Tag: chemical kinetics

Questions Related to chemical kinetics

What does the exponential factor represent?

chemistry chemical kinetics introduction to chemical kinetics understanding chemical kinetics rate of chemical reaction

  1. The total number of reactants in a reaction.

  2. The amount of energy needed to start a chemical reaction.

  3. The fraction of reactants that have approached the activation energy hill and made it over per number of attempts.

  4. The fraction of reaction energy given off per unit of time.

  5. The fraction of products that have approached the activation energy hill and made it over per number of attempts.

Show answer
Correct Option: C
Explanation:

The exponential factor represents the fraction of "reactants" that have approached the activation energy hill and made it over per number of attempts.

In a reaction mechanism consisting of elementary reaction steps where the relative rate of each is given, which of the following is most likely to be the rate-determining step?

chemistry chemical kinetics introduction to chemical kinetics understanding chemical kinetics rate of chemical reaction

  1. A step labeled fast

  2. A step labeled moderate

  3. A step labeled slow

  4. It is not possible to tell which step is rate determining from this information.

Show answer
Correct Option: C
Explanation:

In a multi-step reaction, the elementary step having the slowest-rate is always the rate determining step of the overall reaction.

Therefore, a step labeled slow will be the rate determining step.

Statement: The rate of instantaneous reactions can be determined experimentally.


State whether the given statement is true or false.

chemistry chemical kinetics introduction to chemical kinetics understanding chemical kinetics rate of chemical reaction

  1. True

  2. False

Show answer
Correct Option: A
Explanation:

Instantaneous rate: It can be determined graphically by using the concentrations of reactant or product at different time intervals.


Hence, the given statement is $\text{true}$

If the rate with respect to $O _2, NO, NO _2$ are $\displaystyle \frac{- \Delta [O _2]}{\Delta t}$ $= \dfrac{-1}{2}\dfrac{\Delta [NO]}{\Delta t}$, $= \dfrac{+1}{2} \dfrac{\Delta [NO _2]}{\Delta t}$, then the corresponding chemical equation is $2NO+O _2  \rightarrow NO _2$.
chemistry chemical kinetics introduction to chemical kinetics understanding chemical kinetics rate of chemical reaction

  1. True

  2. False

Show answer
Correct Option: B

Rate of formation of ${ SO } _{ 3 }$ according to the reaction $2{ SO } _{ 2 }+O _{ 2 }\rightarrow 2{ SO } _{ 3 }\quad is\quad 1.6\times { 10 }^{ -3 }kg\quad min^{ -1 }$. Hence rate at which $SO _{ 2 }$ reacts is:

chemistry chemical kinetics introduction to chemical kinetics understanding chemical kinetics rate of chemical reaction

  1. $1.6\times { 10 }^{ -3 }kg\quad { min }^{ -1 }$

  2. $8.0\times { 10 }^{ -4 }kg\quad { min }^{ -1 }$

  3. $3.2\times { 10 }^{ -3 }kg\quad { min }^{ -1 }$

  4. $1.28\times { 10 }^{ -3 }kg\quad { min }^{ -1 }$

Show answer
Correct Option: D
Explanation:

$2 SO _2 + O _2 \rightleftharpoons  2 SO _3$


$\dfrac{1}{2}\dfrac{d[SO _3]}{dt} = 1.6 \times 10^{-3} kg/min = \dfrac{1.6 \times 10^{-3}}{80} mol/min$                             $[\because M _{SO _3}= 80 \times 10^{-3} Kg/mol]$


$\dfrac{-1}{2}\dfrac{d[SO _2]}{dt} = \dfrac{+1}{2}\dfrac{d[SO _3]}{dt}$

$= -\dfrac{1.6 \times 10^{-3}}{80 \times 10} $ mol/min

$= -\dfrac{1}{50} \times 64 \times 10^{-3} = -1.28 \times 10^{-3} $ kg/min

For a reaction
$2A+B\rightarrow C+D$, the active mass of $B$ is kept constant but that of $A$ is tripled. The rate of reaction will -

chemistry chemical kinetics introduction to chemical kinetics understanding chemical kinetics rate of chemical reaction

  1. Decrease by $3$ times

  2. Increased by $9$ times

  3. Increase by $3$ times

  4. Unpredictable

Show answer
Correct Option: B
Explanation:

According to the given reaction, $ Rate = k[A]^{2}[B]^{1} $

When A becomes 3A i.e. triples, the rate will become: $ Rate^{'} = k[3A]^{2}[B]^{1} = 9k[A]^{2}[B] $
Thus, rate of reaction is increased by 9 times. 

The study of chemical kinetics becomes highly complicate if there occurs:

chemistry chemical kinetics introduction to chemical kinetics understanding chemical kinetics rate of chemical reaction

  1. reversible reaction

  2. side reaction

  3. surface reaction

  4. none of these

Show answer
Correct Option: B,C
Explanation:

The study of chemical kinetics becomes highly complicate if there occurs side reactions or surface reactions.These reactions bring changes in the value of rate constant and not help to proceeds the reaction in the right manner.

The container of $2$ litrer contains $4$ mole of $N _{2}O _{5}$. On heating to $100^{\circ}C, N _{2}O _{5}$ undergoes complete dissociation to $NO _{2}$ and $O _{2}$. Select the correct answers if rate constant for decomposition of $N _{2}O _{5}$ is $6.2\times 10^{-4}sec^{-1}$.
1. The mole ratio before and after dissociation is $4 : 2$
2. Half life of $N _{2}O _{5}$ is $1117\ sec$ and it is independent of concentration.
3. Time required to complete $40$% of reaction is $824\ sec$.
4. If volume of container is doubled, the initial rate of decomposition becomes half of the initial rate.

chemistry chemical kinetics introduction to chemical kinetics understanding chemical kinetics rate of chemical reaction

  1. $1, 3, 4$

  2. $1, 2, 3, 4$

  3. $3, 4$

  4. $2, 3, 4$

Show answer
Correct Option: C
Explanation:

Because reaction is first order

$\vartheta =k[A]$
$A=A _{0}e^{-kt}$

for which

$t _{40}=824$sec
if$v _{f}=2V _{i}\Rightarrow C _{f}=\frac{C _{i}}{2}\Rightarrow \vartheta _{f}=\frac{\vartheta _{i}}{2}$

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.