Title: First, system reaches equilibrium
1Chemical Equilibrium
- Introduction
- 1.) Equilibria govern diverse phenomena
- Protein folding, acid rain action on minerals to
aqueous reactions - 2.) Chemical equilibrium applies to reactions
that can occur in both directions - reactants are constantly forming products and
vice-versa - At the beginning of the reaction, the rate that
the reactants are changing into the products is
higher than the rate that the products are
changing into the reactants. - When the net change of the products and reactants
is zero the reaction has reached equilibrium. -
Then, system continually exchanges products and
reactants, while maintaining equilibrium
distribution.
First, system reaches equilibrium
Product
Reactants
At equilibrium the amount of reactants and
products are constant, but not necessarily equal
2Chemical Equilibrium
- Equilibrium Constant
- 1.) The relative concentration of products and
reactants at equilibrium is a constant. - 2.) Equilibrium constant (K)
- For a general chemical reaction
Equilibrium constant
Where - small superscript letters are the
stoichiometry coefficients - A concentration
chemical species A relative to standard state
3Chemical Equilibrium
- Equilibrium Constant
- 2.) Equilibrium constant (K)
- A reaction is favored when K gt 1
- K has no units, dimensionless
- - Concentration of solutes should be expressed as
moles per liter (M). - - Concentrations of gases should be expressed in
bars. - ? express gas as Pgas, emphasize pressure
instead of concentration - ? 1 bar 105 Pa 1 atm 1.01325 bar
- - Concentrations of pure solids, pure liquids and
solvents are omitted - ? are unity
- ? standard state is the pure liquid or solid
- 3.) Manipulating Equilibrium Constants
Consider the following reaction
Reversing the reaction results in a reciprocal
equilibrium reaction
4Chemical Equilibrium
- Equilibrium Constant
- 3.) Manipulating Equilibrium Constants
If two reactions are added, the new K is the
product of the two individual K values
K1
K2
K3
5Chemical Equilibrium
- Equilibrium Constant
- 3.) Manipulating Equilibrium Constants
- Example
Given the reactions and equilibrium constants
Kw 1.0 x 10-14
KNH3 1.8 x 10-5
Find the equilibrium constant for the reaction
Solution
K1 Kw
K21/KNH3
K3Kw1/KNH35.6x10-10
6Chemical Equilibrium
- Le Châteliers Principal
- 1.) What Happens When a System at Equilibrium
is Perturbed? - Change concentration, temperature, pressure or
add other chemicals - Equilibrium is re-established
- Reaction accommodates the change in products,
reactants, temperature, pressure, etc. - Rates of forward and reverse reactions
re-equilibrate
7Chemical Equilibrium
- Le Châteliers Principal
- 1.) What Happens When a System at Equilibrium
is Perturbed? - Le Châteliers Principal
- - the direction in which the system proceeds
back to equilibrium is such that the change is
partially offset.
Consider this reaction
At equilibrium
To return to equilibrium (balance), some (not
all) CO and H2 are converted to CH3OH
Add excess CO(g)
If all added CO was converted to CH3OH, then
reaction would be unbalanced by the amount of
product
8Chemical Equilibrium
- Le Châteliers Principal
- 2.) Example
Consider this reaction
At one equilibrium state
9Chemical Equilibrium
- Le Châteliers Principal
- 2.) Example
What happens when
According to Le Châteliers Principal, reaction
should go back to left to off-set dichormate on
right
Use reaction quotient (Q), Same form of
equilibrium equation, but not at equilibrium
10Chemical Equilibrium
- Le Châteliers Principal
- 2.) Example
Because Q gt K, the reaction must go to the left
to decrease numerator and increase
denominator. Continues until Q K 1. If the
reaction is at equilibrium and products are added
(or reactants removed), the reaction goes
to the left 2. If the reaction is at
equilibrium and reactants are added ( or products
removed), the reaction goes to the right
11Chemical Equilibrium
- Le Châteliers Principal
- 3.) Affect of Temperature on Equilibrium
1. Equilibrium constant of an endothermic
reaction (DHo ) increases if the
temperature is raised. 2. Equilibrium
constant of an exothermic reaction (DHo
-)decreases if the temperature is
raised.
D
DH
D
DH -
12Chemical Equilibrium
- Solubility Product
- 1.) Equilibrium constant for the reaction
which a solid salt dissolves to give its
constituent ions in solution - Solid omitted from equilibrium constant because
it is in a standard state - Example
13Chemical Equilibrium
- Solubility Product
- 1.) Saturated Solution contains excess,
undissolved solid - Solution contains all the solid capable of
dissolving under the current conditions - EXAMPLE
- Find Cu2 in a solution saturated with
Cu4(OH)6(SO4) if OH- is fixed at 1.0x10-6M.
Note that Cu4(OH)6(SO4) gives 1 mol of SO42- for
4 mol of Cu2?
14Chemical Equilibrium
- Solubility Product
- 2.) If an aqueous solution is left in contact
with excess solid, the solid will dissolve until
the condition of Ksp is satisfied - Amount of undissolved solid remains constant
- Excess solid is required to guarantee ion
concentration is consistent with Ksp - 3.) If ions are mixed together such that the
concentrations exceed Ksp, the solid will
precipitate. - 4.) Solubility product only describes part of the
solubility of a salt - Only includes dissociated ions
- Ignores solubility of solid salt
15Chemical Equilibrium
Common ion effect a salt will be less soluble
if one of its constituent ions is already present
in the solution.
Decrease in the solubility of MgF2 by the
addition of NaF
PbCl2 precipitate because the ion product is
greater than Ksp.
16Chemical Equilibrium
- Common Ion Effect
- 1.) Affect of Adding a Second Source of an Ion
on Salt Solubility - Equilibrium re-obtained following Le Châteliers
Principal - Reaction moves away from the added ion
- EXAMPLE
Find Cu2 in a solution saturated with
Cu4(OH)6(SO4) if OH- is fixed at 1.0x10-6M and
0.10M Na2SO4 is added to the solution.
17Chemical Equilibrium
- Complex Formation
- 1.) High concentration of an ion may
redissolve a solid - Ion first causes precipitation
- Forms complex ions, consists of two or more
simple ions bonded to each other
ppt. formation
Complex forms and redissolves solid
18Chemical Equilibrium
- Complex Formation
- 2.) Lewis Acids and Bases
- M acts as a Lewis acid ? accepts a pair of
electrons - X- acts as a Lewis base ? donates a pair of
electrons - Bond is a coordinate covalent bond
adduct
ligand
Lewis base
Lewis acid
19Chemical Equilibrium
- Complex Formation
- 3.) Affect on Solubility
- Formation of adducts increase solubility
- Solubility equation becomes a complex mixture of
reactions - - dont need to use all equations to determine
the concentration of any species
Ksp
Implies low Pb2 solubility
Only one concentration of Pb2 in solution
Concentration of Pb2 that satisfies any one of
the equilibria must satisfy all of the equilibria
All equilibrium conditions are satisfied
simultaneously
20Chemical Equilibrium
- Complex Formation
- 3.) Affect on Solubility
- Total concentration is dependent on each
individual complex species
Total solubility of lead depends on I- and the
solubility of each individual complex formation.
21Chemical Equilibrium
- Complex Formation
- 3.) Affect on Solubility
- EXAMPLE
Given the following equilibria, calculate the
concentration of each zinc-containing species in
a solution saturated with Zn(OH)2(s) and
containing OH- at a fixed concentration of
3.2x10-7M. Zn(OH)2 (s) Ksp
3.0x10-16 Zn(OH) b1 2.5 x104 Zn(OH)3- b3
7.2x1015 Zn(OH)42- b4 2.8x1015
22Chemical Equilibrium
- Acids and Bases
- 1.) Protic Acids and Bases transfer of H
(proton) from one molecule to - another
- Hydronium ion (H3O) combination of H with
water (H2O) - Acid is a substance that increases the
concentration of H3O - Base is a substance that decreases the
concentration of H3O - - base also causes an increase in the
concentration of OH- in aqueous solutions - 2.) Brønsted-Lowry definition does not require
the formation of H3O - Extended to non-aqueous solutions or gas phase
acid
acid
base
salt
23Chemical Equilibrium
- Acids and Bases
- 3.) Salts product of an acid-base reaction
- Any ionic solid
- Acid and base neutralize each other and form a
salt - Most salts with a single positive and negative
charge dissociate completely into ions in water - 4.) Conjugate Acids and Bases
Products of acid-base reaction are also acids and
bases
A conjugate acid and its base or a conjugate base
and its acid in an aqueous system are related to
each other by the gain or loss of H
24Chemical Equilibrium
- Acids and Bases
- 5.) Autoprotolysis acts as both an acid and
base - Extent of these reactions are very small
water
- - H3O is the conjugate acid of water
- - OH- is the conjugate base of water
- Kw is the equilibrium constant for the
dissociation of water
Acetic acid
25Chemical Equilibrium
- Acids and Bases
- 6.) pH negative logarithm of H
concentration - Ignores distinction between concentration and
activities (discussed later) - A solution is acidic if H gt OH-
- A solution is basic if H lt OH-
26Chemical Equilibrium
- Acids and Bases
- 6.) pH
- Example
What is the pH of a solution containing 1x10-6 M
H?
What is OH- of a solution containing 1x10-6 M
H?
27Chemical Equilibrium
- Acids and Bases
- 7.) Strengths of Acids and Bases
- Depends on whether the compound react nearly
completely or partially to produce H or OH- - strong acid or base completely dissociate in
aqueous solution - - equilibrium constants are large
- - everything else termed weak
Strong ? no undissociated HCl or KOH
28Chemical Equilibrium
- Acids and Bases
- 7.) Strengths of Acids and Bases
- weak acids react with water by donating a proton
- - only partially dissociated in water
- - equilibrium constants are called Ka acid
dissociation constant - - Ka is small
- weak bases react with water by removing a proton
- - only partially dissociated in water
- - equilibrium constants are called Kb base
dissociation constant - - Kb is small
Ka
Equivalent
Ka
Kb
Equivalent
Kb
29Chemical Equilibrium
Some Common Weak Acids (carboxylic acids)
30Chemical Equilibrium
Some Common Weak Acids (Metals cations)
31Chemical Equilibrium
Some Common Weak Bases (amines)
- The Ka or Kb of an acid or base may also be
written in terms of pKa or pKb - As Ka or Kb increase ? pKa or pKb decrease
- - a strong acid/base has a high Ka or Kb and a
low pKa or pkb
32Chemical Equilibrium
- Acids and Bases
- 8.) Polyprotic Acids and Bases can donate or
accept more than one proton - Ka or Kb are sequentially numbered
- - Ka1,Ka2,Ka3 Kb1,Kb2,Kb3
33Chemical Equilibrium
- Acids and Bases
- 8.) Relationship Between Ka and Kb
34Chemical Equilibrium
- Acids and Bases
- 8.) Relationship Between Ka and Kb
- EXAMPLE
Write the Kb reaction of CN-. Given that the Ka
value for HCN is 6.2x10-10, calculate Kb for CN-.