about chemical kinetics

1. ISC 2015-16

2. About the Scientist
Scientist who give this was Henry
Louis Le Châtelier .
He gives the principle i.e. Le
Châtelier’s principle.
He was born on 8 October 1850 in
Paris and his nationality was French.
He was died on 17 September.
He was also elected to the Royal
Swedish Academy of Sciences in
1907.

3. Some important terms
Irreversible reaction – The chemical
reaction which proceed mainly in one
direction i.e. from right to left or left to
right.
e.g. 2Na(s) + 2H2O(l) 2NaOH(aq) + H2
(g).
Reversible reaction – The reaction
which proceeds simultaneously in both
the direction i.e. from left to right as well
as in the direction from right to left.
e.g. 2Na(g) + 3H2(g) 2NH3(g)

4. Forward reaction – The reaction
proceeding from left to right .
A + B C + D
Backward reaction – T he reaction
proceeding from right to left.
A + B C + D
Chemical Equilibrium – chemical
equilibrium is a particular state of a
reversible reaction in which the forward
and backward reactions occurs at the
same rate and the concentration of all
reactants and products remain constant,
i.e. do not change with time.

5. Dynamic nature of chemical
equilibrium.
As we have seen that the at state of chemical
equilibrium the concentration of all reactant and
product constant and do not change in with
time. Besides concentrations, all other
measurable properties of the system also
acquire constant values at equilibrium. But that
leads us to think that the forward as well as
backward reaction have stopped. But, this is not
true actually at equilibrium both forward as well
as backward reaction continue to occur.
However, they take in such a way that their
rates are exactly equals. Now we can say that

6. Le Châtelier’s Principle
According to Le Châtelier’s Principle “If a
chemical system at equilibrium
experiences a change in
concentration, temperature or
total pressure, the equilibrium will
shift in order to minimize that
change or to counteract- -s the
changes ”.

7. It’s factor’s - chemical and
physical equilibria.
It includes it’s factors i.e.
a) Change in concentration.
b) Change in pressure.
c) Change in Temperature.
d) Effect of catalyst.
e) Effect of Addition of Inert
gas.

8. a) Change in concentration.
According to Le-chatlier’s principle:
“When a reactant or product is added the
system shifts away from that added
component”.
“If a reactant or product is removed, the
system shifts toward the removed
component.”
It also can be understand with this also

9. Example of change in
concentrationWhat happens if the concentration of CO is
increased?
 To relieve the “stress” of added CO, according
to Le Châtelier’s principle, the extra CO must
be used up. In other words, the rate of the
forward reaction must increase to consume
CO.
 Think of the CO added on the left as “pushing”
the equilibrium to the right:

10. Continues…….
 The forward and reverse reaction rates adjust
until they are again equal and equilibrium is
reestablished.
 At this new equilibrium state, the value of [H2]
will be lower, because more has reacted with
the added CO, and the value of [CH3OH] will
be higher.
 The changes offset each other, however, so the
value of the equilibrium constant K remains
constant.

11. b) Change in pressure.
According to Le-chatlier’s principle:
 If the volume decreases, the concentration increases,
and there will be a shift to the side with the less amount
of moles.
 If the volume increases, the concentration decreases,
and there will be a shift to the side with the more
amount of moles.
 It also can be understand with this also

12. Example of change in pressure.
What happen when the pressure is increase
is act on Haber's process?
According to Le Châtelier’s principle
 Stress of pressure is reduced by reducing the
number of gas molecules in the container. While in
increasing the pressure.
 There are 4 molecules of reactants vs. 2 molecules
of products.
 Thus, the reaction shifts to the product ammonia.

13. Continues…….
What happen when the pressure is
decrease is act on Phosphorus penta
chloride(Pcl5)?
 Stress of decreased pressure is reduced by
increasing the number of gas molecules in the
container.
 There are two product gas molecules vs. one
reactant gas molecule.
 Thus, the reaction shifts to the products .
PCl5(g) = PCl3(g) +
Cl (g)

14. c) Change in Temperature.
According to Le-chatlier’s principle:
 Exothermic reaction – produces heat (heat is a product)
Adding energy shifts the equilibrium to the left (away from the
heat term).
 Endothermic reaction – absorbs energy (heat is a reactant)
Adding energy shifts the equilibrium to the right (away from the
heat term).
 It also can be understand by this also.

15. Example of change in
temperature.
F o r e x o t h e r m i c r e a c t i o n s .
According to Le Chatelier prinicple
N2(g) + 3H2(g) 2NH3(g); H = – 92.4 kJ/mol
if increase in temperature brings a net change in
the equilibrium state in that direction where this
extra heat is consumed. The net change is in
the backward direction and some ammonia will
decompose producing nitrogen and hydrogen.
Similarly if the temperature is decreased the
equilibrium shifts to the forward direction.
◦ Add heat  Shift to reactants
◦ Remove heat  Shift to products

16. Continues…….
F o r e n d o t h e r m i c r e a c t i o n s .
According to Le-chateliers principle
N2(g) + O2(g) 2NO(g); H = + 180.7 kJ/mol–1
If the temperature is increased the added heat
will be absorbed by the reactant and the net
change takes place to the equilibrium in the
forward direction. If the temperature in
decreased it will bring a 'net' change to
equilibrium in the backward direction i.e.
direction in which it is exothermic
Add heat  Shift to products
Remove heat  Shift to
reactants

17. d) Effect of catalyst.
A Catalyst lowers the activation
energy and increases the
reaction rate.
It will lower the forward and
reverse reaction rates,
Therefore, a catalyst has NO
EFFECT on a system at
equilibrium!
It just gets you to equilibrium

18. e) Effect of Addition of Inert
gas.
 An inert substance is a substance
that is not- reactive with any species
in the equilibrium system.
 These will not affect the equilibrium
system.
 If the substance does react with a
species at equilibrium, then there will
be a shift!
 Addition of an inert gas at constant
volume has no effect on equilibrium.

19. Applications of Le-chateliers
principle.
The Haber Process for Industrial
Production of Ammonia

20. Quick revision

21. Lets Have Some Question.
Given
S8(g) + 12O2(g)  8 SO3(g) + 808 kcals.
Predict what will happen when…..
 Oxygen gas is added?
 shift to product
The reaction vessel is cooled?
 Shifts to Products – to replace heat.
The size of the container is increased?
V increases, Pressure decreases, shifts to more
particles – to reactants!
Sulfur trioxide is removed?
Shift to products to replace it!
A catalyst is added to make it faster?
No change!