Chemical Kinetics Introduction
The basics of kinetics - definitions of rates and
orders.
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Chemical Kinetics - Introduction
Definitions
Definitions of Chemical Kinetics:
Chemical kinetics is the study of the speed of chemical reactions
and application of the knowledge and the theories obtained from
the observations.
Chemical Kinetics - Introduction
Definitions
Defining the rate of chemical reactions
Just as with thermodynamics, in chemical kinetics one needs to
make clear what is meant by a rate of a chemical reaction. There
could be several ways to do this. To illustrate this a classic
example will be used, that is the reaction:
H2(g) + I2(g) ! 2HI(g)
Chemical Kinetics - Introduction
Definitions
Defining the rate of chemical reactions illustrated with the reaction:
H2(g) + I2(g) ! 2HI(g)
The rate could be defined as the rate of change in the hydrogen
pressure, which is negative, or (keeping rates positive):
RateH
2
ΔP
ΔP
= ! Δt since Δt < 0
PH
2
ΔP
Δt
Time
Chemical Kinetics - Introduction
Definitions
Defining the rate of chemical reactions illustrated with the reaction:
H2(g) + I2(g) ! 2HI(g)
On the other hand, the rate could be defined as the rate of change
in the iodine pressure, which is also negative, or:
RateI
2
ΔP
ΔP
= ! Δt since Δt < 0
PI
2
PH
2
ΔP
Δt
Time
Chemical Kinetics - Introduction
Definitions
Defining the rate of chemical reactions illustrated with the reaction:
H2(g) + I2(g) ! 2HI(g)
The rate could also be defined as the rate of change in the product
pressure, that is the HI pressure, which is positive:
RateHI
ΔP
ΔP
= + Δt since Δt > 0
Δt
ΔP
PI
PHI
2
PH
2
Time
Chemical Kinetics - Introduction
Definitions
Defining the rate of chemical reactions illustrated with the reaction:
H2(g) + I2(g) ! 2HI(g)
nH
nI
n HI
By stoichiometry these are related since:
1 = 1 = 2
2
2
ΔPH
ΔPI
1 ΔPHI
Thus, recognizing the negative sign: !
Δt = ! Δt = + 2 Δt
2
n
PI
PHI
2
PH
2
Time
2
Chemical Kinetics - Introduction
Rate Laws
By observation, most reactions may be expressed in the form of a
rate law. These laws relate the reaction rate to multiplicative
expression of the reactant and product concentrations or amounts.
The equation form for a reaction expressed as:
aA + bB + ... ! cC + dD
is:
Rate = k[A]n[B]m...[C]p[D]q...
where the brackets “[]” can indicate not just concentration but also
pressure, mole fraction or amount.
The values for n, m, p, q, etc., are usually integer numbers, positive
or negative, but could also be rational fractions such as ½, a, etc.
The constant k is referred to as the “rate constant”.
Chemical Kinetics - Introduction
Rate Laws
Rate = k[A]n[B]m...[C]p[D]q...
The numbers n, m, p, q are referred to as the order of the reaction
with respect to A, B, C and D. Usually the Cardinal name is used,
that is first, second, third, etc., if the form exists. (For example,
the one half order doesn’t have a cardinal name.)
One says that the reaction is are certain order with respect to A, B,
C or D or that the overall order of the reaction is the sum of the
individual orders
The only reaction orders that make sense for simple, one-step
reactions in terms of molecular theory are 0th, 1st and 2nd order
overall. Other orders arise from complications where reactions
may have multiple steps or preconditions attached.
Rate = k[A]n[B]m...[C]p[D]q...
As an example, the reaction given for illustration:
H2(g) + I2(g) ! 2HI(g)
is observed under most conditions to obey the law:
Rate = kPH PI
2
2
This indicates that the reaction is 1st order in H 2, 1st order in I2 and
2nd order overall.
Order with respect to H2 = 1st
Order with respect to I2 = +1st
Overall reaction order = 2nd order
Rate = k[A]n[B]m...[C]p[D]q...
For another example, consider the reaction:
3H2 + 2U ! 2UH3
It is observed to obey the law:
Rate = kPH½ PU
2
This indicates that the reaction is ½ order in H 2, 1st order in U and
1½ order overall.
Order with respect to H2 = 1/2
Order with respect to U = +1st
Overall reaction order = 1½ order
Rate = k[A]n[B]m...[C]p[D]q...
...and for the reaction:
2NO2 ! N2O4
The observed rate law is:
Rate = kPNO2
2
This indicates that the reaction is 2nd order in NO 2, the only
individual order and thus 2nd order overall.
Order with respect to NO2 = 2nd
Overall reaction order
= 2nd order
Chemical Kinetics Introduction
The basics of kinetics - definitions of rates and
orders.
THE END