Dr. Asra Hameed
ORDER OF REACTION
Kinetics is the study of the
rate of reactions, or how fast
they occur under different
It usually includes a study of
the mechanisms of reactions,
which is a look at how the
reacting molecules break apart
and then form the new
This knowledge allows chemists
to control reactions and/or
design new or better ways to
produce the desired products.
ORDER OF REACTION:
In chemical kinetics, the order of reaction with respect to
certain reactant is defined as the index, or exponent, to
which itsconcentration term in the rate equation is raised.
given a chemical reaction with a rate equation,
2A + B → C
r = k[A]2[B]1
The reaction order with respect to A in this case is 2 and
with respect to B in this case is 1; the overall reaction
order is 2 + 1 = 3.
It is not necessary that the order of a reaction be a whole
number – zero and fractional values of order are possible –
but they tend to be integers.
A first-order reaction is a reaction that
proceeds at a rate that depends linearly only
on one reactant concentration.
Rate is the reaction rate and k is the reaction
In first order reactions, the units of k are 1/s.
However, the units can vary with other order
Graphing First-order Reactions
The following graph
reactants versus time
for a first-order
In a pseudo-1st
-order reaction, we can manipulate the initial concentrations
of the reactants. One of the reactants, A, for example, would have a
significantly high concentration, while the other reactant, B, would have a
significantly low concentration. We can then assume that reactant A
concentration effectively remains constant during the reaction because its
consumption is so small that the change in concentration becomes
negligible. Because of this assumption, we can multiply the reaction rate,
k′, with the reactant with assumed constant concentration, A, to create a
new rate constant (k=k′[A]) that will be used in the new rate equation,
as the new rate constant so we can treat the 2nd
order reaction as a 1st
One way to create a pseudo-1st
-order reaction is to manipulate the
physical amounts of the reactants
In a second-order reaction, the sum of the exponents in the rate
law is equal to two.
The two most common forms of second-order reactions are
Case 1: Two of the same reactant (A) combine in a single
The reaction rate for this step can be written as
where k is a second order rate constant with units of M-1min-
1 or M-1s-1.
Two different reactants (A and B)
combine in a single elementary step.
The reaction rate for this step can be
where the reaction order with respect to
each reactant is 1.
For a second-order
the rate of reaction
increases with the
square of the
upward curving line
in the rate-
A zero-order reaction is a reaction that proceeds at
a rate that is independent of reactant concentration.
In other words, increasing or decreasing the
concentration of reactants will not speed up or slow
down the reaction, respectively.
This means that the rate of the reaction is equal to
the rate constant, k, of that reaction.
Graphing Zero-Order Reactions
If we plot rate as a function of
time, we obtain the graph below.
this only describes a narrow
region of time.
The slope of the graph is equal
to k, the rate constant.
Therefore, k is constant with
time. In addition, we can see
that the reaction rate is
completely independent of how
much reactant you put in.
Rate vs. time of a zero-order
The rate constant, k, has units
of mole L-1 sec-1.
If we increase the temperature
in the endothermic reaction
the rate of reaction will
increase and vise versa.
If we increase the temperature
in the exothermic reaction the
rate of reaction will decrease
and vise versa.
The De-bye Huckel equation may be used to
demonstrate that increased ionic strength would be
expected to decrease the rate of reaction between
oppositely charged ions and increase the rate of
reaction between similarly charged ions .
Thus, the hydrogen ions catalyzed hydrolysis of
sulphate esters is inhibited by increasing electrolyte
ROSO3 + H2O ROH + HSO4
Enzymes are affected by changes
The most favorable pH value - the
point where the enzyme is most
active - is known as the optimum
Extremely high or low pH values
generally result in complete loss
of activity for most enzymes.
pH is also a factor in the stability
of enzymes. As with activity, for
each enzyme there is also a
region of pH optimal stability.
Enzyme pH Optimum
Lipase (pancreas) 8.0
Lipase (stomach) 4.0 - 5.0
Lipase (castor oil) 4.7
Pepsin 1.5 - 1.6
Trypsin 7.8 - 8.7
Maltase 6.1 - 6.8
Amylase (pancreas)6.7 - 7.0
Amylase (malt) 4.6 - 5.2
Light energy may be aborbed by certain molecules
which then sufficiently activated for participation in a
Only frequencies in the visible and ultra-violet
region can provide sufficint energy to cause
Since the for enery for activation is provided by light
in these reaction the rates of the latter are
independent of temperature.
Photochemical reaction involve the absorption of
In acid catalysis and base catalysis a
chemical reaction is catalyzed by an acid or a base.
The acid is often the proton and the base is often a
hydroxyl ion. Typical reactions catalyzed by proton
transfer are esterfications and aldol reactions. In
these reactions the conjugate acid of the carbonyl
group is a better electrophile than the neutral
carbonyl group itself. Catalysis by either acid or
base can occur in two different ways: specific
catalysis and general catalysis.
In specific acid catalysis taking place in solvent S, the reaction rate is
proportional to the concentration of the protonated solvent molecules
.The acid catalyst itself (AH) only contributes to the rate
acceleration by shifting the chemical equilibrium between solvent S
and AH in favor of the SH+
S + AH → SH+
For example in an aqueous buffer solution the reaction rate for
reactants R depends on the pH of the system but not on the
concentrations of different acids.
This type of chemical kinetics is observed when reactant R1 is in a
fast equilibrium with its conjugate acid R1H+
which proceeds to react
slowly with R2 to the reaction product; for example, in the acid
catalysed aldol reaction.
In general acid catalysis all species capable of
donating protons contribute to reaction rate
The strongest acids are most
effective. Reactions in which proton transfer is rate-
determining exhibit general acid catalysis, for
example diazonium coupling reactions.
When keeping the pH at a constant level but
changing the buffer concentration a change in rate
signals a general acid catalysis. A constant rate is
evidence for a specific acid catalyst.
Hydrolysis is a reaction involving the
breaking of a bond in a molecule using
water. The reaction mainly occurs
between an ion and water molecules
and often changes the pH of a
solution. In chemistry, there are three
main types of hydrolysis: salt
hydrolysis, acid hydrolysis, and base
An oxidation-reduction (redox) reaction is a type
of chemical reaction that involves a transfer of
electrons between two species. An oxidation-
reduction reaction is any chemical reaction in
which the oxidation number of a molecule, atom,
or ion changes by gaining or losing an electron.
Redox reactions are common and vital to some
of the basic functions of life,
including photosynthesis, respiration,
combustion, and corrosion or rusting.
A chemical reaction that takes
place between a metal ion and a
molecular or ionic entity known as
a ligand that contains at least
one atom with an unshared pair
This is a sample reaction coordinate of a
Note that it involves an intermediate
and multiple transition states.
A complex reaction can be explained in
terms of elementary reactions.