September 22, 2004

• These are the lost lecture notes-- not quite what I actually covered in class
• There will be a new problem set distributed today in recitation
  • it will be due Monday (by 1 PM, in class, in my mailbox, to my office or to Maria)
  • this material is very problem oriented-- work lots of sample problems and learn to be able to work problems without flipping through the chapter.
  • Don't continue to struggle with a problem that you can't figure out. After about 20 minutes the likelihood of success in the next 20-60 minutes is very low and you get frustrated and discouraged. Even worse, you begin to reinforce the wrong relationships.
    • leave the problem and return in an hours or so; maybe it now makes sense
    • forget the problem, but perhaps other material will trigger an "ahah" moment
    • stop by and ask for a hint or help and then go back and work it and a similar problem
• I'm also promoting the talk Thursday night (7:00 in Olscamp 115.)
  • Robotic Geologist on Mars -- Rover Spirit and Opportunity
  • Link to NASA site, wonderful images
  • also description of the instruments

Equilibrium---

• for a reversible reaction
• first balance the equation
• then write Keq = (products on top),
  • (reactants on bottom)
  • use concentrations for solutions
  • use 1 for solids, liquids
  • usually use P for gases

Henry's Law

• [O₂] = k P(O₂)
  • process is O₂ (g) ----dissolves--> O₂ (aq)
  • K = [O₂] / P(O₂)
    • so Henry's K is same as Kequil
  • if this K_c or K_p ?
    • not important distinction here (K_p actually)

Example of Blood and Hemoglobin (Hm)

• back of the chapter
• plot of dissolved O₂ vs P(O₂) (see plot in text)
  • does not follow Henry's Law (would be a straight line)
• reaction is (simplified)
  • O₂ (g) + H(aq) ----> H-O2 (aq)
  • K = [Hm-O2] / ([Hm] * P(O2) )
    • not the form of Henry's Law
  • as O2 dissolves, Hm is removed
  • and less O2 can be bound

Role of a Catalyst

• (actually covered in skipped chapter 12)
• species that makes reaction go faster
• isn't used up in the reaction
• doesn't affect equilibrium
  • might make equilibrium possible-- we might never see equilibrium without the catalyst (even in our lifetime)

• H₂ and O₂ react, explosively
  • equilibrium constant extremely large at 20^oC
    • IF spark or flame
    • but O2 and H2 mix, stay that way for years
  • touch of Pt or Pd metal, and boom!

NH3 synthesis (Haber Process)

• N₂ + 3 H₂ < === > 2 NH₃ (gas)
• delta_H reaction = delta_- 0 - 3*0 = 2(-46.1) kJ
  • exothermic, so K decreases at higher temperatures
    • (but process is so slow cold ... usually run hot)
  • (needs a catalyst)
  • form reaction mixture ... some NH₃, some stays H₂ and N₂
  • pump it off (remove from catalyst)
    • cool annd condense NH3 (collect it)
    • rewarm gas, add catalyst
      • Q < K so more N2 + H2 --> NH3
  • (removing a product continues to make more product)
  • Example of manipulation of an equilibrium

Kc vs Kp for a gas reaction

• Ammonia... Kp = P(NH3)² / P(N2) * P(H2)³
  • P_i = (n_i/V)RT = [ i ] RT

• Kp = { [NH₃]RT}² / { [N₂]RT} * { [H₂]RT}³
• Kp = Kc / (RT)²

• DN (gas) = 2 -1 - 3 = -2
  • be careful, this refers only to gases
• Kp = Kc * (RT)^DN

LeChatlier and Shifting an Equilibrium

• NH3 is case: compressing a mixture makes more NH3
  • stress leads to reduced number of gas molecules
    • Kp = P(NH3)² / P(N2) * P(H2)³
  • if we suddenly cut volume in half we'd double the P of each species
    • would cause Q to be 1/4 what it was (equilibrium)
    • form more product, less reactant

Another way to manipulate equilibrium

• eliminate product (through use of another reaction) Ag₂[SO₄] ---> 2 Ag+ + SO₄2-
  • Ksp = [Ag⁺]² [SO₄2-]
  • limited solubility

• add H+ ion (H3O+)
  • H⁺ + SO₄2- --> HSO₄- (a weak acid)
    • HSO₄- plays no role in solubility equilibrium
  • but we have reduced SO₄2-,
  • so more salt dissolves (in H+ than in H₂O)
• could also add ammonia (a ligand)
  • forms coordinate covalent bond to Ag+
  • forms Ag(NH₃)₄+
    • this is not Ag+ so it does not enter into the reaction: Ag₂[SO₄] ---> 2 Ag+ + SO₄2-
    • this reduces [Ag+]
    • more salt dissolves to achieve equilibrium
    • actually, much more salt dissolves

Hydrates

• CuSO₄ + 5 H₂O ---> CuSO₄.5H₂O
• Keq = 1 / P(H₂O)⁵
• so if both CuSO₄ and CuSO₄.5H₂O present
  • the P(H₂O) is fixed , at Keq^1/5

• can control humidity (to very low level)
• called a desiccant
• if we run out of CuSO₄, could get higher humidity
  • (silica gel packets often sed in medicines, herbs to keep the humidity low in the bottle)

• Next few chapters.... we've done much of it
  • equilibrium in aqueous solution
    • weak acids and bases
    • common situation, important,
  • solubility of salts (works best if low solubility)
  • dissociation of water itself
  • metal ions (Cu, Fe, Ni) and ligands (NH₃, H₂O, Cl-)

• return to the chem 127 home site