Chemical Equilibrium
Reversible Reactions
Freezing Ice?
Cooking an egg?
Melting water?
• Reversible reactions are those in which the
products can react to re-form reactants.
• Ex: 2HgO (s) + heat -------------> 2Hg (l) + O2 (g)
2Hg (l) + O2 (g) -------------> 2HgO (s) + heat
• Chemical Equilibrium:
When the rate of the forward reaction = the rate
of the reverse reaction AND the concentrations
of product and reactants remain the same.
Equilibrium
• If product is favored, the reaction is represented as:
+ (aq) + Br- (aq)
HBr (aq) + H2O(l) -------->
H
O
3
<--• If reactant is favored, the reaction is represented as:
+
H2CO3 (aq) + H2O (l) --->
<------ H3O (aq) + HCO3 (aq)
• If concentrations of reactants = products:
H2SO3 (aq) + H2O (l) ------->
<------- H3O+ (aq) + HSO3- (aq)
•
The extent to which reactants ----> product is
determined from the equilibrium constant.
The Equilibrium
Expression
• Since the concentrations of reactants and products
after equilibrium has been established remain
constant, so does the ratio of reactant to product.
Ex: nA(aq)+ mB(aq) -------> xC(aq) + yD(aq)
• K = [C]x[D]y K is determined experimentally
[A]n[B]m K is dependent on temperature
No solids or liquid included in the equilibrium expression
If K > 1, product is favored
If K < 1, reactant is favored
If K = 1, reactant = product
Question
• What is the equilibrium expression for the
following reaction at equilibrium?
------>
• H2(g) + I2 (g) <------ HI (g)
• K = [HI]2
[H2][I2]
• The equilibrium expression can be used to
calculate concentrations of reactants and
products at equilibrium. Let’s try…
Predicting Direction of a
Shift in Equilbrium
• Le Chatelier’s Principle: If a stress is applied
to a system at equilbrium, the system will react
in a way to minimize that stress.
• Changes in pressure:
– Affect gases only!
• Changes in temperature
• Changes in concentration:
– Solids and liquids not affected.
- K is not affected.
Reactions that Go to
Completion
• In these reactions, ions are almost
completely removed from solution.
• Formation of a gas
– Ex: H2CO3 (aq) H20 (l) + CO2 (g)
• Formation of a precipitate
– Ex: NaCl(aq) + AgNO3(aq)  NaNO3 + AgCl (s)
• Formation of a slightly ionized product
(acids and bases)
Ex: HCl(aq) + NaOH(aq)  NaCl(s) + H2O(l)
Common Ion Effect
• When the addition of an ion common to
2 solutes brings about precipitation.
-------->
• Ex: NaCl (s) <-------- Na+ (aq) + Cl- (aq)
• If I now add HCl to this solution, which
ion will be affected?
• HCl(g) + H2O(l) -----> H3O+(aq) + Cl-(aq)
• Equilibrium will shift to the left
(production of Solid NaCl)
Equilibria of Acids, Bases
and Salts
• Ionization of a Weak Acid
------->
CH3COOH + H2O <------- H3O+ + CH3COO• K = [H3O+ ][CH3COO-]/[CH3COOH][H2O]
• We can assume that the [H2O] remains
constant.
• K[H2O] = [H3O+ ][CH3COO-] = Ka Acid
Ionization
Constant
[CH3COOH]
Ka
• Constant at a given temperature.
• For weak acids, Ka is < 1.
• To determine the Ka, the [ ] of
involved reactants/products must be
known.
• [ ] are found by measuring pH.
• Kb used for bases.
Buffers
Resist changes in pH
• Weak acid + its salt (or weak base + salt)
• Salts of a weak acid can combine with
the extra H3O+ that is produced.
– Ex: CH3COOH/NaCH3COO solution
– CH3COO- + H3O+ ----> CH3COOH + H2O
Buffered Solution
Acid
first ―CH COO3
Buffered Solution
second ―CH
No pH change
+ H+ + OH-  H20
Base
- + H O+
COOH
+
H
0

CH
COO
3
2
3
3
Offsets
original base
Hydrolysis of Salts
• Salts formed during neutralization
• Hydrolysis occurs when water molecules
combine with ions of a dissolved salt.
• Can form acidic, basic or neutral solutions:
–
–
–
–
Strong Acid + Strong Base -----> Neutral
Strong Acid + Weak Base -----> Acidic
Strong Base + Weak Acid -----> Basic
Weak Base + Weak Acid ------> Unpredictable
Solubility Equilibrium
• Used to predict whether precipitation occurs
when solutions are combined.
• Soluble (>1 g/100 g water)
• Ex: AgCl (s) -----> Ag+ (aq) + Cl-(aq)
• Ksp = [Ag+][Cl-] = Solubility Product Constant
– If Ksp > ion product, the substance
DISSOLVES (unsaturated- no solid present)
– If Ksp < ion product, there is
PRECIPITATION
−
Pg. 579 Table 18-3