Chemical Kinetics

"Two-third life" ( ) is the time required for the reaction to be 2/3 complete.
This means the amount consumed is , so the amount remaining is of .
So, .
Formula:
Substitute values ( s ):

We know .



s.

For a first-order reaction, the half-life ( ) is given by the formula:

Given:
min .
Substitute the value:

min.
Rounding to reasonable precision: min (or approx 5.2 seconds).

For a first-order reaction, the integrated rate equation is:

Given:
s .
We want the final concentration to be of initial .
So, .
Substitute values:

Since :

s.
Rounding to significant figures, s.

1. Definition/Determination: Order is the sum of powers of concentration terms in the experimental rate law; it is an experimental quantity. Molecularity is the number of reacting species colliding simultaneously in an elementary step; it is a theoretical concept.
2. Values: Order can be zero, fractional, or an integer. Molecularity can only be a positive integer (1, 2, 3) and cannot be zero or fractional.

For a reaction of -th order, the rate law is Rate .
The units of the rate constant are given by the general formula .
For a first order reaction, .
Substituting into the formula: .
Alternatively, since Rate (mol L s ) Concentration (mol L ), must have units of s .

Concept: Activation energy is calculated using the Arrhenius equation, which relates rate constant with temperature.
Answer:
The Arrhenius equation is: Taking logarithm for two temperatures: Formula for Activation Energy: where:

• = rate constants
• = temperatures in Kelvin

Example:
If , , , , substitute values to calculate .

Concept: Order and molecularity are important kinetic concepts that describe reaction behavior, but they differ in definition and applicability.
Answer:
Order of Reaction:

• It is the sum of powers of concentration terms in the rate law.
• It is determined experimentally.
• It can be zero, fractional, or integer.
• It applies to the overall reaction.

Molecularity of Reaction:

• It is the number of molecules colliding in an elementary step.
• It is a theoretical concept.
• It is always a whole number (never zero or fractional).
• It is defined only for elementary reactions.

Differences:

• Order is experimental; Molecularity is theoretical.
• Order can be fractional; Molecularity is always an integer.
• Order applies to overall reaction; Molecularity applies to elementary steps only.