Aqueous Solutions

1

• Aqueous Solutions
• Chemical equilibrium - law of mass action
• For the general equation aA bB zZ
  xX
• species molar concentration of dissolved
  species involved in the reaction
• partial pressure of a gas in atmospheres of
  gaseous reactants
• 1 if the species is a pure liquid, pure
  solid or solvent for a dilute solution
• At equilibrium, the rate of the forward reaction
  rate of reverse reaction
• The position of the equilibrium is independent of
  how it is established
• Example for the equilibrium 1A 1B

2

• Aqueous Solutions
• Chemical Equilibrium - Le Châteliers Principle
• When a stress is imposed on a system at
  equilibrium the system will respond in such a
  way as to relieve the stress.
• Addition of a reactant
• Change in pressure or volume of a gas
• Change in temperature
• Types of equilibria
• Dissociation of water
• 2H2O H3O OH-
• H2O H OH-

3

• Aqueous Solutions
• Types of equilibria
• Dissociation of water
• Example pure water
• 2H2O H3O OH-
• H3O OH-
• Example An aqueous solution 0.100 M NaOH
• 2H2O H3O OH-
• H3O
  OH-0.100-H3O
• Kw H3OOH- H3O(0.100-H3O)
• if H3O ltlt 0.100
• H3O x 0.100 1.00x10-14
• H3O 1.00x10-13
• OH- 0.100 1.00x10-13 0.100

4

• Aqueous Solutions
• Types of equilibria
• Brønsted-Lowry acids and bases
• An acid is a substance that donates H to a
  proton acceptor
• A base is a H acceptor from a proton donor
• Conjugate acid - base pairs
• acid1 base1 acid2 base2
• base2 is the conjugate base of acid1
• Example
• acid1 base1 acid2 base2
• H2O NH3 NH4 OH-

conjugate pair
conjugate pair
OH- is the conj. Base of the acid H2O and NH4 is
the conj. acid of the base NH3 H2O is the conj.
Acid of the base OH- and NH3 is the conj. base of
NH4
5

• Aqueous Solutions
• Types of equilibria
• Brønsted-Lowry acids and bases
• Example
• acid1 base1 acid2 base2
• HAc H2O H3O Ac-

• Ac- is the conj. base of Hac and H3O is the
  conj. acid of H2O
• H2O is the conj. base of H3O and HAc is the
  conj acid of Ac-
• Note H2O was an acid in the previous example
• Water is amphiprotic.
• Other amphiprotic substances
• HCO3- H2O H2CO3 OH-
• HCO3- H2O CO32- H3O-

6
Aqueous Solutions Relationship between Ka and Kb
for conjugate a acid - base pair HAc H2O
H3O Ac- Ac- H2O HAc OH-
2H2O H3O OH- KwH3OOH-
The same result is obtained for a weak base in
water
7
Aqueous Solutions Amine bases
8

• Aqueous Solutions
• Calculate the H in an aqueous solution of a
  weak acid or base
• Example what is H in a 1.00 M NH4Cl solution
• NH4 H2O NH3 H3O
• Assume the 2nd equilibrium is suppressed
• by the 1st equilibrium
• NH3H3O

2H2O H3O OH-
KwH3OOH-
9

• Aqueous Solutions
• Calculation of H for a solution of a weak acid
  or base often leads to a quadratic equation as
  in previous problem
• The assumption that H ltlt CHA simplifies the
  algebra and arithmetic
• An error in the calculated H results
• See the table and figure showing errors in
  calculated H as a function of Ka and CHA in
  FAC7
• Sometimes the problem requires solution to a
  complex equation in one unknown
• ax3 bx1/2 c 0
• Use the method of successive approximations - or
  Mathcad

10

• Aqueous Solutions
• Types of equilibria
• Slightly soluble inorganic solids
• Assume dissolved solid is 100 dissociated into
  ions
• PbI2(s) Pb2(aq) 2I-(aq)

1
Stoichiometry demands I- 2Pb2 at
equilibrium Find the solubility of PbI2 in
concentration terms, how many moles PbI2
dissolves in 1 L of solution - find the C of
PbI2 in moles per liter of solution Solubility
S Pb2 since I- 2Pb2 at
equilibrium Pb2I-2 Pb2(2Pb2)2
7.1x10-9 4Pb23 7.1x10-9 Pb2 1.2x10-3
S
11

• Aqueous Solutions
• Types of equilibria - slightly soluble inorganic
  solids
• Common ion effect - dissolve a solid in a
  solution containing one of the ions common to
  the solid
• Example What is the solubility of PbI2 in a
  solution that is 0.100 M in KI?

PbI2(s) Pb2(aq) 2I-(aq) Ksp
Pb2I-2 7.1 x 10-9 S Pb2 I- 0.100
2Pb2 Pb2(0.100 2Pb2)2 7.1 x
10-9 assume 2Pb2 ltlt 0.100 Pb2(0.0100) 7.1
x 10-9 Pb2 7.1 x 10-7 S Check assumption
2 x Pb2 1.42 x 10-6 ltlt 0.100
12

• Aqueous Solutions
• Types of equilibria
• Complex ion formation
• Ag 2NH3 Ag(NH3)2
• Fe3 SCN- FeSCN2
• Oxidation - reduction equilibria
• 5Fe2 MnO4- 8H 5Fe3 Mn2 4H2O