Transcript CHEMICAL KINETICS CLASS- XII

CHEMICAL KINETICS
CLASS- XII
VINAY KUMAR
PGT CHEMISTRY
KV NTPC KAHALGAON
PATNA REGION
• It is the branch of physical chemistry which deals
with the study of the rate of a chemical reaction
and the mechanism by the reaction occur.
• RATE OF THE CHEMICAL REACTION OR AVERAAGE
RATE OF REACTION :- it is the change in the
concentration of reactant or product with time in
which a chemical reaction proceed.
Rate of reaction = Decrease in the concentrationof R
time taken
Or
Increase in the concentrationof P
time taken
Unit of rate is Mol L-1 S-1 or atm S-1 (For gaseous reaction)
Or
Rate of reaction = -R]
t
= +P]
t
• INSTANTANIOUS RATE OF REACTION:- it is
the rate of the reaction at the particular
moment of time and measured as a very small
concentration change over a very small time
interval. t 0 for a reaction R P
Instantaneous rate =
-dR]
dt
• r inst. = -dR] = - slope of R
dt
• r inst. = +dP] = + slope of P
dt
= +dP]
dt
• FACTORS AFFECTING THE RATE OF A CHEMICAL REACTION(I)
(II)
(III)
(IV)
(V)
Nature of reactant
Concentration of reactant
Temperature
Surface area of reactant
Radiation
• GENERAL EXPRESSION FOR RATE OF REACTION:For a general chemical reaction
aA + bB  cC + dD
Rav. = -1 A] = -1 B] = 1 C] = 1 D]
a t
b t
c t
d t
Rinst. = -1 dA] = -1 dB] = 1 dC] = 1 dD]
a dt
b dt
c dt
d dt
• RATE LAW:- It is experimentally determined
expression which relates the rate of reaction
with the concentration of reactants.
For a hypothetical reaction
A + B  Products
Rate  A]m B]n
Rate = k A]m B]n
Where k is the rate constant .
If A] = B] = 1 Mol L-1 than Rate = k
Thus rate constant is the rate of reaction when
concentration of each reactant in the reaction is
unity.
• ORDER OF REACTION:- It may be defined as
the sum of the power of the concentration of
reactants in the rate law expression. Order of
chemical reaction can be 1,2 or 3 and even
may be fractional.
• MOLECULARITY OF REACTION:- The total
number of reacting species( molecules, atoms
or ions) taking part in an elementary chemical
reaction. The molecularity of a reaction may
not be fractional.
• INTEGRATED RATE LAW FOR ZERO ORDER
CHEMICAL REACTION:Consider a general zero order reaction
RP
1
1
2
2
• Comparing eq-2 with strait line equation
y = m x + c , if we plot [R] against t we get a
strait line with slope= -k and intercept equal
to [R]0.
• Further simplify equation 2 we can get the
rate constant k
• Half life for zero order reaction-
• INTEGRATED RATE LAW FOR FIRST ORDER
CHEMICAL REACTION:-
1
1
2
3
4
5
5
4
6
7
2
t
3
• Half life for the first order of reaction:-
• PSEUDO FIRST ORDER REACTION:-
• DETERMINATION OF ORDER OF REACTION:1. Graphical Method:This method is applicable to those
reactions wherein only one reactant is
involved.
2. Initial rate Method:This method is used for those reactions
where more than one reactant is involved.
In this method we carried out some series
of experiments.
• We change the one reactant’s concentration
and determine the rate of reactions by
keeping the constant concentration of each
other reactants and compare the rate from
initial concentration rate.
• Similarly, we repeat the experiments for all
other reactants and compare the rate from
initial concentration rate and finally determine
the overall rate of reaction.
3. Integrated rate law Method:• In this method we put the data of the reaction
under investigation in all the integrated rate
equation one by one .
• The expression which gives a constant value of
rate constant decide the order of reaction.
• Temperature dependence of a rate of a
reaction:Most of the chemical reactions are
accelerated by increase in temperature. For
example, in decomposition of N2O5, the time
taken for half of the original amount of
material to decompose is 12 min at 50oC, 5 h
at25oC and 10 days at 0oC. We also know that
in a mixture of potassiumpermanganate
(KMnO4) and oxalic acid (H2C2O4), potassium
permanganate gets decolourised faster at a
higher temperature than that at a lower
temperature.
• It has been found that for a chemical reaction
with rise in temperature by 10°, the rate
constant is nearly doubled.
• The temperature dependence of the rate of a
chemical reaction can be accurately explained
by Arrhenius equation.
• It was first proposed by Dutch chemist, J.H.
van’t Hoff but Swedish chemist, Arrhenius
provided its physical justification and
interpretation.
1
k = A e -Ea /RT
• where A is the Arrhenius factor or the
frequency factor. It is also called preexponential factor. It is a constant specific to a
particular reaction. R is gas constant and Ea is
activation energy measured in joules/mole (J
–1
mol ).
• It can be understood clearly using the
following simple reaction
2H2(g) + I2(g)→ 2HI(g)
• According to Arrhenius, this reaction can take
place only when a molecule of hydrogen and a
molecule of iodine collide to form an unstable
intermediate.
• It exists for a very short time and then breaks
up to form two molecules of hydrogen iodide.
According to Arrhenius, this reaction can take
place only when a molecule of hydrogen and a
molecule of iodine collide to form an unstable
intermediate.
• It exists for a very short time and then breaks
up to form two molecules of hydrogen iodide.
• The energy required to form this intermediate,
called activated complex (C), is known as
activation energy (Ea). Reaction coordinate
represents the profile of energy change when
reactants change into products. Some energy
is released when the complex decomposes to
form products. So, the final enthalpy of the
reaction depends upon the nature of
reactants and products.
• Ludwig Boltzmann and James Clark Maxwell
used statistics to predict the behaviour of
large number of molecules. According to
them, the distribution of kinetic energy may
be described by plotting the fraction of
molecules (NE/NT) with a given kinetic energy
(E) vs kinetic energy. Here, NE is the number of
molecules with energy E and NT is total
number of molecules.
• The peak of the curve corresponds to the
most probable kinetic energy, i.e., kinetic
energy of maximum fraction of molecules.
• There aredecreasing number of molecules
with energies higher or lower than this value.
When the temperature is raised, the
maximum of the curve moves to the higher
energy value and the curve broadens out, i.e.,
spreads to the right such that there is a
greater proportion of molecules with much
higher energies.
• The plot of ln k vs 1/T gives a straight line.
Thus, it has been found from Arrhenius
equation that increasing the temperature or
decreasing the activation energy will result in
an increase in the rate of the reaction and an
• slope = – Ea/ R and intercept = ln A. So we can
calculate Ea and A using these values. At
temperature T1, equation (1) is
ln k1 = – Ea/RT1 + ln A
(2)
• At temperature T2 eq.(1) is
(3)
• A is the constant for this particular reaction.
• K1 and k2 are the rate constant for the
temperatures T1 and T2 respectively.
• Substracting eq(2) from eq(3)
(4)
• Effect of Catalyst on the rate of a chemical
reaction:• A catalyst is a substance which alters the rate
of a reaction without itself undergoing any
chemical change at the end of the chemical
reaction. For example MnO2 increases the
rate of decomposition of potassium chlorate
• According to intermediate complex theory a
catalyst participate in a chemical reaction by
forming temporary bonds with the reactants
resulting in a intermediate complex.
• This has a transitary existence and decompose
to yield products and the catalyst.
• Collision Theory of a Chemical Reaction:• According to this theory the molecules of
reactants are having sufficient kinetic energy
so they may collide with each other and make
product molecules.
• The number of collisions per second per unit
volume of the reaction mixture is known as
collision frequency (Z).
A + B →Products
• rate of reaction can be expressed as
• Rate = P ZAB e –Ea/RT
• Where ZAB = the collision frequency of
reactants A & B.
• P= Probability or steric factor.
• e –Ea/RT = fractions of molecules with energies
equal to greater than Ea.