3 Marks Question

Question 13 Marks

The mechanism of the reaction $2 NO + Br _2 \rightarrow 2 NOBr$ is :
(i) $NO + Br _2 \rightleftharpoons NOBr _2$ (fast term)
(ii) $NOBr _2+ NO \rightarrow 2 NOBr$ (slow term)
Write the rate equation for this reaction.

Answer

On the basis of the mechanism of reaction, the rate of reaction is determined from the slow term hence,
$
\text { Rate }=k\left[NOBr_2\right][NO]
$
Since, $NOBr _2$ is not a reactant but a secondary compound and the first term is reversible, hence,
Equilibrium constant $K _{ c }=\frac{\left[ NOBr _2\right]}{[ NO ]\left[ Br _2\right]}$
or $\quad\left[ NOBr _2\right]= K _{ c }[ NO ]\left[ Br _2\right]$
Put the value in eq. (i)
$
\begin{aligned}
\text { Rate } & =k K_{c}[NO]\left[Br_2\right][NO] \\
\text { Rate } & =k^{\prime}\left[NO^2\right]^2\left[Br_2\right] \\
k^{\prime} & =k \cdot K_{c}
\end{aligned}
$
Here,
Hence, the rate equation for the reaction will be as follows:
$
\text { Rate }=k^{\prime}[NO]^2\left[Br_2\right]
$

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Question 23 Marks

(a) The molecularity of the slow term of a complex reaction is the molecularity of entire reaction. Explain.
(b) High order reactions generally do not occur, why?

Answer

(a) In the mechanism of complex reactions, the slowest term is considered as the rate determining step since the value of molecularity of any reaction is not more than 3, hence the molecularity of a reaction is determined from the molecules present in slow step even if the number of molecules is a completely balanced equation is more.
(b) While a reaction occurs, there is a collision between those molecules which are oriented in a certain direction and in complex reactions, the amount of change in the concentration of molecules in the slow step is the order of the reaction. Generally, not more than three molecules takes part in the collision, hence higher order reactions do not takes place.

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Question 33 Marks

Determine the formula for the half life of a first order reaction and prove that the half life of a first order reaction does not depends on the initial concentration of the reactant.

Answer

Rate constant for first order reaction
$
k=\frac{2.303}{t} \log \frac{[R]_0}{[R]}
$
At $t _{1 / 2},[ R ]=\frac{[ R ]_0}{2}$
On putting value of $[R]$
$
k=\frac{2.303}{t_{\frac{1}{2}}} \log \frac{[R]_0}{[R]_0 / 2}
$
or $\quad t _{\frac{1}{2}}=\frac{2.303}{ k } \log 2$
$
=\frac{2.303}{k} \times 0.3010(\because \log 2=0.3010)
$
or $\quad t _{\frac{1}{2}}=\frac{0.693}{ k }$
Therefore, the value of half life for any first order reaction is fixed. That is, it does not depend on the initial concentration of reactant.

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Question 43 Marks

Differentiate between order and molecularity of a reaction.

Answer

There are following differences in the order and molecularity of a reaction :
(1) The order of a reaction is an experimental quantity while molecularity is a theoretical quantity.
(2) Order can also be zero or fractional whereas molecularity is always an integer.
(3) The mechanism of reaction is known from the order not from molecularity.
(4) Order of a reaction is applicable for both primary and complex reactions but molecularity of reaction is applicable only for primary reactions. The molecularity of complex reactions has no meaning. In complex reactions, the order is given by the slowest term and generally, the molecularity and order of the slowest term are the same.
(5) Order depends on temperature, pressure or reaction conditions but not molecularity.
(6) Order is related to concentration whereas molecularity is related to the number of molecules.

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