1. 18-2 Shifting Equilibrium
Using Le Chatelier’s Principle

2. Le Chatelier’s Principle
If a system at equilibrium is subjected to stress, the equilibrium is shifted in the direction that tends to relieve the stress
Applies to changes in:
Pressure
Concentration
Temperature

3. Changes in Pressure
Only effects equilibrium systems where there are gases involved
In order to affect the system, the total number of moles of gas on the left side must be different than the number of moles on the right side
CaCO3(s) CaO(s) + CO2(g)
reactant side – 0 moles of gas, product side – 1 mole of gas
Higher pressure favors reverse reaction (Fewer CO2 are produced
Lower pressure more CO2 produced

4. Changes in Pressure N2(g) + 3H2(g) 2NH3(g)
Moles of gas on reactant side?
Moles of gas on product side?

5. Changes in Pressure N2(g) + 3H2(g) 2NH3(g)
Moles of gas on reactant side? 4
Moles of gas on product side? 2
Increase in pressure = increase in concentration of all species involved
To relieve this stress, shift the equilibrium to the right
Produce more NH3, using up N2 and H2 will relieve the stress
Value of Equilibrium Constant does not change

6. Changes in pressure
What would happen to the following reaction systems if the pressure is decreased?
3O2(g) O3(g)
NH4Cl(s) NH3(g) + HCl(g)
H2(g) + Cl2(g) HCl(g)

7. Changes in Concentration
An increase in concentration of a reactant is a stress on the system
It causes increased collision frequency and an increase in reaction rate (Remember Rate Laws )
A + B C + D
An increase in A would shift equilibrium to the right
A and B are used up faster and more C and D is produced
An increase in C or D would shift the equilibrium to the left
Decreasing C or D (same as increasing A or B) shifts the equilibrium to the right
Do not change the equilibrium constant

8. Changes in Temperature
Reversible reactions are exothermic in one direction and endothermic in the other
The effect of changing the temperature depends on which of the reactions is endothermic and which is exothermic

9. Changes in Temperature
Adding heat will shift the equilibrium so that energy is absorbed (favors endothermic)
Removing heat favors exothermic
Raising temperature always increases the rate of reaction
Equilibrium constant is affected!

10. Changes in Temperature
N2(g) + 3H2(g) NH3(g) + 92 kJ
Endothermic or Exothermic?

11. Changes in Temperature
N2(g) + 3H2(g) NH3(g) + 92 kJ
Endothermic or Exothermic?
Increase in temperature will favor the endothermic reaction so that energy is absorbed
Equilibrium will shift to the left

12. Changes in Temperature
556 kJ + CaCO3(s) CaO(s) + CO2(g)
Endothermic or Exothermic?

13. Changes in Temperature
556 kJ + CaCO3(s) CaO(s) + CO2(g)
Endothermic or Exothermic?
What will decreasing the temperature do?

14. Changes in Temperature
556 kJ + CaCO3(s) CaO(s) + CO2(g)
Endothermic or Exothermic?
What will decreasing the temperature do?
Will want to go towards the exothermic reaction (Favors the reverse reaction)
Equilibrium will shift to the left