1. Reversible Reactions – Part II

2. Le Chatelier’s Principle
For a system at equilibrium, an alteration in the conditions will cause the state of balance to be disturbed
The position of equilibrium will shift until a new state of balance is attained.
Therefore Le Chatelier’s Principle states that when a system in equilibrium is disturbed, the system will react in such a manner so as to partially remove the disturbance.
This leads to a new position of equilibrium.
Henri Louis Le Chatelier (1850 – 1936)

3. Factors affecting Chemical Equilibrium
Changes in Concentration or Partial Pressure
Example: N2(g) + 3H2(g) NH3(g)
When more nitrogen gas is added to the equilibrium mixture, concentration of nitrogen increases
By the Le Chatelier’s Principle, the equilibrium position shifts to the right so as to use up the nitrogen and decrease its concentration.
The concentration of hydrogen gas will also decrease while that of ammonia gas will increase.

4. Factors affecting Chemical Equilibrium
Effect of adding more N2 to the equilibrium mixture

5. Factors affecting Chemical Equilibrium
Question
FeCl3 + 3NH4CNS Fe(CNS)4 + 3NH4Cl
(ammonium (iron (III)
thiocyanate) thiocyanate)
An equilibrium mixture of the above 4 compounds is red in colour. The red colouration is due to iron(III) thiocyanate. Explain your observations if 25cm3 the following was added into 1dm3 of the mixture.
Iron(III) chloride
Ammmonium chloride solution

6. Factors affecting Chemical Equilibrium
Answer
Adding iron(III) chloride deepens the colour, as the concentration of iron(III) chloride increases. By the Le Chatelier’s Principle, the equilibrium position would shift to the right so as to use up iron(III) chloride and decrease its concentration. Thus more iron(III) thiocyanate, which gives the red colour, is formed and so the redness is deepened.

7. Factors affecting Chemical Equilibrium
Answer
Adding ammonium chloride lightens the colour, as the concentration of ammonium chloride increases. By the Le Chatelier’s Principle, the equilibrium position would shift to the left so as to use up ammonium chloride and decrease its concentration. Thus less iron(III) thiocyanate, which gives the red colour, is formed and so the redness is lightened.

8. Factors affecting Chemical Equilibrium
Changes in Temperature
This is a twofold effect
An increase in temperature quickens a chemical reaction
Heat is supplied to the reaction

9. Endothermic & Exothermic Process
An Endothermic Process is a process in which there heat is absorbed from the surrounding. Thus there would be a decrease in temperature in the system.
An Exothermic Process is a process in which there heat is released to the surrounding. Thus there would be a increase in temperature in the system.

10. Factors affecting Chemical Equilibrium
Changes in Temperature
Example 1: N2 + 3H2 2NH3
The forward reaction is exothermic. (heat is released to surroundings, hence temperature rises)
If the equilibrium mixture is heated, by Le Chatelier’s Principle, the position of equilibrium shifts to the left.
This is to decrease the temperature as the reverse reaction is endothermic.
Hence heating favours the decomposition of ammonia.
If the mixture of nitrogen and hydrogen is heated, equilibrium is reached sooner, as heating quickens the reaction and less ammonia is produced.

11. Factors affecting Chemical Equilibrium
Question
Reaction: 2NO2(g) N2O4(g)
Explain why cooling favours the conversion of nitrogen dioxide to dinitrogen teroxide (forward reaction).

12. Factors affecting Chemical Equilibrium
Answer
The forward reaction is exothermic. (heat is released to surroundings, hence temperature rises)
If the equilibrium mixture is cooled, by Le Chatelier’s principle, position of equilibrium shifts to the right.
This is to increase the temperature as forward reaction is exothermic.
Hence cooling favours the conversion of nitrogen dioxide to dinitrogen teroxide.

13. Factors affecting Chemical Equilibrium
Changes in Total Pressure
Example 1: N2(g) + 3H2(g) NH3(g)
(2 vols. of reactants) (1 vol. of product)
When total pressure increases, position of equilibrium shifts in a direction to lower the pressure of the mixture of gases.
In this case, the pressure of gases is lowered by forming more ammonia gas, due to the reduction of volume.
Thus an increase in external pressure shifts the equilibrium to the right in the equation above.

14. Factors affecting Chemical Equilibrium
Effect of increasing pressure on equilibrium. Increase in total pressure (by reducing volume) favours the side with a fewer number of moles of gases.

15. Factors affecting Chemical Equilibrium
Changes in Total Pressure
Example 2: H2(g) + I2(g) HI(g)
The number of gases molecules on both sides of the equation are the same.
Hence an increase in pressure has no effect on the position of the equilibrium.

16. Factors affecting Chemical Equilibrium
Presence of a Catalyst
A catalyst is a substance that increase the rate of a reaction but is itself left chemically unaltered at the end of the process.
When a catalyst is added to an equilibrium system, it increases both forward and reverse rates by the same extent, because it lowers the same activation energy of both forward and backward reactions.
Hence a catalyst does not affect the position of equilibrium. It only enables the state of equilibrium to be reached more rapidly.