Scope of chemical kinetics
Chemical kinetics is the study of the rates and the mechanism of
chemical reactions. Commonly the measure of how fast the products are formed
and the reactants consumed is given by the rate values.
The study of chemical kinetics has been highly
useful in determining the factors that influence the rate, maximum yield and
conversion in industrial processes. The mechanism or the sequence of steps by
which the reaction occurs can be known. It is also useful in selecting the optimum
conditions for maximum rate and yield of the chemical process.
1. Rate of chemical reactions
The rate of a reaction tells us how fast the
reaction occurs. Let us consider a simple reaction.
A + B -- > C + D
As the reaction proceeds, the concentration of the reactant A and B
decreases with time and the concentration of the products C + D increase with
time simultaneously. The rate of the reaction is defined as the change in the
concentration of any reactant or product in the reaction per unit time.
For the
above reaction,
Rate of
the reaction
1. Rate of disappearance of A
2. Rate of disappearance of B
3. Rate of appearance of C
4. Rate of appearance of D
During the reaction, changes in the concentration is infinitesimally
small even for small changes in time when considered in seconds. Therefore
differential form of rate expression is adopted. The negative sign shows the
concentration decrease trend and the positive sign shows the concentration
increase trend.
Rate = concentration
change / time taken = - ∆ [A] / ∆t
=
-d[A] / dt = -d[B] / dt = -d[C] /
dt = -d[D] / dt
For a general balanced reaction, written with stoichiometries like x,y,
for the reactant and l,m for the product, such as
xA + yB -- > lC + mD.
The reaction rate is
Rate = -
(1/ x) d[A] / dt = - (1/ y)
d[B] / dt = + (1/ l) d[C] / dt
=
+ (1/ m) d[D] / dt
For
example : In the reaction,
H2 + Br2 -- > 2HBr
The
overall rate of the reaction is given by
Rate =
- d[H2] / dt = -
d[Br2] / dt = ½ d[HBr] / dt
Consider
the reaction, 2NO + 2H2 -- > N2 + 2H2O
Rate = -
1/2 d[NO] / dt =- 1/2 d[H2]
/ dt =
d[N2] / dt = 1/2 d[H2O] / dt
Units
of Rate
Reaction rate has units of concentration divided
by time. Since concentration is expressed in mol lit-1 or mol dm-3
the unit of the reaction rate is mol lit-1 s-1 or mol dm-3
s-1.
2. Factors
influencing reaction rates
There are number of factors which influence the rate of the reaction.
These are :
i.
Nature of the reactants and products
ii.
Concentration of the reacting species
iii.
Temperature of the system
iv.
Presence of catalyst
v.
Surface area of reactants
vi.
(vi) Exposure to radiation
(i) Effect of nature of the reactant and product
Each reactant reacts with its own rate. Changing the chemical nature of
any reacting species will change the rate of the reaction. For example,
in halogenation reactions, the reactions involving iodine is found to be slower
than those involving chlorine.
In case of products, some of them are capable of
reacting back to form reactants or some other kind of products. In such cases,
the overall rate will be altered depending on the reactivity of the products.
(ii) Effect of reacting species
As the
initial concentration of the reactants increase in the reaction mixture, the
number of reacting molecules will increase. Since the chemical reaction occurs
when the reacting species come close together and collide, the collisions are
more frequent when the concentrations are higher. This effect increases the
reaction rate.
(iii)
Effect of temperature
Increase in temperature of the system increases the rate of the
reaction. This is because, as the temperature increases the kinetic energy of
the molecules increases, which increases the number of collisions between the
molecules. Therefore the overall rate of the reaction increases. This condition
is valid only for endothermic reaction. For exothermic reaction the overall
rate decreases with increasing temperature.
(iv)
Effect of presence of catalyst
A catalyst is a substance that alters the rate of a chemical reaction,
while concentration of catalyst remaining the same before and after the
reaction. The addition of catalyst generally increases the rate of the reaction
at a given temperature. Also, catalyst is specific for a given reaction.
(v) Effect
of surface area of reactants
In case of reactions involving solid reactants and in case of
heterogeneous reactions, surface area of the reactant has an important role. As
the particle size decreases surface area increases for the same mass. More
number of molecules at the surface will be exposed to the reaction conditions
such that the rate of the reaction increases. Thus the reactants in the
powdered form (or) in smaller particles react rapidly than when present in
larger particles.
(vi)
Effect of radiation
Rates of certain reactions are increased by absorption of photons of
energy. Such reactions are known as photochemical reactions. For example, H2
and Cl2 react only in the presence of light. With increase in the intensity of
the light (or) radiation, the product yield increases. For photosynthesis light
radiation is essential and the process does not proceed in the absence of
light.
3. Rate law
According to concepts of chemical kinetics, the
rate of the reaction is proportional to the product of the initial
concentration of all the reactants with each reactant concentration raised to
certain exponential powers.
Consider a general reaction pA + qB cC + dD.
The rate
law is given by the expression,
Rate . [A]p
[B]q \Rate =
k[A]p [B]q
where k is
proportionality constant also known as the rate constant or velocity constant
of the reactions. p and q represent the order of the reaction with respect to A
and B. The values of k, p and q are experimentally determined for a given
reaction. Values of p and q need not be same as the stoichiometric coefficients
of the reaction.
Rate
constant
In the above general equation k represents the rate constant. Rate constant
or velocity constant (or) specific reaction rate is defined as the rate of the
reaction when the concentration of each of the reactants is unity in the
reaction.
When concentration of A and B is unity then, the rate constant is equal
to the rate of the reaction. When the temperature of the reaction mixture
changes, the value of rate constant changes.
4. Order of
the reaction
Order of a reaction is defined as the sum of the exponential powers to
which each concentration is raised in the rate expression. For example, if the
overall rate is given by the expression
Rate = k[A]p [B]q
Then, the
overall order of the reaction is (p+q). The order with respect to A is p. The
order with respect to B is q. If p=1; q=0 and vice versa, the order of the
reaction is 1, and the reaction is called first order. If p=1, q=1, the order
of the reaction is 2 and the reaction is called second order and so on.
A zero order reaction is one where the reaction rate does not depend
upon the concentration of the reactant. In this type of reaction, the rate
constant is equal to the rate of the reaction.
5. Unit of
rate constant
In general, rate expression for the reaction,
pA + qB -- >
cC + dD
Rate = k [A]p
[B]q
K= Rate / [A]p [B]q
The unit for the rate constant `k' depends upon the
rate of the reaction, the concentration of the reactants and the order of the
reaction.
In the case of the first order reaction.
K= Rate / [A]1 [B]2
K= mol dm -3 sec-1 / mol dm-3
Unit of
k = sec-1 for first order reaction.
Similarly unit of k = mol-1 dm3 sec-1
for second order reaction unit of k = mol-(n-1) dm3(n-1)
sec-1 for nth order reaction
Following are
the important differences between rate and rate constant of a reaction
Rate of reaction
1. It represents the speed at which the
reactants are converted into products at any instant.
2. At any instant of time, the rate depends
upon the concentration of reactants at that instant.
3. It decreases as the reaction proceeds.
4. Rate of rate determining step determines
overall rate value.
Rate constant of reaction
1. It is the constant of proportionality in
the rate law expression.
2. It refers to the rate of a reaction at the
specific point when concentration of every reacting species is unity.
3. It is constant and does not depend on the
progress of the reaction.
4. It is an experimental value. It does not
depend on the rate determining step.
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