�����CHEMICAL KINETICS����
CHAPTER-4
CLASS-XII
*
1
Prepared by
Ashitha A A
PGT Chemistry
JNV Kottayam
Hyderabad region
Three aspects of chemical reaction
feasibility
extent
Rate of the reaction
*
2
Types of reactions
*
3
Content of the chapter
*
4
Rate of a chemical reaction:
Change in concentration of either reactant or product per unit time.
Unit of rate of reaction Mol L ‾¹S‾¹
Chemical Kinetics:-
Study of reaction rates and their mechanisms
*
5
Definitions
*
6
Rate of change in concentration of either reactant or product per unit time.
Unit of rate of reaction Mol L ‾¹S‾¹
Rate of a chemical reaction:
A → B
*
7
Rate of a reaction
Graphical representation of rate of a reaction
*
8
*
9
Total change in concentration of reactant or product by the elapsed time
Change in concentration of reactant or product at particular instant of time
Denoted by ΔX/ΔT
Denoted by dx/dt.
AVERAGE RATE
INSTANTANEOUS RATE
*
11
Graphical Representation of average rate and instantaneous rate
?
For the reaction R → P, the concentration of a reactant changes from 0.03 M to 0.02M in 25 minutes. Calculate the average rate of reaction using units of time both in minutes and seconds
*
12
Average rate of a reaction
*
13
-Δ[ R]/ Δt
= -( 0.02-0.03)/25 M min‾¹
=4X10‾⁴ M min‾¹
=4X10‾⁴/ 60M sec‾¹
=6.67x10 ‾⁶M sec‾¹
A
A
aA+bB→cC+dD
For a reaction
*
14
Rate of a reaction=
-1/a d[ A]/dt =-1/b d[ B]/dt
=1/c d[C/dt =1/d d[D]/dt
2HI(g) → H2(g) + I2(g)
r=-d[N2 ]//dt = -1/3 d [ H2 ]/dt =+1/2 d [ NH3 ]/ /dt
*
15
Examples
r= -1/2 d [ HI ]/dt=+d [ H2 ]/dt =d[ I2 ]/dt
N2 +3 H2 → 2NH3
Factors affecting rate of reaction
*
16
*
17
Effect of concentration
When concentration of reactant increases rate of reaction increases
Rate Law
Rate of a reaction is directly proportional to concentration of reactants raised to some power
Which may or may not be same as stoichiometric coefficient of reactants in a balanced chemical equation.
*
18
Rate = k[A]x[B]y
For a reaction:
aA + bB → Product
k is called rate constant
x and y may or may not be equal to stoichiometric coefficient a and b
Differrential rate equation
Example
2NO+O2→ 2NO2
rate=k [NO ]
*
19
2
[ O2 ]
Sum of the powers of the concentration of the reactant terms in experimentally determined rate equation.
*
20
Order of a reaction
It can have a value of zero, integral values as well as fractional values
Rate = k [A]x [B]y
x and y represent the order with respect to the reactants A and B respectively
x + y the overall order of a reaction
*
21
?
A
*
22
Examples of zero order reaction
*
23
Examples of First Order reaction
•A reaction is zero order in a reactant if the change in concentration of that reactant produces no effect.
• A reaction is 1st order if doubling the concentration causes the rate to double.
• A reaction is 2nd order if doubling the concentration causes a quadruple increase in rate.
Order of reactions…
The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate
of formation of Y?
X→ Y follows second order kinetics
r =k [X] 2
X is increase 3 times ,rate will become 9 times
r =k [3X] 2 i.e., r= 9x k X2
?
A
Depending on the order of the reaction unit of rate constant changes
*
26
Unit of rate constant
For a general reaction aA+bB → cC+dD
Unit can be expressed as
( mol L-1 ) (1- n) s-1
*
27
Units of rate constant
*
28
?
Identify order of the reaction if the units of its rate constant are i)L‾¹ mol sec‾¹ ii)L mol‾¹ sec‾¹
A
i)Zero order ii)Second order
The decomposition reaction of NH3
on platinum surface is given
N2 +3 H2 → 2NH3
What are the rates of production of N2 and H 2
if K= 2.5 X10-4mol l-1 s-1
.
?
*
30
A
der
Elementary reactions are those which complete
in one step
NH4NO2 → N2 + 2H2O
Decomposition of dinitrogen tetra oxide
N2O4(g) → 2NO2(g)
Elementary Reaction
*
32
Molecularity of a reaction is simply the number of reacting species (atoms, ions or molecules) involved in an elementary reaction which must collide simultaneously.
Molecularity
e.g. dissociation of hydrogen iodide.
2HI → H2 + I2
e.g. 2NO + O2 → 2NO2
Molecularity….
*
34
Why the reactions having molecularity greater than three rare ?
Since the chances of collision and reaction of more than three molecules at a time are very less, the molecularity greater than three is rare.
Definition:
Multistep reactions where products are obtained after completion of a sequence of elementary reactions
*
35
Complex reactions
The first step being slow, is the rate determining
step ,therefore order is two
*
36
Slow
fast
Difference between order and molecularity
�
order | molecularity |
Experimental quantity | theoretical |
can be zero and even a fraction | cannot be zero or a non integer |
applicable to elementary as well as complex reactions | applicable only for elementary reactions. |
For complex reaction, order is given by the slowest step | individual steps has its own molecularity |
*
38
Integrated Rate Equations
Equation obtained by integrating the differential rate equation to give a relation between directly measured experimental data, i.e., concentrations at different times and rate constant is called integrated rate equation.
Zero Order Reactions
R → P
Rate ∝ (Conc. Of Reactant)0
*
39
Integrated rate equations
*
40
Integrating both sides
[R] = – k t + I
where, I is the constant of integration.
At t = 0, the concentration of the reactant [R]=[R]o
where [R]o is initial concentration of the reactant.
Substituting in equation )
[R] = –k × 0 + I ]
[R]o=1
Substituting the value of I in the equation )
[R] = -kt + [R]0
*
41
[R0]
Time
Concentration of R
Concentration vs Time plot for zero order reaction
[R] = -kt + [R]o
First Order Reactions
*
42
Consider the reaction,
R → P
*
43
*
44
Concentration vs Time plot for first order reaction
y=mx+c
*
45
The half-life of a reaction is the time in which the concentration of a reactant is reduced to one half of its initial concentration. It is represented as t1/2
*
46
Half Life of a reaction
*
47
*
48
Half life for a first order reaction
Half life of first order reaction is independent of initial concentration
*
49
*
50
*
51
*
52
*
53
Time required to decompose SO2Cl2 to half its initial amount is 60 minutes. If the decomposition is a first order reaction, calculate the rate constant of the reaction.
?
*
54
A
*
55
k-rate constant
Ea-activation energy
*
56
The temperature dependence on the rate of a
chemical reaction can be accurately explained by Arrhenius equation
Temperature dependence of the rate of reaction:
For a chemical reaction with rise in temperature by 10°
the rate constant is nearly doubled.
T –temperature
R-universal gas constant
The time taken for half of the original amount of material to decompose is 12 min at 500C, 5 h at 250C and 10 days at 00C
*
57
During decomposition of N2O5,
Temperature dependence of the rate of reaction:
Reaction between oxalic acid and potassium permanganate occur at higher temperature
*
58
Transition State Theory
According to Arrhenius, reaction takes place through formation of unstable Intermediate called activated complex which exist for short time and then breaks up to form product.
*
59
*
60
The energy needed to form activated complex is called energy of activation. It is very low for some reactions and very high for others.
Activation energy (Ea)
*
61
Increasing the temperature of the substance increases the fraction of molecules, which collide with energies greater than Ea.
*
62
It is clear from the diagram that in the curve at (t + 10), the area showing the fraction of molecules having energy equal to or greater than activation energy gets doubled leading to doubling the rate of a reaction.
*
63
For a chemical reaction with rise in temperature by 10°, the rate constant is nearly doubled
The ratio is called the temperature
coefficient and its value is 2 or 3
k(T+10)
kT
*
64
Arrhenius Equation
k = A e-Ea /RT
e-Ea /RT = Fraction of molecule having K.E. > Ea
Natural logarithm of Arrhenius equation gives
Plot of ln k vs 1/T is straight line shows that increasing the temperature or decreasing the activation energy will
result in an increase in the rate of the reaction and an exponential
increase in the rate constant.
*
65
Arrhenius Equation
Plot of ln k vs 1/T is straight line shows that increasing the temperature or decreasing the activation energy will
result in an increase in the rate of the reaction and an exponential increase in the rate constant.
*
66
Arrhenius Equation
*
67
A catalyst is a substance which alters the rate of a reaction without itself undergoing any permanent chemical change.
For example, MnO2 catalyse the following reaction so as to increase its rate considerably.
Effect of catalyst on rate of reaction
*
68
Effect of Catalyst on Rate of reaction
*
69
Collision theory
Rate = ZABe-Ea/RT
The reactant molecules are assumed to be hard spheres
Reaction occurs when molecules collide with each other.
Z AB the number of collisions per second per unit volume of the reaction mixture.
e-Ea /RT the fraction of molecules with energies equal to or greater than Ea.
*
70
The proper orientation of reactant molecules lead to bond formation
Improper orientation makes them simply bounce back
and no products are formed.
Collision theory continued…..
Diagram showing molecules having proper and improper orientation
*
71
*
72
Collision theory…
Rate = PZAB e-Ea/RT
In collision theory activation energy and proper orientation of the molecules together determine the criteria for an effective collision and hence the rate of a chemical reaction.
By considering orientation factor
P=steric factor or probability factor
*
73
Thank you