Equilibrium is a dynamic state where the rates of the forward and reverse reactions are equal, resulting in constant concentrations over time. The equilibrium constant, Kc, relates the concentrations of products and reactants at equilibrium and can be used to determine whether a reaction favors products or reactants. Factors like temperature, pressure, and concentration can influence the position of equilibrium according to Le Chatelier's principle, shifting it to counteract imposed changes and relieve stress on the system.

1. Equilibrium
By: Ali Wagih

2. Dynamic Equilibrium
Definition:
• Equilibrium is dynamic (reactions doesn’t stop)
• Equilibrium is achieved in a closed system
• At equilibrium there's no change in macroscopic properties (no
change in color and density)
• Equilibrium can be reached from either direction (doesn’t matter if
started with all reactants or all products)
Physical Systems:
• There will come a time when the rate of the forward reaction is
equal to the rate of the reverse reaction. At this point, the system
has reached equilibrium.
Chemical Systems:
• The concentrations of both reactants and products remain constant
over time

3. Equilibrium Constant
• For the hypothetical homogeneous chemical reaction:
aA+bB cC+dD
The equilibrium constant is defined as
KC = [C]^c [D]^d , where [A] signifies the molar
concentration of species
[A]^a [B]^b
• Note that the expression for the equilibrium constant
includes only solutes and gases; pure solids and liquids
do not appear in the expression.

4. Magnitude of the Equilibrium Constant
• When Kc >> 1, the reaction goes almost to completion
• When Kc << 1, the reaction hardly proceeds
If the value of Kc is large (Kc >> 1 ):
• the equilibrium lies to the right hand side - i.e. there is
a much greater concentration of the products than the
reactants
If the value of Kc is small (Kc << 1):
• the equilibrium lies to the left hand side - i.e. there is a
much greater concentration of reactants than
products.

5. Le Chatelier’s Principle
• States: when a stress is brought to bear on a
system at equilibrium, the system will react in
the direction that serves to relieve the stress.

6. Temperature
• The only thing which can change the value of Kc for a given
reaction is a change in temperature.
• The effect of a change of temperature on a reaction will
depend on whether the reaction is exothermic or
endothermic
• When the temperature increases, Le Chatelier's principle
says the reaction will proceed in such a way as to
counteract this change, i.e. lower the temperature.
• Endothermic reactions (Hrxn > 0 )will move forward, and
exothermic reactions (Hrxn < 0 )will move backwards
• The reverse is true for a lowering of temperature.

7. Pressure
• According to Le Chatelier, the position of equilibrium
will move in such a way as to counteract the change.
That means that the position of equilibrium will move
so that the pressure is reduced again.
• Increasing the pressure on a gas reaction shifts the
position of equilibrium towards the side with fewer
molecules.
• In the case of the same number of molecules on both
sides of the equilibrium reaction, increasing the
pressure has no effect whatsoever on the position of
the equilibrium.

8. Concentration
• When the concentration of a product is
increased, the reaction proceeds in reverse to
decrease the concentration of the products.
a+b c+d , reaction moves backwards
• When the concentration of a reactant is
increased, the reaction proceeds forward to
decrease the concentration of reactants.
a+b c+d , reaction moves forward

9. Value of the Equilibrium Constant
Equilibrium constants aren't changed if you
change the pressure of the system.
Equilibrium constants aren't changed if you add
a catalyst.
Temperature:
• Equilibrium constants are changed if you
change the temperature of the system
• As the temperature increases, the value of Kc
falls.

10. Catalyst
• Adding a catalyst makes absolutely no
difference to the position of equilibrium
• Le Chatelier's Principle doesn't apply to them.
• A catalyst speeds up the rate at which a
reaction reaches dynamic equilibrium.

11. The Haber Process
• Ammonia: (Haber Process)
• N2 + 3H2 2NH3 -92kJ/mol
• 400-450 degrees Celsius is not a low
temperature, which defies the Le Chatelier’s
principle, as it would be expected to have a lower
temperature to produce more ammonia.
However, this was done to speed up the rate of
the process
• 200 atm is not a very high pressure, but it was
used because very high pressures are expensive
to achieve

12. Sulphur Trioxide Formation
• 2SO2(g) + O2(g) <=> 2SO3(g) ∆H = -196 kJ/mol
Conditions required for the reaction:
• 1. Temperature: 450 oC
• 2. Pressure : 1 atm
• 3. Catalyst: Vanadium (V) oxide