chemistry-class-12-unit-04-chapter-14-chemical-kinetics-l-14-16-hefbhqal78k

Chemical kinetics Lecture-14

Elementary reaction

Example
$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br}+\mathrm{OH}^-$ $\rightarrow \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OH}+\mathrm{Br}^{-1}$

As these molecules (reactants) come close, these interact

$\quad \quad \quad \downarrow$

Chemical bonds distort

Chemical kinetics Lecture-14

Elementary reaction

Chemical bonds distort

$\quad \quad \quad \downarrow$

Potential energy increases

$\quad \quad \quad \downarrow$

At distances typical of bond lengths, the reactant species become partially bonded

$\quad \quad \quad \downarrow$

New chemical bonds form

Chemical kinetics Lecture-14

Elementary reaction

Potential energy reaches maximum
The state where potential energy is maximum known transition state
Transition state is represented as $\left.\begin{array}{c}\text { (double }\text { dagger }\end{array}\right) \rightarrow \stackrel{\downarrow}{\neq}$

Chemical kinetics Lecture-14

Elementary reaction

Consider a reaction
$A_2+B_2 \rightarrow 2 A B$
$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br}+\mathrm{OH}^{-}$
Ea $\rightarrow$ attained through collisions between the reactants is known kinetic energy

Chemical kinetics Lecture-14

Elementary reaction

$\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Cl}$ collides with it own molecules
$\mathrm{C}_6 \mathrm{H}_3 \mathrm{CH}_2 \mathrm{Cl}$ can also collide with the solvent molecules
Difference in potential energy between the products and reactants
Potential energy of products - Potential energy of reactant = enthalpy change

Chemical kinetics Lecture-14

Elementary reaction

For reactions involving single reactant $\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Cl}$
Energy is needed to break the $\mathrm{C}-\mathrm{Cl}$ bond
There is no other reactant so Will collision be ruled out

Chemical kinetics Lecture-14

Activated complex

The molecular species present at the transition state refered as ‘Activated complex’(Transient species)
Activated complex is not on intermediate

Chemical kinetics Lecture-14

Examples of elementary reaction

$\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Cl}+\mathrm{OH}^{-} \rightarrow \mathrm{C}_6\mathrm{H}_5 \mathrm{CH}_3 \mathrm{OH}\ +\mathrm{Cl}^{-}$
Step 1. $\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Cl} \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2^{+}+\mathrm{Cl}^{-}$ …(3)
Step 2. $\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2{ }^{+}+\mathrm{OH}^{-} \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{OH}$ …(4)

Chemical kinetics Lecture-14

Molecularity

$A \rightarrow P$ $\text {unimolecular}$
$r=k[A] \text { unimolecular }$
$A+B \rightarrow P$ $\text {bimolecular}$
$r=k[A][B] \text { bimolecular }$
$2 A+B \rightarrow P$ $\text {termolecular}$
$r=k[A]^2[B]\text { termolecular }$
For an elementary reaction, molecularity and order are the same

Chemical kinetics Lecture-14

Molecularity

The experimental overall order of an elementary reaction is the same as molecularity
Molecularity is a theoretical concept
Order is an experimental concept

Chemical kinetics Lecture-14

Elementary reaction

Single - step reaction
Must proceed through only one transition state
Molecularity = overall order

Chemical kinetics Lecture-14

Examples

$2 \mathrm{Br} \rightarrow \mathrm{Br}_2$
$\mathrm{Br}+\mathrm{Br} \rightarrow \mathrm{Br}_2$
$r=k[\mathrm{Br}][\mathrm{Br}]$
$r=k\left[\mathrm{Br}\right]^2$
$I_2 \rightarrow I+I$
$r=k\left[I_2\right]$

Chemical kinetics Lecture-14

Important question of Molecularity

Question 1. First order reaction what can be said about its molecularity?
Answer- It is not told whether the first order reaction is elementary or not

Chemical kinetics Lecture-14

Important points of molecularity

Molecularity applicable for an elementary reaction
This has no existence for a complex or composite reaction

Chemical kinetics Lecture-14

Complex or composite reaction

Detection of reaction intermediates
Difficult to identify intermediates
Form of experimental rate equation
$\mathrm{ClO}^{-}\left(\mathrm{aq}\right)+I^{-}\left(\mathrm{aq}\right) \rightarrow \mathrm{Cl}^{-}\left(\mathrm{aq}\right)+I O^{-}\left(\mathrm{a}_q\right)$
$r=R\left[\mathrm{ClO}^{-}\right]\left[I^{-}\right]$
$r=k \frac{\left[\mathrm{ClO}^{-}\right]\left[I^{-}\right]}{\left[\mathrm{OH}^{-}\right]}$
Reaction is composite or complex in nature

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