Transcript Kinetics - A Study o..

Collision Rate Model
Three conditions must be met at the nanoscale level if a reaction is to occur:
• the molecules must collide;
• they must be positioned so that the reacting
groups are together in a transition state
between reactants and products;
• and the collision must have enough energy
to form the transition state and convert it
into products.
Transition State:
Activated Complex or
Reaction Intermediates
• an unstable arrangement of atoms that has
the highest energy reached during the
rearrangement of the reactant atoms to give
products of a reaction
Activation Energy
the minimum energy required to start a reaction
In fact, usually only a VERY SMALL fraction of the total number of
collisions are successful……..resulting in product formation.
The 3 conditions must be fulfilled
for a successful collision resulting in
product formation.
In reality we are mainly limited by
the overall kinetic energy of the
molecules (1st condition).
……..molecules are often moving too
slowly which prevents a close enough
approach for rxn. to occur….
Only some collisions are SUCCESSFUL………..
Most collisions DON’T result in a
Unsuccessful Collision
reaction……………..so even though
molecules are constantly in motion
and colliding…….only a few of these
collisions are actually successful.
Why ?????
…..because there are THREE
CONDITIONS that must be met
for a reaction to occur.
THREE conditions must be met for a reaction to occur :
1st Condition: Two molecules must
have sufficient kinetic energy
(meaning they must be moving fast
enough)
Successful Collision
2nd Condition: The sum of the
molecular momentum vectors must be
near zero
(meaning molecules have to hit each
other directly)
3rd Condition: There must be overlap
of the involved electronic orbitals of
the molecules
(meaning the molecules must be
properly oriented)
Product
What affect does
TEMPERATURE have
on the rate of a
reaction?
Temperature & Reaction Rate
What is a Catalyst?
Catalyst
• substance which speeds up the rate of a
reaction while not being consumed
What are the two types of catalysts?
Catalyst
• substance which speeds up the rate of a
reaction while not being consumed
Homogeneous Catalysis - a catalyst which is
in the same phase as the reactants
Heterogeneous Catalysis - a catalyst which is
in the different phase as the reactants
catalytic converter
– solid catalyst working on gaseous materials
Enzymes
any one of many specialized organic
substances, composed of polymers of
amino acids, that act as catalysts to
regulate the speed of the many
chemical reactions involved in the
metabolism of living organisms.
ln k = ln A – (Ea / R T)
y = B + mx
The parameter Ea is obtained from the slope which is equal to (–Ea/R)
Ea is called the activation energy.
A and Ea are together referred to as the Arrhenius parameters.
• Ea is given by slope of ln k versus 1/T ……..thus the higher the
activation energy the stronger the temperature dependence of
of the rate constant
(meaning the steeper the slope)
• If a reaction has a zero activation energy, its rate is independent
of temperature.
• In some cases the temperature dependence of reactions is
“non-Arrhenius like” meaning a straight line is not obtained when
ln k is plotted against 1/T
Potential Energy Profile for an Exothermic Reaction
How does the potential energy change
in the course of a reaction ?
…..consider a reaction where reactants
A and B react to form products
……A and B collide, come into contact
and distort, begin to exchange or
discard atoms………………..
…….potential energy rises to maximum
and cluster of atoms at this point
is termed activated complex…………..
……potential energy then falls and
reaches characteristic value for
products
Activated
Complex
Arrhenius Parameters
Activation Energy
ln k = ln A – (Ea / R T)
or
Pre-exponential Factor
……the pre-exponential factor
is a measure of the rate at which
collisions occur irrespective of
their energy.
k=A e
– Ea/RT
Fraction of collisions
with a kinetic energy
in excess of an Ea is
given by the Boltzmann
distribution as e – Ea/RT
….this is fraction of collisions
with enough kinetic energy
to lead to reaction
What fraction of the molecules have the minimum amount of kinetic
energy required for a successful collision ?
………to define number of molecules with sufficient KE to react we use
the Boltzmann distribution with Ea as the energy difference……………
(*Remember we discussed Boltzmann Distribution in Section 2)
The ratio of the populations of molecules in any two energy
levels (E1 and E2) is given by the Boltzmann Distribution Law:
N2 = e
N1
–(E)/(kT)
………where  E = Ea
kB = Boltzmann Constant = 1.381 x 10-23 JK-1
N2 = e
N1
–(Ea)/(k T)
B
……so probability of molecules being in higher energy state is:
probability

e
– (ΔE/RT)
..….the rate of our reaction (successful collisions per unit
time) is also proportional to this fraction:
RATE  e
– (Ea/RT)
Arrhenius Parameters
Activation Energy
ln k = ln A – (Ea / R T)
or
Pre-exponential Factor
……the pre-exponential factor
or frequency factor is a
measure of the rate at which
collisions occur irrespective of
their energy.
k=A e
– Ea/RT
Fraction of collisions
with a kinetic energy
in excess of an Ea is
given by the Boltzmann
distribution as e –Ea/RT
….this is fraction of collisions
with enough kinetic energy
to lead to reaction
• activation energy (Ea) is the minimum amount of energy required to
initiate the chemical reaction.
• factor e-Ea/RT resembles Boltzmann distribution law….and represents
the fraction of molecular collisions that have energy equal to or
greater than the activation energy, Ea
• A, the frequency factor represents the frequency of collisions
between reactant molecules
……….A is temperature dependent.
• plot of ln k versus 1/T gives straight line with…………
slope = - Ea/R
We can calculate the rate constant
at different temperatures……..
ln k1 = ln A – (Ea / RT1)
ln k2 = ln A – (Ea / RT2)
Combine these two equations…………
ln (k2/k1) = - (Ea/R) (1/T2 - 1/T1)
……….if we know Ea and rate constant at one temperature
we can calculate the rate constant at another temperature.