1. Standard 9: Chemical Equilibrium chapter 18
Vocabulary:
Equilibrium position
Equilibrium constant
Reversible reaction
Rate
Concentration
Le Chatelier’s Principle
Chemistry.
Ms. Siddall

2. Standard 9b: equilibrium conditions
Reversible Reactions
Most reactions are ‘reversible’
Forward reaction: reactants make products
e.x. 3O2(g)  2O3(g)
Reverse reaction: products make reactants
e.x. 2O3(g)  3O2(g)
In a reversible reaction the forward and reverse reactions occur at the same time
e.x. 3O2(g)  2 O3(g)

3. Summary 1
What is a reversible reaction?

4. Reversible reactions reach
equilibrium: a balance between reactants and products
Conditions of Equilibrium:
rate of forward reaction = rate of reverse reaction
Concentration of reactants and products is constant (does not change)
NOTE:
Rate = speed
Concentration = number of particles or moles
example: [HCl] = concentration of HCl
6M HCl = 6mole/L HCl
= 6 moles of HCl per liter of solution

5. Summary 2
What is equilibrium?

6. Reversible reaction: X  Y
concentration
rate
[Y]
Y  X
time
time
Concentrations are constant
Reaction rates are equal
equilibrium

7. Summary 3
Describe the part of each graph that illustrates equilibrium conditions.

8. Le Chatelier’s Principle
9a: Le Chatelier’s Principle
Le Chatelier’s Principle
A system in equilibrium will react to relieve stress (change) and re-establish equilibrium
Stress:
Adding reactants or products
Removing reactants or products
Changing temperature
Changing pressure (for gases only)

9. Summary 4
According to Le Chatelier’s Principle, what will happen to a system at equilibrium if more reactants or products are added?

10. Example: N2(g) + 3H2(g)  NH3(g) + heat
Stress: Add N2
Stress relief:
Forward reaction (→) to get rid of N2
H2 is used up (↓)
NH3 and Heat are produced (↑)

11. Summary 5: N2(g) + 3H2(g)  NH3(g) + heat
Stress: remove N2
Stress relief:
Which way does equilibrium shift?
What happens to [H2]?
What happens to [NH3]?
What happens to heat?

12. Stress relief. Adding products or reactants
Equilibrium shifts to remove addition
Removing products or reactants
Equilibrium shifts to replace what has been removed
Gasses
Equilibrium shifts to produce:
more gas at low pressure
Less gas at high pressure

13. Summary 6
Why would a gas equilibrium system produce more gas at low pressure and less gas at high pressure?

14. Haber Process: N2(g) + 3H2(g)  NH3(g) + heat
Change
Equilib.
[N2]
[H2]
[NH3]
↑ [N2]
↓ [N2]
↑ [H2]
↓ [H2]
↑ [NH3]
↓ [NH3]

15. Haber Process: N2(g) + 3H2(g)  2NH3(g) + heat
Change
Equilib.
[N2]
[H2]
[NH3]
↑ heat
↓ heat
↑pressure
↓pressure

16. Summary 7 According to Le Chatelier’s Principle:
Increasing reactant concentration will cause:
other reactants to __________?
products to __________?
Decreasing reactant concentration will cause:

17. A(g) + B(g)  AB(g) + heat

18. Summary 8 ↑ heat ↓ [A] ↑pressure ↓[AB]
A(g) + B(g)  AB(g) + heat
Complete the table of equilibrium changes
Change
Equilib.
[A]
[B]
[AB]
↑ heat
↓ [A]
↑pressure
↓[AB]

19. Equilibrium Constant: Keq
HONORS Standard 9c: equilibrium constant
Equilibrium Constant: Keq
At equilibrium concentrations are constant
Keq represents concentrations of reactants and products at equilibrium
Example: aA + bB  cC + dD
Keq = [C]c[D]d
[A]a[B]b

20. Summary 9
write Keq expression for the Haber-Bosch Process: N2(g) + 3H2(g)  2NH3(g)

21. Concentrations calculated in mol/L (M)
Only solutions(aq) & gases(g) are considered
No solids (s)
No liquids (l)
Example: 2H2O(l)  2H2(g) + O2(g)
Keq = [H2]2[O2]

22. Summary 10
Fe(OH)2(aq) + 2HSO3(aq)  Fe(SO3)2(aq) + 2H2O(l)
Find Keq

23. What Keq tells us
If Keq ≤ 1 There are more reactants than products at equilibrium
If Keq ≤ 1/100 There are mostly reactants at equilibrium
If Keq ≥ 1 There are more products than reactants at equilibrium
If Keq ≥ 100 There are mostly products at equilibrium

24. Summary 11 CO(g) + 2H2(g)  CH3OH(g) Keq=290 at 430°C
Write the expression for Keq
Reaction is…
(mostly products or reactants?)

25. Solubility Ksp is the equilibrium constant for solubility
Example: AgCl(s)  Ag+(aq) + Cl-(aq)
Ksp AgCl = 1.77 x 10-10
Does not really dissolve, mostly solid
Example: AgNO3(s)  Ag+(aq) + NO3-(aq)
Ksp AgNO3 ~ 1 x 1010
Very soluble

26. Summary 13
Write the balanced equation for the dissolving of sodium sulfate.
Write a Ksp expression for the reaction.