ISC 2015-16

About the ScientistScientist 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.

Some important termsIrreversible 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)

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.

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

that “The chemical equilibrium is dynamic in

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 ”.

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.

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

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:

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.

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

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.

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) +

Cl2(g)(1 mole of gas) (2 mol of gas)

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.

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

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 exothermicAdd heat Shift to productsRemove heat Shift to reactants

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

faster!

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.

Applications of Le-chateliers

principle.

The Haber Process for Industrial

Production of Ammonia

Quick revision

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!