Physical chemistry 2
۲ ‫كيمياء فيزيائية‬
Dr. Tahani M Bawazeer
BSc, MSc, PhD
Room: 372/256
E-mail: [email protected]
Umm Al-Qura University
College of Applied Science
Chemistry Department
Outline
² Pseudo order reaction
² Half-life of a Reaction
² Calculation of Time for Completing any Fraction of the
Reaction
² Comparison between the different order reactions
Umm Al-Qura University
College of Applied Science
Chemistry Department
Pseudo Order Reaction
A+B
p •  The experimental order which is not the actual one.
Hydrolysis of an ester :
rate = k[A][B]
rate = k`[A]
CH3COOC2H5 + H2O
CH3COOH + C2H5OH
Rate = k'[CH COOH]
Umm Al-Qura University
College of Applied Science
Chemistry Department
Half-life of a Reaction
The time required for the concentration of a reactant to decrease to half
its initial value.
First order:
(1) half-life for a first order reaction is
independent of the initial
concentration.
(2) it is inversely proportional ot k, the
rate-constant.
Umm Al-Qura University
College of Applied Science
Chemistry Department
Half-life of a Reaction
Concentration of A as a function of
time for a 1st reaction.
Umm Al-Qura University
College of Applied Science
Chemistry Department
Concentration of A as a function of
time for a 2ed reaction.
Calculation of Time for Completing any Fraction of
the Reaction
Umm Al-Qura University
College of Applied Science
Chemistry Department
Example:
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College of Applied Science
Chemistry Department
PROBLEM 1.
Compound A decomposes to form B and C the reaction is first order. At
25°C the rate constant for the reaction is 0.450 s–1. What is the half-life
of A at 25°C ?
SOLUTION
•  We know that for a first order reaction, half-life t1/2, is given by the
expression
t1/2 = 0.693 /k
where k = rate constant
•  Substituting the value of k = 0.450 s–1
•  t1/2 = 0.693/ 0.450s-1 = 1.54s-1
Umm Al-Qura University
College of Applied Science
Chemistry Department
PROBLEM 2.
The half-life of a substance in a first order reaction is 15 minutes.
Calculate the rate constant.
SOLUTION
•  For a first order reaction
•  t1/2 = 0.693/ k
•  Putting t1/2 = 15 min in the expression and solving for k,
•  k = 0.693/t1/2 = 0.693/ 15mins = 4.62 × 10–2 min–1
Umm Al-Qura University
College of Applied Science
Chemistry Department
Comparison between the different order reactions
Umm Al-Qura University
College of Applied Science
Chemistry Department
Examples of First order Reactions
•  From the following data for the decomposition of N2O5 in CCl4
solution at 48°C, show that the reaction is of the first order
t (mins)
10
15
20
∞
Vol of O2 evolved
6.30
8.95
11.40
34.75
Umm Al-Qura University
College of Applied Science
Chemistry Department
Examples of First order Reactions
Umm Al-Qura University
College of Applied Science
Chemistry Department
Examples of second order Reactions
•  Hydrolysis of ethyl acetate by NaOH using equal concentration of the
reactants, was studied by titrating 25ml of the reaction mixture at
different time intervals against standard acid. From the data given
below, establish that this is a second order reaction.
t (mins)
0
5
15
25
ml acid used
16
10.24
6.13
4.32
Umm Al-Qura University
College of Applied Science
Chemistry Department
Examples of second order Reactions
Umm Al-Qura University
College of Applied Science
Chemistry Department
Examples of second order Reactions
Umm Al-Qura University
College of Applied Science
Chemistry Department
End of Lecture 4
Umm Al-Qura University
College of Applied Science
Chemistry Department