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)
begin & end with same substance:
only phase is different

6. Examples - Phase Equilibrium
water & water vapor in sealed
container
ice cubes & water in insulated container
perfume in partially full, sealed flask

7. Solution Equilibrium: Solids
saturated solution = dynamic equilibrium
NaCl(s)  NaCl(aq)
dissolving & solidification occur at equal rates

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

9. Reversible Reactions N2(g) + 3H2(g)  2NH3(g) forward rxn:
N2 & H2 consumed; NH3 produced
2NH3(g)  N2(g) + 3H2(g)
reverse rxn:
NH3 consumed; N2 & H2 produced

10. 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

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

12. Reaction Rate depends on concentration of reactants
as concentration reactants ↓,
rate forward reaction ↓
as concentration product ↑,
rate reverse reaction ↑

13. 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!

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

15. 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

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

17. Reversible Reactions
once reach equilibrium, don’t produce any more product
bad news if product is what you’re selling

18. What you would really like to see…
can you change the equilibrium concentrations?
if so how can it be done?
for example, how can you maximize product?
What you would really like to see…

19. New equilibrium point
lots of product created as fast as possible

20. equilibrium can be changed or affected by:
anything that affects forward and reverse reactions differently

21. What factors affect rate of rxn?
concentration/pressure (gases only)
temperature
presence of catalyst

22. 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

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

24. 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

25. System only 2 possible actions: 1. shift to right & form more product
forward reaction speeds up more than reverse reaction
2. shift to left & form more reactant
reverse reaction speeds up more than forward reaction

26. A + B  C + D (at equilibrium)
If ↑ concentration A, how will system react?
wants to get rid of excess [A]
Use logic:
If you ↑ [A]: the system wants to ↓ [A]
must use A up, so forward reaction speeds up
How does new equilibrium mixture compare to original equilibrium mixture?
concentrations will be different but still constant

27. A + B  C + D ______ left [B] right [C] [D] [A] [D] [C] [B] [A]
increase 
[D]
increase 
[A]
[D]
[C]
[B]
[A]
equilibrium shift
stressor
decrease 
increase 
increase 
decrease 
decrease 
decrease 
increase 
increase 
decrease 
decrease 

28. Changes in Temp exothermic reaction: A + B  C + D + heat
If ↑ temperature, system shifts to consume heat
shifts to left
endothermic reaction: A + B + heat  C + D
shifts to right

29. 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 causes shift to right
If ↓ pressure:
system shifts to side with more moles of gas
↓ pressure causes shift to left

30. 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