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

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?

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

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

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

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.

  1. 20 min

  2. 24 min

  3. 28 min

  4. 32 min
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

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

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

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

  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. $\displaystyle CH _3CO _2C _2H _5\, +\, H _2O\, \overset{OH^{\oplus}}{\rightarrow}\, CH _3CO _2H\, +\, C _2H _5OH$
Reveal answer Fill a bubble to check yourself
A,B,C Correct answer
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.

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

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:

  1. 1

  2. 2

  3. 3

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

Multiple choice chemistry chemical equilibrium relationship between equilibrium constant, reaction quotient and gibbs energy law of mass action chemical equilibrium and acids-bases

Which of the following is true about the reaction quotient?

  1. It relates the ratio of the concentrations of products to reactants once the reaction has reached chemical equilibrium.

  2. It is always equal to the equilibrium constant.

  3. It can never be equal to the equilibrium constant.

  4. It relates the concentrations of products to reactants at any point in time.

  5. None of these answers are correct
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The reaction quotient relates the concentrations of products to reactants at any point in time.
When equilibrium is attained, the reaction quotient is equal to the equilibrium constant.

Multiple choice chemistry how far? how fast? collision theory collision theory of chemical reactions rate of chemical reaction

A chemical reaction occurs as a result of collisions between reacting molecules. Therefore, the reaction rate is given by:

  1. total number of collision occuring in a unit volume per second

  2. fraction of molecules which possess energy less than the threshold energy

  3. total number of effective collisions

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

According to collision theory, the reaction occurs when molecules collide with each other. The rate is given by:

Rate=${ Z } _{ AB }\times f$

Where $Z _{AB}$=collision frequency of reactants $A$ & $B$.

i.e. total number of collisions occurring in a unit volume per second. &

$f$=fraction of effective collisions.

So, the rate depends on both (A) & (C)

Multiple choice chemistry how far? how fast? collision theory collision theory of chemical reactions rate of chemical reaction

The fraction of collisions that posses the energy $E _a$ is given by:

  1. $f = e^{\frac{-Ea}{RT}}$
  2. $f = e^{\frac{Ea}{RT}}$
  3. $f = e^{- Ea.RT}$
  4. $f = e^{Ea.RT}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
$k=Pze^{\cfrac {-Ea}{RT}}$
$\cfrac kA$ or $\cfrac k{Pz}= $ Fraction of collision possessing $E _a$
$\therefore$ Fraction of collision possessing energy $E _a= \cfrac kA$
$=\cfrac {Ae^{-E _a/RT}}{A}$
$f=e^{-E _a/RT}$
Multiple choice chemistry how far? how fast? collision theory collision theory of chemical reactions rate of chemical reaction

Among the following which will decrease the rate of the reaction?
i. Using highly concentrated reactants
ii. Decreasing the temperature by $25\ K$
iii. Stirring the reactants

  1. i only

  2. ii only

  3. i and iii only

  4. ii and iii only

  5. i, ii, and iii
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\bullet $  Using highly concentrated reactants will increase the rate of the reaction as rate is directly proportional to the 

    concentration of reactants.
$\bullet $  Decreasing the temperature by 25 K will decrease the rate constant and hence the rate of reaction will decrease.
$\bullet $  Stirring the reactants increases the rate of interaction between the reactants and hence the rate of reaction increases.
$\therefore $ The correct answer is (ii) only.

Multiple choice zoology respiratory system do plants breathe? plant respiration respiration-plants

Free radicals are formed through

  1. Oxidation

  2. Reduction

  3. Hydrolysis

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

A.Oxidation – Release of electrons and protons or reaction with oxygen

B.Reduction – Acceptance of electrons and protons or release of oxygen.

C. Hydrolysis – Breaking of molecules in presence of water.

D.Synthesis – Process of formation of biomolecules.

So, the correct option is ‘oxidation’.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

For a chemical reaction, $A \rightarrow products$, the rate of reaction doubles when the concentration of A is increased by a factor of 4, the order of reaction is :

  1. 2

  2. 0.5

  3. 4

  4. 1
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
let x be order of reaction and K be rate of constant.
* A$\rightarrow $ products.
$ \cfrac { K\cfrac { { \left( A \right)  }^{ x } }{ time }  }{ K\cfrac { { \left( 4A \right)  }^{ x } }{ time }  } or,\quad \cfrac { 1 }{ 2 } = { \left( \cfrac { A }{ 4A }  \right)  }^{ x }\\ or,\quad \cfrac { 1 }{ 2 } ={ \left( \cfrac { 1 }{ 4 }  \right)  }^{ x }$      (reciprocating)
$\\ or,\quad 2={ \left( 4 \right)  }^{ x }\\ or,\quad 2={ { 2 }^{ 2 } }^{ x }\\ or,\quad 2x=1\\ or,\quad x=\cfrac { 1 }{ 2 } \\ x=0.5$
Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

The term $-\dfrac{dc}{dt}$ in a rate equation refers to:

  1. the concentration of a reactant

  2. the decrease in concentration of the reactant with time

  3. the velocity constant of reaction

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

The term $-\dfrac{dc}{dt}$ in a rate expression indicates the decrease in concentration of the reactant with time. It is the minus sign which is used to show the decrease in concentration of the reactant.

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

For a first order reaction, A$\rightarrow$ products, the concentration of A changes from $0.1$M to $0.025$ M $80$ minutes. The rate of reaction when the concentration of A is $0.01$M, is:

  1. $1.73\times 10^{-5}$M/min
  2. $3.47\times 10^{-4}$M/min
  3. $3.47\times 10^{-5}$M/min
  4. $1.73\times 10^{-4}$M/min
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

K$= \frac { 2.303 }{ t } \log { \left[ \dfrac { { A } _{ 0 } }{ A }  \right]  } $


$\quad =\dfrac { 2.303 }{ 80 } \log [{ \dfrac { 0.1 }{ 0.025 }  }]$

$k= 0.0173\quad { min }^{ -1 }$

$\therefore $ Rate$= K\left[ A \right]$

$\quad \quad \quad =0.0173\left[ 0.01 \right]$

$\quad \quad \quad = 1 .73 \times { 10 }^{ -4 } $

$\quad \quad \quad =1.73\times{ 10 }^{ -4 }M/min$

Hence, the correct option is $(D)$

Multiple choice chemistry rates of reaction introduction to rate of reaction rate of chemical reaction rate of reaction

Which of the following is (are) true for first order reaction?

  1. Rate of reaction is fastest at the beginning of reaction.

  2. Rate of reaction is fastest when (reactants)=(products)

  3. Rate of reaction increases with temperature

  4. Ea decreases considerably as temperature as temperature increases, hence the reaction becomes faster
Reveal answer Fill a bubble to check yourself
A,C Correct answer
Explanation

A] True. for first order reactions, products are formed at the fastest rate during the  beginning of the reaction.

B] False. When reactants>Products, the rate of reaction is fast.
C] True. Increasing the temperature increases the rate of reaction.
D] False. Activating energy does not decrease, but increasing the temperature allows crossing of energy barrier.