CHEMISTRY 59-320
ANALYTICAL CHEMISTRY
Fall - 2010
Lecture 8
6-1 The equilibrium constant
• For an arbitrary reaction
where [A] in the above equation really
means [A]/(1 M) if A is a solute, or (pressure in
bars)/(1 bar) if A is a gas.
• All terms in the above equation is dimensionless.
The equilibrium constant is more correctly
expressed as a ratio of activities.
• 1. Concentrations of solutes
should be expressed as moles
per liter.
• 2. Concentrations of gases
should be expressed in bars.
• 3. Concentrations of pure
solids, pure liquids, and
solvents are omitted because
they are unity.
• These conventions are
arbitrary, but you must use
them if you wish to use
tabulated values of equilibrium
constants, standard reduction
potentials, and free energies.
• In the thermodynamic
derivation of the equilibrium
constant, each quantity in
Equation is expressed as the
ratio of the concentration of a
species to its concentration in
its standard state.
• For solutes, the standard state
is 1 M. For gases, the standard
state is 1 bar (≡105 Pa; 1 atm ≡
1.013 25 bar).
• For solids and liquids, the
standard states are the pure
solid or liquid.
• If the direction of a reaction is reversed, the new value of
K is simply the reciprocal of the original value of K.
• If two reactions are added, the new K is the product of
the two individual values.
• If n reaction are added, the overall equilibrium constant is
the product of n individual equilibrium constants.
• Examples:
Problem 6-6 From the equations
HOCl
↔ H+ + OCl- K = 3.0 x 10-8
HOCl + OBr- ↔ HOBr + OClK = 15
find the value of K for the reaction HOBr ↔ H+ + OBr-.
All species are aqueous.
Answer:
6-2 Equilibrium and thermodynamics
• Enthalpy and entropy affect independently on the degree
of a reaction process.
• Gibbs free energy determines whether a reaction process
is favoured:
• Reaction Gibbs energy:
G  G  RT ln Q
0
Q
activity of products
activity of react an ts
• Standard reaction Gibbs energy
0  G  RT ln K
0
• The relationship between equilibrium constant K
and thermodynamic properties:
• Le Chatelier’s principle: If a system at
equilibrium is subjected to a change that
disturbs the system, the direction in which the
system proceeds back to equilibrium is such that
the change is partially offset.
• Chemical equilibrium constant deals with
thermodynamic predictions, not kinetic
prediction.
Problem 6-9: The formation of tetrafluoroethylene from its elements is highly
Exothermic:
2F2(g) + 2C(s) ↔ F2C=CF2(g)
(a) If a mixture of F2, graphite, and C2F4 is at equilibrium in a closed container, will
the reaction go to the right or to the left if F2 is added?
(b) Rare bacteria from the planet Teflon eat C2F4 and make teflon for their cell wall.
will the reaction go to right or to the left if these bacteria are added?
(d) Will the reaction go right or left if the container is crushed to one-eighth of
its original volume? ( how does the K value change in this case?)
(e) Does the equilibrium constant become larger or smaller of the container
is heated?
Solution:
in class discussion
6-3 Solubility product
• The solubility product is the equilibrium constant for the reaction in
which a solid salt dissolves to give its constituent ions in solution.
• Solid is omitted from the equilibrium constant because it is in its
standard state with an activity value of 1. For example:
• A solution containing excess, undissolved solid is said to be
saturated.
• Physical meaning of solubility:
• Solubility is governed by more than the solubility product.
Separation by precipitation
• Calculate quotient Q, if Q is larger than equilibrium constant K, precipitation takes
place.
6-4 Complex formation
• A Lewis acid accepts a pair of electrons from a Lewis
base when the two form a bond:
• The product of the reaction between a Lewis acid and
Lewis base is called an adduct.
• The bond between a Lewis acid and a Lewis based is
called a dative or coordinate covalent bond.
• The formation of complex ions increases solubility.
Notation for formation constants
• Formation constants are the equilibrium constants for complex
formation.
• The stepwise formation constant, designed Ki, are defined as
follows:
M + X ↔ MX K1 = [MX]/([M][X])
MX + X ↔ MX2 K2 = [MX2]/([MX][X])
• The overall formation constants are denoted βi, see eqs 6-13 , 6-14,
6-15
βn = K1K2… Kn
6-5 Protic acids and bases
• Acid: a substance that increases the
concentration of hydronium ion when added tp
water.
• Base: a substance that decreases the
concentration of hydronium ion.
HCl + H2O ↔ H3O+ + Cl• Bronsted-Lowry acid: proton donor.
• Bronsted-Lowry base: proton acceptor
HCl(g) + NH3(g) ↔ NH4+Cl-(s)
(acid)
(base)
(salt)
Conjugate acids and bases
Conjugate acids and bases are related to each other by
the gain or loss of one H+.
The nature of H+ and OH-
Autoprotolysis
• Protic solvents have a reactive H+.
• All protic solvents undergo autoprotolysis, such
as
•The extent of autoprotolysis is very small.
6-6 pH
6-7 Strengths of acids and bases
Ka=4.2 x 10-13
Ka=6.3 x 10-8
Ka=7.1 x 10-3
Monovalent metal ions are very weak acids (Na+, Ka = 10−13.9).
Divalent ions tend to be stronger
(Fe2+, Ka = 10−9.4) and trivalent ions are stronger yet (Fe3+, Ka = 10−2.19).