1. Le Chatelier’s Principle Common Ion Effect Predicting Precipitates
Chapter 17 & 18, Part III
Le Chatelier’s Principle
Common Ion Effect
Predicting Precipitates

2. LeChatelier’s Principle
Henri LeChatelier studied shifts in the equilibrium of reactions and came up with his principle. He said:
If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.

3. LeChatelier’s Principle
Let’s look at how several factors will shift the equilibrium point.

4. Concentration ↑ [ ] of reactant causes rxn to speed up to the right →
↑ [ ] of product - rxn speeds up toward reactant ←
the opposite is also true . . .
↓[ ] of reactant causes rxn to make more reactant ←
↓[ ] of product - rxn tries to replace the lost product →

5. H2CO3 D H2O + CO2 Which way will equilibrium shift if adding: CO2
Which way will equilibrium shift if removing:

6. Temperature reversible rxns are exothermic one direction
and endothermic the other direction
↓ in temp favors exothermic
↑ in temp favors endothermic

7. 2SO2 + O2 D 2SO3 + heat
Which way will equilibrium shift if heat is added?
Which way will it shift if heat is removed?

8. Pressure (and volume) only affects gases
↓ volume results in ↑ pressure
when pressure ↑ rxn proceeds toward fewer particles, compare # of particles (coefficients) of reactant gas(es) with # of particles (coefficients) of product gas(es)

9. N2 + 3H2 D 2NH3
Assuming all the products and reactants are gases, which way does equilibrium shift when pressure is added?
Which way does it shift when pressure is relieved?

10. Reactions that run to Completion
Some rxns never work in reverse.
They don’t reach equilibrium b/c they only go one way. Reasons why:
1. one of the products is a gas, which escapes
2. a product precipitates (is insoluble)
3. one product won’t ionize enough to reform the reactant
ex: 2HCl NaOH  2NaCl + H2O

11. LeChâtelier’s Practice
X(g) + 2Y(g)  Z(g) + heat
How would each of the following changes shift equilibrium?
a. remove Z by continuous condensation
b. introduce an inert gas
c. decrease volume of container
d. add heat
e. add extra Y

12. LeChâtelier’s Practice
H2(g) + Cl2(g)  2HCl(g) ∆H = kJ
Predict how each of these would affect equilibrium
a. increasing volume of container
b. take away some chlorine
c. what could you do to get more HCl?

13. Common Ion Effect
When you combine two cmpds that have a “common ion” you actually make it seem like there is a huge concentration of that ion.

14. Common Ion Effect ex: HCl(aq) + NaCl(s)
when dissolved in water will ionize to form
H Cl Na Cl-
that’s twice as much chlorine as sodium or hydrogen, remember ↑ concentration causes rxn to speed up →

15. Common Ion Effect
ex: BaSO4(s) + Na2SO4(aq) →

16. Predicting Precipitates
an insoluble solid that will separate from
a solution during a reaction (double displacement).
~ using solubility rules, we can predict precipitates.
Compound
Solubility
Exceptions
Salts of group 1A and ammonia (NH3)
Soluble (aq)
Some lithium compounds
Ethanoates, nitrates, chlorates, and perchlorates
Few Exceptions
Sulfates (SO4)
Compounds of Pb, Ag, Hg, Ba, Sr, and Ca
Chlorides, Bromides, and Iodides
Compounds of Ag and some of Hg and Pb
Sulfides and Hydroxides
Insoluble (s)
Group 1A compounds. Compounds of Ba, Sr, and Ca (slightly)
Carbonates, Phosphates, and Sulfites
Group 1A and ammonia (NH3) compounds

17. Predicting Precipitates
Compound
Solubility
Exceptions
Salts of group 1A and ammonia (NH3)
Soluble (aq)
Some lithium compounds
Ethanoates, nitrates, chlorates, and perchlorates
Few Exceptions
Sulfates (SO4)
Compounds of Pb, Ag, Hg, Ba, Sr, and Ca
Chlorides, Bromides, and Iodides
Compounds of Ag and some of Hg and Pb
Sulfides and Hydroxides
Insoluble (s)
Group 1A compounds. Compounds of Ba, Sr, and Ca (slightly)
Carbonates, Phosphates, and Sulfites
Group 1A and ammonia (NH3) compounds
Predict the precipitate…
*precipitate* +1 -1 +1 -1
LiI (aq) + AgNO3 (aq)
LiNO
AgI
(aq)
(s)
*precipitate* +1 -1 +3 -2
NaOH (aq) + Cr2(SO4)3 (aq)
Na2SO
Cr(OH)3
(aq)
(s)

18. Net Ionic Equations
a net ionic equation shows the chemical change in a
solution (formation of precipitate).
~ all soluble (aq) compounds are separated into ions.
~ insoluble (s) compounds are written as compounds.
To write…
~ write the complete ionic equation.
separate (aq) compounds.
leave (s) compounds intact.
~ cross out any spectator ions…
ions that appear on both sides of equation.

19. Net Ionic Equations Ex… LiI (aq) + AgNO3 (aq) LiNO3 (aq) + AgI (s)
write complete ionic equation…
Li+1 +
I-1 +
Ag+1 +
NO3-1
Li+1 +
NO3-1 +
AgI (s)
cross out spectator ions…
what is left is the net ionic equation!
I-1 +
Ag+1
AgI (s)
Ex…
NaOH (aq) + Cr2(SO4)3 (aq) Na2SO4 (aq) +
Cr(OH)3 (s)
Na+1 +
OH-1 +
Cr+3 +
SO4-2
Na+1 +
SO4-2 +
Cr(OH)3 (s)
OH-1 +
Cr+3
Cr(OH)3 (s)