Topic 7: Equilibrium

Use this outline in conjunction with the IB syllabus.

 

7.1 Dynamic Equilibrium

Equilibrium can only happen at the following conditions:

1. Closed system – none of the products or reactants can escape the system
2. Constant temperature – T does not change throughout the reaction
3. Constant pressure (for gases)
4. No change in macroscopic properties – color, pH, concentration…

In the dynamic equilibrium, the concentration of the products and reactants remains constant although the reaction never goes to completion. It is because the rate of the forward reaction equals the rate of the reverse reaction. Equilibrium is induced by the constant macroscopic properties. 

The dynamic equilibrium can occur in both physical and chemical systems, i.e. during the changes of state and chemical reactions, respectively. 

Examples:

Physical change: Evaporation x Condensation of Water H₂O (l) ⇄ H₂O (g)

Chemical change: Haber process N₂ (g) + 3H₂ (g) ⇄ 2NH₃ (g)

7.2 The position of equilibrium

7.2.1 Equilibrium constant expression

 

7.2.2 Magnitude of the equilibrium constant

The equilibrium constant describes how far would a reaction proceed at a given temperature. Hence, only temperature affects the magnitude of the equilibrium constant. Pressure would not affect K_C, but would be able to shift the equilibrium.

If the K_C is much larger than 1, the reaction goes almost to completion because the concentration of products must be higher than the concentration of reactants.

If the K_C is much smaller than 1, the reaction hardly proceeds because the concentration of reactants is much higher than the concentration of the products.

7.2.3 Qualitative effect of shifts in temperature, pressure and concentration on the equilibrium and 7.2.4 Effect of catalyst

Le Chatelier’s Principle states that when a system in equilibrium is agitated by a change in the temperature, pressure or concentration, it will shift in such a way to minimize or counteract the effect of the disturbance.

Effect of:	Position of equilibrium	Value of KC
Concentration	If the concentration is increased on one side, the equilibrium will shift to the other side to balance the stress. Increased concentration of products speeds up the reaction (collision theory) in the one way to produce more reactants.
Pressure	With the increasing pressure, the equilibrium will always shift in such a ways to produce less gas particles. If the number of gas particles is the same on the both sides, there will be no change.
Temperature	The direction of the endothermic reaction will be always favored if the heat will be added (system tries to use up the energy)
Catalyst	Catalyst lowers the activation energy for the both reaction directions. i.e. a catalyst speeds up both forward and reverse reaction equally.

 K_C changes for the shift in temperature, but if other kind of a disturbance (pressure or concentration) is applied, the system will shift in such a way to reduce the effects of the stress and reach the original equilibrium constant (K_C) again.

7.2.5 Haber process, Contact process

Haber process – industrial production of ammonia

N₂ (g) + 3H₂ (g) ⇄ 2NH₃ (g) ΔH= negative

Conditions:

1. High temperature: 450 °C
2. High pressure: 200 atm = 20,200 kPa
3. Catalyst: iron solid

Use of ammonia:

1. Fertilizers
2. Explosives
3. Dyes
4. Household cleaners
5. Manufacturing of nitric acid
6. Nylon

Contact process – industrial production of sulfur trioxide (used for the production of sulfuric acid)

2SO₂ (g) + O₂ (g) ⇄ 2SO₃ (g) ΔH= negative

 

Conditions:

1. High temperature: 450 °C
2. Standard or slightly high pressure: 1-2 atm = 101 - 202 kPa
3. Catalyst: Vanadium Oxide

Use of sulfuric acid (H₂SO₄):

1. Acid in the car batteries
2. Fertilizers
3. Dehydrating agent
4. Catalyst
5. Measure of economic development