CHEMICAL EQUILIBRIUM

Two types of reactions:

Reversible reaction
Irreversible reaction

- A reversible reaction never goes to completion.

- A reversible reaction is endothermic in one direction and exothermic in the other direction.

A state of equilibrium is reached:

In a closed system.
When the rates of the forward and reverse reactions are equal.
The concentration of the products and reactants remains unchanged.

A + B C

E.g. The Evaporation-Condensation cycle of a volatile liquid in a closed container

(The rate of evaporation = the rate of condensation)

Closed System - No matter can be exchanged with the surroundings.

*concentration of the reactant and product will become constant

"RATE OF FORWARD REACTION = RATE OF BACKWARD REACTION"

CHARACTERISTICS OF EQUILIBRIUM

1. Dynamic (reaction occurring at microscopic level)

2. In a closed system

3. The concentration of products & reactants is constant

4. No macroscopic change in properties

5. Can be achieved from either direction

6. The products and reactants do not need to be equal for equilibrium.

Equilibrium Constant 'Kc' & Reaction Quotient 'Q'

Equilibrium Position

- A reaction with an equilibrium mixture with mostly products is said to lie to the right (large values of Kc).

- A reaction with an equilibrium mixture with mostly reactants is said to lie to the left (small values of Kc).

For some time, t, when the system is not at equilibrium, the reaction quotient, Q, takes the place of K. It can be used to find how the reaction will proceed. (Same formula as Kc).

Large values of K signify the reaction is product favored.
Small values of K signify the reaction is reactant favored.
If Q = K the system is at equilibrium.
If Q > K the system shifts to the left, consuming products and forming reactants until equilibrium is achieved.
If Q < K the system shifts to the right, consuming reactants and forming products until equilibrium is achieved.

THE ONLY THING THAT AFFECTS Kc IS TEMPERATURE!

A catalyst will have no affect on the position of equilibrium (will speed up reactions on both sides)

LeChatlier's Principle

"When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress."

E.g. A closed container of ice and water is at equilibrium. The temperature is then raised.

Ice + Energy Water

The system temporarily shifts to the right to restore equilibrium (more water is formed from the extra heat energy)

E.g. A closed container of N2O4 and NO2 is at equilibrium. NO2 is added to the container.

N2O4(g) + Energy 2NO2(g)

The system temporarily shifts to the left to restore equilibrium as there is not enough N2O4 to balance out the extra NO2.

E.g. A closed container of water and its vapor is at equilibrium. Vapor is removed from the system.

Water + Energy Vapor

The system temporarily shifts to the right to restore equilibrium as more water is being converted to vapor to replace the removed vapor.

E.g. A closed container of N2O4 and NO2 is at equilibrium. The pressure is increased.

N2O4(g) + Energy 2NO2(g)

The system temporarily shifts to the left to restore equilibrium because there are fewer moles of gas on that side of the equation.

HABER PROCESS - MAXIMIZING YIELD:

N2(g) + 3H2 (g) 2NH3(g) + heat

- Remove NH3 as it is produced

- Increasing pressure

- Lower temperature

- Adding a catalyst

IN A REVERSIBLE REACTION:

*Increase in temperature favors endothermic reaction

*Decrease in temperature favors exothermic reaction