Chapter 17 & 18, Part
III
Le Chatelier’s Principle
Common Ion Effect
Predicting Precipitates
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.
LeChatelier’s Principle
Let’s look at how several factors will shift
the equilibrium point.
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 →
H2CO3 D H2O + CO2
Which way will equilibrium shift if
adding:
CO2
H2CO3
H 2O
Which way will equilibrium shift if
removing:
CO2
H2CO3
Temperature
reversible rxns are exothermic one
direction
and endothermic the other direction
↓ in temp favors exothermic
↑ in temp favors endothermic
2SO2 + O2 D 2SO3 + heat
Which way will equilibrium shift if heat is
added?
Which way will it shift if heat is removed?
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)
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?
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 + 2NaOH  2NaCl + H2O
LeChâtelier’s Practice
X(g) + 2Y(g) D Z(g) + heat
How would each of the following changes
shift equilibrium?
a.
b.
c.
d.
e.
remove Z by continuous condensation
introduce an inert gas
decrease volume of container
add heat
add extra Y
LeChâtelier’s Practice
H2(g) + Cl2(g) D 2HCl(g) ∆H = -185 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?
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.
Common Ion Effect
ex: HCl(aq) + NaCl(s)
when dissolved in water will ionize to form
H+ + Cl- + Na+ + Clthat’s twice as much chlorine as sodium or
hydrogen, remember ↑ concentration
causes rxn to speed up →
Common Ion Effect
ex: BaSO4(s) + Na2SO4(aq) →
Predicting
Precipitates
• precipitate: 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
Soluble (aq)
Few Exceptions
Sulfates (SO4)
Soluble (aq)
Compounds of Pb, Ag, Hg, Ba, Sr,
and Ca
Chlorides, Bromides, and
Iodides
Soluble (aq)
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,
Group 1A and ammonia (NH )
Predicting
Compound
Solubility
Precipitates
Exceptions
Salts of group 1A and ammonia (NH3)
Soluble (aq)
Some lithium compounds
Ethanoates, nitrates, chlorates, and
perchlorates
Soluble (aq)
Few Exceptions
Sulfates (SO4)
Soluble (aq)
Compounds of Pb, Ag, Hg, Ba, Sr,
and Ca
Chlorides, Bromides, and Iodides
Soluble (aq)
Compounds of Ag and some of Hg
and Pb
Insoluble (s)
Group 1A compounds.
Compounds of Ba, Sr, and Ca
(slightly)
Carbonates, Phosphates,
and Sulfites
Insoluble (s)
• Predict
the precipitate…
Group 1A and ammonia (NH3)
*precipitate*
compounds
Sulfides and Hydroxides
+1
-1
+1
-1
LiI (aq) + AgNO3 (aq)
+1
-1
+3
-2
NaOH (aq) + Cr2(SO4)3 (aq)
LiNO3 (aq) +
AgI (s)
*precipitate*
Na2SO4 (aq) + Cr(OH)3 (s)
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.
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
• Ex…
NaOH (aq) + Cr2(SO4)3
AgI (s)
(aq)
Na+1 + OH-1 + Cr+3 + SO4-2
OH-1 + Cr+3
Na2SO4 (aq) + Cr(OH)3 (s)
Na+1 + SO4-2 + Cr(OH)3 (s)
Cr(OH)3 (s)