Chemical Kinetics 3 Flashcards
thumb rule for change in rate of reacn with temperature? what is its basis?
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
what is arrhenius equation
k = A e^-Ea /RT
where A is the Arrhenius factor or the frequency factor. It is also called pre-exponential factor. It is a constant specific to a particular reaction.
R is gas constant and Ea is activation energy measured in joules/mole (J mol –1).
T is temperature in kelvin
define activation energy
The minimum extra amount of energy required by reactant molecules for converting
into products.
It is the extra amount of energy required by reactants to reach the activated complex/ transition state having higher energy.
general characteristics of maxwell-boltzmann curve
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 (Fig. 3.8).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 are decreasing 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 area under the curve
must be constant since total
probability must be one at all times.
why does the rate double with 10 rise in temperature
Increasing the temperature of the substance increases the fraction
of molecules, which collide with energies greater than Ea
. 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.
Thus, it has been found from Arrhenius equation (3.18) 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.
what is a catalyst?
A catalyst is a substance which increases the rate of a reaction without
itself undergoing any permanent chemical change. For example, MnO2
catalyses the following reaction so as to increase its rate considerably.
2KClO3—–MnO2—-> 2KCl + 3O2
AN inhibitor reduces the rate of reaction
action of catalyst using intermediate complex theory
According to this theory, a catalyst participates in a chemical reaction by
forming temporary bonds with the reactants resulting in an intermediate
complex. This has a transitory existence and decomposes to yield products
and the catalyst.
It is believed that the catalyst provides an
alternate pathway or reaction mechanism by
reducing the activation energy between
reactants and products and hence lowering
the potential energy barrier.
It is clear from Arrhenius equation (3.18)
that lower the value of activation energy faster
will be the rate of a reaction.
does catalyst alter the gibbs free energy of a reaction
A small amount of the catalyst can catalyse
a large amount of reactants. A catalyst does
not alter Gibbs energy, DG of a reaction. It
catalyses the spontaneous reactions but does
not catalyse non-spontaneous reactions.
It is also found that a catalyst does not change the equilibrium constant of
a reaction rather, it helps in attaining the equilibrium faster, that is, it
catalyses the forward as well as the backward reactions to the same
extent so that the equilibrium state remains same but is reached earlier
Collision Theory of Chemical Reactions
collision theory, which was developed by Max Trautz
and William Lewis in 1916 -18, provides a greater insight into the
energetic and mechanistic aspects of reactions. It is based on kinetic
theory of gases.
state the collision theory of chem reactions
According to this theory, the reactant molecules are assumed to be hard spheres and reaction is postulated to occur when
molecules collide with each other. The number of collisions per
second per unit volume of the reaction mixture is known as
collision frequency (Z).
Equation (3.23) predicts the value of rate constants fairly
accurately for the reactions that involve atomic species or simple
molecules but for complex molecules significant deviations are
observed.
2 barriers for making products
The reason could be that all collisions do not lead to the formation of products.
The collisions in which molecules collide with
sufficient kinetic energy (called threshold energy*) and proper
orientation, so as to facilitate breaking of bonds between reacting
species and formation of new bonds to form products are called as
effective collisions.
The proper orientation of
reactant molecules lead to bond
formation whereas improper
orientation makes them simply
bounce back and no products are
formed.
probability or steric factor
To account for effective collisions,
another factor P, called the probability
or steric factor is introduced. It takes into account the fact that in a
collision, molecules must be properly oriented
Rate= PZ e^-E/RT
Thus, 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.
drawbacks of collision theory
Collision theory also has certain drawbacks as it considers atoms/
molecules to be hard spheres and ignores their structural aspect.
