chemical kinetics-kinetic vs thermodynamic

Physical Chemistry Chemical Kinetic
DR FARHAD M. ALI 00-CHEMICAL KINETICS- KINETIC VS THERMODYNAMIC-2010-2021
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Zakho University-Faculty of Science-Chemistry Department
2020 / 2021
 00-Chemical Kinetics-Thermodynamic vs Chemical Kinetic
Lecturer: Dr Farhad M. Ali
 “What changes in a chemical reaction?”
 “How fast does it change?” and
 “How complete is the change?”

Physical Chemistry Chemical Kinetic
DR FARHAD M. ALI 00-CHEMICAL KINETICS- KINETIC VS THERMODYNAMIC-2010-2021
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Thermodynamic vs Kinetic
The study of chemical reactivity may be broadly divided into the subject areas
of reaction stoichiometry, reaction kinetics, and reaction thermodynamics.
Reaction Stoichiometry deals with the classiﬁcation of chemical reactions,
their expression as properly balanced net chemical equations, and the various
quantitative calculations that are based upon these balanced equations.
Reaction Kinetics deals with the determination of rate laws and the deduction
of reaction mechanisms.
Reaction Thermodynamics deals with reaction efﬁciency and chemical
equilibrium as a function of the relative stabilities of the various reactants and
products, their concentrations, and the ambient temperature and pressure.
Thermodynamic: Thermodynamics is the study of energy changes
accompanying physical and chemical changes. The term itself clearly suggests what
is happening -- "thermo", from temperature, meaning energy, and "dynamics",
which means the change over time. Thermodynamics can be roughly encapsulated
with these topics:
 Heat and Work
 Energy
 Enthalpy
 Entropy
 Gibbs Free
Energy
Thermodynamics: all about “if”
 Tells whether or not a process or a reaction can occur (is there a decrease in
free energy?)
 Applicable to systems in stable or metastable equilibrium

Physical Chemistry Chemical Kinetics
DR FARHAD M. ALI
00-CHEMICAL KINETICS-KINETIC VS THERMODYNAMIC-2020-2021
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 Sufficient driving force is needed to enforce a favourable transformation.
What is Chemical Kinetic concern about?
The area of chemistry that is concerned with the speeds, or rates, of reactions is
called chemical kinetics. Chemical kinetics is a subject of broad importance. It
relates, for example, to how quickly a medicine is able to work, to whether the
formation and depletion of ozone in the upper atmosphere are in balance, and to
industrial problems such as the development of catalysts to synthesize new materials.
It is to understand how to determine the rates at which reactions occur, but also to
consider the factors that control these rates.
For example:
▪ What factors determine how rapidly food spoils?
▪ How does one design a fast-setting material for dental fillings?
▪ What determines the rate at which steel rusts?
▪ What controls the rate at which fuel burns in an automobile engine?
Kinetics: all about “how”
 how fast or slow a process can occur, i.e., determining the rate
 applicable to systems in transition from no equilibrium to equilibrium, or
between two equilibrium states
 kinetics of a process is generally about how to overcome the energy barrier to
finish the transformation from the starting (reactant) state to the final (product)
state

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Thermodynamics points the way and makes it possible
Chemical change is driven by the tendency of atoms and molecules to rearrange
themselves in a way that results in the maximum possible dispersion of thermal
energy into the world. The observable quantity that measures this spreading and
sharing of energy is the free energy of the system.
As a chemical change takes place, the quantities of reactants and products change in
a way that leads to a more negative free energy. When the free energy reaches its
minimum possible value, there is no more net change and the system is said to be
in equilibrium.
The beauty of thermodynamics is that it enables us to unfailingly predict the net
direction of a reaction and the composition of the equilibrium state even without
conducting the experiment; the standard free energies of the reactants and products,
which can be independently measured or obtained from tables, are all we need.
...but thermodynamic says nothing about how long it will take to get there!
How fast!
It is worth noting that the concept of "time" plays no role whatsoever in
thermodynamics. But kinetics is all about time.
The "speed" of a reaction — how long it takes to reach equilibrium — bears no
relation at all to how spontaneous it is (as given by the sign and value of ΔG°) or
whether it is exothermic or endothermic (given by the sign of ΔH°). Moreover,
there is no way that reaction rates can be predicted in advance; each reaction must
be studied individually. One reason for this is that

DR FARHAD M. ALI
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 the stoichiometric equation for the reaction says nothing about
its mechanism
A process concerned for Kinetics could be a chemical reaction, or a transformation
between two material structures (or phases), for which only the crystalline structure
(atomic arrangement) changes, while the chemical compositions (concerned
elements, ionic valence state, etc) remain the same.
Typical examples of “phase transformation” include freezing of water, eutectoid
transformation of steel between austenite (-Fe) and pearlite (-Fe + Fe3C),
transition between graphite and diamond (but takes forever), etc.
Kinetic and thermodynamic considerations
A chemical reaction can be defined by two important parameters- the Gibbs free
energy associated with a chemical transformation and the rate of such a
transformation. These parameters are independent of each other. While free energy
change describes the stability of products relative to reactants, the rate of any
reaction is defined by the energy of the transition state relative to the starting
material. Depending on these parameters, a reaction can be favourable or
unfavourable, fast or slow and reversible or irreversible.
A favourable reaction is one in which the change in free energy ∆G° is negative.
∆G° can be written as a function of change in enthalpy (∆H°) and change
in entropy (∆S°) as ∆G°= ∆H°-T∆S°. Practically, enthalpies, not free energy, are
used to determine whether a reaction is favourable or unfavourable, because ∆H°
is easier to measure and T∆S° is usually too small to be of any significance (for
T<100°C).

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When a reactant can form two different products depending on the reaction
conditions, it becomes important to choose the right conditions to favour the
desired product.

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➔ If a reaction is carried out at relatively lower temperature, then the
product formed is one lying across the smaller energy barrier. This is called kinetic
control and the ratio of the products formed depends on the relative energy barriers
leading to the products. Relative stabilities of the products do not matter.
➔ At higher temperatures the molecules have enough energy to cross over
both energy barriers leading to the products. In such a case, the product ratio is
determined solely by the energies of the products and energies of the barrier do not
matter. This is known as thermodynamic control and it can only be achieved
when the products can inter-convert and equilibrate under the reaction condition.
A reaction coordinate diagram can also be used to qualitatively illustrate kinetic and
thermodynamic control in a reaction.
In the following diagram a rock rolls down a valley. You can guess that the
rock is eventually going to roll downhill, and eventually come to rest at the
bottom of the valley.
You know that it is not going to move unless something moves it. In other words,
it needs some kinetic energy to get going.

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Now it is really obvious that the
rock isn't going anywhere until
it gains enough kinetic energy to
overcome the little hill between
the valley it is in, and the deeper
valley to the right.
We call the first valley a local
minimum in the potential
energy surface. In mathematical
terms, this means that the first derivative
of potential energy with respect to
position is zero:
dE / dx = 0 (the potential energy in
relative to this point is zero. In other words, the slope is zero and the shape is
concave up (or convex).

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The deeper valley to the right is the global minimum. It has the same
mathematical properties, but the magnitude of the energy is lower - the valley is
deeper.
If it doesn't have enough kinetic energy to move out of its current position, we say
that it is kinetically stable or kinetically trapped. If it has reached the global
minimum, we say it is thermodynamically stable. The one we are in has the
lowest thermodynamic potential energy, then we are in a thermodynamically
stable state.
Thermodynamic Versus Kinetic Control of Reactions
Conjugated hydrohalogenation is one of the reactions that undergoes
kinetic and vs thermodynamic control
When a conjugated diene undergoes an electrophilic addition reaction, two factors:
the temperature at which the reaction is carried out and the structure of the
reactant—determine whether the 1,2-addition product or the 1,4-addition product
will be the major product of the reaction. When a reaction produces more than one
product, the product that is formed most rapidly is called the kinetic product, and
the most stable product is called the thermodynamic product.
⚫ Reactions that produce the kinetic product as the major product are said to
be kinetically controlled.
It needs at least enough kinetic energy to overcome all the local maxima
along the path between its current local minimum and the global
minimum.

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⚫ Reactions that produce the thermodynamic product as the major product are
said to be thermodynamically controlled.
 “The thermodynamic product is the most stable product.”
 ‘‘In chemicals reactions that have more than one product, being one
of them the kinetic product and the other the thermodynamic
product, the temperature dependency is a relevant factor.’’
For many organic reactions, the most stable product is the one that is formed most
rapidly. In other words, the kinetic product and the thermodynamic product are one
and the same. Electrophilic addition to 1,3-butadiene is an example of a reaction in
which the kinetic product and the thermodynamic product are not the same: The 1,2-
addition product is the kinetic product, and the 1,4-addition product is the
thermodynamic product.
 “The kinetic product is the product that is formed most rapidly”
Example 1: addition of HBr to 1,3-butadiene

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For a reaction in which the kinetic and thermodynamic products are not the same,

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the product that predominates depends on the conditions under which the reaction
is carried out:
 If the reaction is carried out under sufficiently mild (low-temperature)
conditions to cause the reaction to be irreversible, the major product will be
the kinetic product.
For example, when addition of HBr to 1,3-butadiene is carried out at -80 °C, the
major product is the 1,2-addition product.
 ‘’The kinetic product predominates when the reaction is irreversible”
 If, on the other hand, the reaction is carried out under sufficiently vigorous
(high-temperature) conditions to cause the reaction to be reversible, the major
product will be the thermodynamic product. When the same reaction is carried out
at 45 °C, the major product is the 1,4-addition product. Thus, the 1,2-addition
product is the kinetic product (it is formed more rapidly), and the 1,4-addition
product is the thermodynamic product (it is the more stable product).
 “The thermodynamic product predominates when the reaction is
reversible.”

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➢ Hot reaction conditions favour the thermodynamic product.
➢ Cold reaction conditions favour the kinetic product
The addition reaction coordinate diagram:
A reaction coordinate diagram helps explain why different products predominate
under different reaction conditions. The first step of the addition reaction—
addition of a proton to C1—is the same whether the 1,2-addition product or the
1,4-addition product is being formed.
It is the second step of the reaction that determines whether the nucleophile (Br-
)
attacks C2 or C4. Because the 1,2-addition product is formed more rapidly, we
know that the transition state for its formation is more stable than the transition state
for formation of the 1,4-addition product. This is a reaction in which the less stable
product has the more stable transition state.

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Example 2: Hydrogen chloride addition to 1,3 butadiene:
Two products form when HCl is added to 1,3 butadiene; 1,3 product (A) and 1,4
product (B):
A is the major product at low temperatures and limited time. B is the major
product at high temperatures and unlimited time.

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Getting Selectivity for either Thermodynamic Product or Kinetic Product
Kinetic products are in general, more easily formed than thermodynamic products
due to the lower energetics required. Since to form a thermodynamic product, you
will have to supply enough energy to the system, which is greater than that for the
kinetic product, you will always get some of the kinetic product in the reaction. But
there are some tricks you could use while doing these reactions in order to produce
more of either
To Get Kinetic Product:
A kinetic product is favored under non-equilibrium conditions.
1. Shortening the reaction time – this will ensure that there is not enough
time for a reversible reaction to occur to lead to the formation of more stable
thermodynamic product.

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2. Use of strong base (or strong reagents) – This will ensure that the process
is not reversible at the intermediate step.
3. Try doing the reaction at lower temperatures – that way you do not
provide more energy to cross the EA of the thermodynamic product. But keep in
mind, that lowering the temperature may also allow equilibrium conditions to make
the reaction reversible and hence allowing formation of thermodynamic conditions.
4. Use of hindered base –where abstraction of less hindered proton would
result in formation of kinetic product. If therefore, we use a hindered or bulky base,
the more hindered proton would not be readily taken up by the base, thus pushing
the kinetic control into the reaction.
To Get Thermodynamic Product:
In order to get thermodynamic control, you need conditions of reversibility at some
stage of the mechanism. If that exists then you can try the following:
1. Lengthening the reaction time – longer reaction times will ensure that the
equilibrium favors formation of more stable product

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2. Use of milder base (or milder reagents) – This will ensure a greater
equilibrium stage allowing reactions to occur in a reversible manner in the
mechanism.
3. Use of higher temperatures

chemical kinetics-kinetic vs thermodynamic

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