$
Hence the answer is $C$.

Multiple choice chemistry reaction kinetics determining rates graphically calculating rate of reaction graphically rate of a chemical reaction

For $S{O _2}C{l _{2\left( g \right)}} \to S{O _{2\left( g \right)}} + C{l _{2\left( g \right)}},$ Pressures of $S{O _2}C{l _2}$ at $t = 0$ and $t = 20$ minutes respectively are $700mm$ and $350mm.$ When $\log \left( {{P _0}/p} \right)$ is plotted against time ($t$), slope equals to:

  1. $1.505 \times {10^{ - 2}}{s^{ - 1}}$
  2. $1.202 \times {10^{ - 3}}{\min ^{ - 1}}$
  3. $1.505 \times {10^{ - 2}}{\min ^{ - 1}}$
  4. $0.3465\ {\min ^{ - 1}}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$k=\dfrac{2.303}{t}log\dfrac{P _0}{P}$ [1st order reaction]

Plot of $log\dfrac{P _0}{P}$ against $t$ will give slope $=  \dfrac{k}{2.303}$
Using given data, $k=\dfrac{2.303}{20}log \dfrac{700}{350}$
$\dfrac{k}{2.303}=\dfrac{log2}{20}=1.5\times 10^{-2}s^{-1}$
Slope $=1.5\times 10^{-2}s^{-1}$

Multiple choice chemistry reaction kinetics determining rates graphically calculating rate of reaction graphically rate of a chemical reaction

From the concentrations of R at different times given below. Determine  the average rate of the reaction range: R $\rightarrow$ P in given intervals of time.

t (s) 0 5 10 20 30
$10^{-3}\, \times\, [R] \,(mol\, L^{-1})$ 160 80 40 10 2.5
  1. $3.5\times\,10^{2}$ to $0.42 \, \times\, 10^{2}$ $mol.L^{-1}\, s^{1}$
  2. $7\times\,10^{2}$ to $0.84 \, \times\, 10^{2}$ $mol.L^{-1}\, s^{1}$
  3. $8\times\,10^{3}$ to $0.37 \, \times\, 10^{3}$ $mol.L^{-1}\, s^{1}$
  4. $16\times\,10^{3}$ to $0.75 \, \times\, 10^{3}$ $mol.L^{-1}\, s^{1}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Average Rate $=\cfrac{ change \ in \ concentration }{ change \ in \ time }$


$\therefore$ here we wanted to find reaction range

Initially avergae rate$=\cfrac{ (160-80) }{ (5-0) }=$$\cfrac{ 80 }{ 5 }$
$=16$

Here, ${ 10 }^{ 3 }$ already given.

$\therefore$ Average rate $=16 \times {10}^{3}\ mol {L}^{-1} {s}^{-1}$

Final average rate $=\cfrac{ (10-2.5) }{(30-20) }=\cfrac{ 7.5 }{ 10 }=0.75$

$\therefore$ Average Rate $=0.75 \times {10}^{3}\ mol {L}^{-1} {s}^{-1}$

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

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

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

         

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

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

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

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

Which of the following is/are correct?

  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
Reveal answer Fill a bubble to check yourself
A,B,C,D Correct answer
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.

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

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

Which of the following is pseudo-unimolecular reaction?

  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. $2{ O } _{ 3 }\longrightarrow 3{ O } _{ 2 }$
Reveal answer Fill a bubble to check yourself
B Correct answer
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 

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

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?

  1. Same

  2. Doubled

  3. Half

  4. Data insufficient
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

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

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

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

  1. $24.2\ min$
  2. $58.2\ min$
  3. $102.2\ min$
  4. $120.2\ min$
Reveal answer Fill a bubble to check yourself
B Correct answer
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 $B$

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

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 ______________.

  1. $12 min$
  2. $34 min$
  3. $40 min$
  4. $43 min$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

For a first order reaction, the rate constant k = (2.303 / t) * log(initial / final). Given 60 percent decomposes in 45 minutes, the remaining fraction is 40 percent (0.4). So k = (2.303 / 45) * log(100 / 40) = (2.303 / 45) * 0.3979 = 0.02036 min^-1. The half life t(1/2) = 0.693 / k = 0.693 / 0.02036 = 34.03 minutes, which rounds to 34 min.

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

The acid hydroylsis of ester is:

  1. first order reaction

  2. bimolecular reaction

  3. pseudo unimolecular reaction

  4. none of the above
Reveal answer Fill a bubble to check yourself
A,B,C Correct answer
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.

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

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:

  1. $2$
  2. $1$
  3. $0$

  4. Unpredictable
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

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

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

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

Which of the following are pseudo unimolecular reactions?

  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) $
Reveal answer Fill a bubble to check yourself
A,C Correct answer
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 

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

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:

  1. $48.1$ s
  2. $24.1$ s
  3. $34.1$ s
  4. $44.1$ s
Reveal answer Fill a bubble to check yourself
A Correct answer
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