Systematic Treatment of Equilibrium

1
Chapter 9

• Systematic Treatment of Equilibrium

2
Chemical Equilibrium

• Basis For
• Titrations
• Chromatography
• Applications of Chemistry in other Sciences

3
Equilibrium Assumptions

• Ion is Naked
• Negated this idea in Chapter 8
• Water Sheath
• Ions other than just the desired exist in
  solution
• Reaction of interest is the only one in solution
• Realistic?

4
Equilibrium Assumptions

• Examples
• Destruction of limestone and marble in buildings
• CaCO3(s) ? Ca2 CO32-
• CO32- H2O ? HCO3- OH-
• HCO3- H2O ? H2CO3 OH-
• or
• Formation of acid rain
• SO2 H2O ? H2SO3
• H2SO3 oxidation ? H2SO4

5
Equilibrium Calculations

• Previous
• BaC2O4 ? Ba2 C2O42-
• Ba2 C2O42-
• But what if oxalate then reacted with H2O
• C2O42- H2O ? HC2O4- OH-
• HC2O4- H2O ? H2C2O4 OH-
• Ba2 ? C2O42-
• because C2O42- C2O42- HC2O4- H2C2O4

6
Systematic Treatment of Equilibrium

• Step 1. Write all pertinent reactions
• Step 2. Write the charge balance equation
• Step 3. Write the mass balance equation
• Step 4. Write the equilibrium constant for each
  chemical reaction (should use activities but we
  will ignore them)
• Step 5. Count the equations and unknowns (
  equations unknowns)
• Step 6. Solve the unknowns

7
Charge Balance Equations

• Charge Balance Equations -
• An algebraic statement of electroneutrality.
• The concentration of the sum of the positive
  charges the concentration of the sum of the
  negative charges.
• General Form of the Equation
• n1C1 n2C2 m1A1 m2A2
• n1 and m1 represent the magnitude of the charge
  of the ion
• C1 and A1 represent the concentration of each
  cation and anion respectively

8
Charge Balance Examples

• Given
• H 5.1 x 10-12 M K 0.0550 M
• OH- 0.0020 M H2PO4- 1.3 x 10-6 M
• HPO42- 0.0220 M PO43- 0.0030 M
• Find the solution for the Charge Balance
  equation.

9
Charge Balance Examples

• Write the charge balance equation for a solution
  containing H2O, H, OH-, ClO4-, Fe(CN)63-,
  CN-, Fe3, Mg2, CH3OH, HCN, NH3, and NH4.

10
Charge Balance Examples

• Write a charge balance equation for aqueous
  solution of glycine, which reacts as follows
• H3NCH2CO2- ? H2NCH2CO2- H
• H3NCH2CO2- H2O ? H3NCH2CO2H OH-

11
Charge Balance Examples

• Write a charge balance equation for a solution of
  Al(OH)3 dissolved in 1 M KOH. Possible species
  are Al3, AlOH2, Al(OH)3, Al(OH)2, and Al(OH)4-

12
Mass Balance Equation

• Mass Balance (Material Balance) -
• Statement of Conservation of Matter - The
  quantity of all species in a solution containing
  a particular atom (or group of atoms) must be
  equal to the amount of that atom (or group of
  atoms) delivered to the solution.

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Mass Balance Example (known concentrations)

• Write the mass balance equation for the acetate
  group of atoms in a 0.05 M solution of acetic
  acid.
• Write the mass balance equation for the phosphate
  group of atoms in a 0.025 M solution of
  phosphoric acid.

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Mass Balance Example (known concentrations)

• Write the mass balances for K and for phosphate
  in a solution prepared by mixing 0.0250 mol
  KH2PO4 plus 0.0300 mol KOH and diluting to 1.0 L.
• Write a mass balance equation for a 0.05 M
  solution of glycine in water.

15
Mass Balance Example (known concentrations)

• Suppose that 0.30 g of AlOOH (FM 59.99) plus
  150 mL of 3.0 M KOH are diluted to 1.0 L to give
  the species Al3, AlOH2, Al(OH)3, Al(OH)2, and
  Al(OH)4-

16
Mass Balance Example (unknown concentrations)

• Write the mass balance equation for La(IO3)3.
• Write the mass balance equation for a saturated
  solution of slightly soluble Ag3PO4, which
  produces PO43- and 3Ag when it dissolves.

17
Systematic Treatment of Equilibrium

• Determining the equations used to calculate
  unknown concentrations is the heart of this type
  of problem
• The hard part, however, is solving for multiple
  unknowns using these equations

18
Systematic Treatment of Equilibrium

• Calculate the concentration of the hydronium and
  hydroxide ions in water.
• Calculate the concentration of Hg22 in a
  saturated solution of Hg2Cl2.
• Find the solubility of CaF2 in water if the pH is
  somehow fixed at 3.0.

19
Systematic Treatment of Equilibrium

• Find the solubility of Ba(C2O4) in water assuming
  a pH of 2.0. Calculate the concentration of all
  species in solution.

20
Chapter 9 - Homework

• Problems 2, 4, 8, 9, 18, 22, 23