2. Equilibrium State of balance
Condition in which opposing forces exactly balance/equal each other
Need 2-way or reversible situation
Need a closed system

3. Dynamic Equilibrium Macroscopic level Microscopic level
looks like nothing is happening
Microscopic level
lots going on

4. 3 Kinds of Equilibria Phase equilibrium – physical
Solution equilibrium – physical
Chemical equilibrium - chemical

5. Phase Equilibrium Phase changes are reversible processes
H2O(l)  H2O(g)
H2O(l)  H2O(s)
Same substance on both sides but
phase is different

6. Examples - Phase Equilibrium
Water & water vapor in sealed water bottle
Perfume in partially full, sealed flask
Ice cubes & water in insulated container
Dry ice & CO2(g) in a closed aquarium

7. Solution Equilibrium: Solids
Saturated solution = dynamic equilibrium
Dissolving & Solidification occur at equal rates

8. Solid in Liquid NaCl(s)  NaCl(aq)
Favored a little bit by higher temperature

9. Solution Equilibrium: Gases
CO2 in water
CO2(g)  CO2(aq)
favored by high pressure & low temperature

10. Reversible Reactions N2(g) + 3H2(g)  2NH3(g) Forward:
N2 & H2 consumed; NH3 produced
2NH3(g)  N2(g) + 3H2(g)
Reverse:
NH3 consumed; N2 & H2 produced

11. Reversible Reactions: 1 Equation
N2(g) + 3H2(g)  2NH3(g)
Forward reaction: reactants on L
Read left to right
Reverse reaction: reactants on R
Read in reverse: right to left
Reaction runs in both directions all the time

12. N2(g) + 3H2(g)  2NH3(g) Concentration H2 NH3 N2 Time
Why is this point significant?
Concentration H2
NH3 N2
Time

13. Reaction Rate Depends on concentration of reactants
As concentration reactants ↓,
rate forward reaction ↓
As concentration product ↑,
rate reverse reaction ↑

14. NO! Chemical Equilibrium
State in which forward & reverse rxns balance each other
Rateforward rxn = Ratereverse rxn
Does this mean concentrations reactants/products are equal?
NO!

15. Chemical Equilibrium Rateforward rxn = Ratereverse rxn
At equilibrium: concentrations all species are constant
stop changing
rarely ever equal

16. Reversible Reactions vs. Reactions that “Go to Completion”
If goal is to maximize product yield:
Easier in reaction that goes to completion
Use up all reactants
Left with only product
Reversible reactions are different
Look at Conc/time picture again

17. N2(g) + 3H2(g)  2NH3(g) Concentration H2 NH3 N2 Time Original
Equilibrium Point
Concentration H2
NH3 N2
Time

18. Reversible Reactions
Once reach equilibrium, don’t produce any more product
bad news if product is what you’re selling
Can you change the equilibrium concentrations? If so how can it be done? For example, how can you maximize product?

19. What you would really like to see…

20. New equilibrium point
Lots of product as fast as possible

21. Affecting Equilibrium
Equilibrium can be changed or affected by:
any factor that affects forward and reverse reactions differently

22. What factors affect rate of reaction?
Concentration/Pressure
Temperature
Presence of catalyst

23. Catalyst same effect on both forward & reverse reactions
Equilibrium reached more quickly, but “equilibrium point” not shifted
equilibrium concentrations are same with or without catalyst

24. Concentration, Pressure, Temperature
Changes in concentration, pressure, temperature affect forward & reverse reactions differently
Composition of equilibrium mixture will shift to accommodate these changes

25. LeChatelier’s Principle
“If system at equilibrium is subjected to stress, the system will act to reduce stress”
stress = change in concentration, pressure,
or temperature
System tries to undo stress

26. System Only 2 possible actions Shift to the right & form more product
forward reaction speeds up more than the reverse reaction
Shift to the left & form more reactant
reverse reaction speeds up more than the forward reaction

27. A + B  C + D (at equilibrium)
If ↑ concentration A, how will system react?
How does new equilibrium mixture compare to original equilibrium mixture?
Use logic:
If you ↑ [A]: the system wants to ↓ [A]
It has to use A up, so it speeds up the forward reaction

28. A + B  C + D DEC  ______ INC  Left [B] Right [C] [D] [A] [D]
Equil. Shift
Stress

29. Changes in Temp Exothermic reaction: A + B  C + D + heat
If ↑ temperature, system shifts to consume heat so shifts to left
Endothermic reaction: A + B + heat  C + D
If ↑ temperature, system shifts to consume heat so shifts to right

30. Changes in Pressure N2(g) + 3H2(g)  2NH3(g)
If ↑ pressure, system shifts to side with fewer moles of gas
left side: 4 moles of gas; right side: 2 moles
↑ pressure cause shift to the right
If ↓ pressure, system shifts to side with more moles of gas
↓ pressure cause shift to the left

31. H2(g) + I2(g)  2HI(g)
This system has 2 moles gas on left & 2 moles gas on right
Systems with equal moles gas on each side cannot respond to pressure changes so no shift occurs