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Chapter 18. Chemical Equilibrium. http://www.learner.org/resources/series61.html?pop=yes&pid=806# - Molecules in Action Starting at 15:30-23:52 (FYI: Co(H 2 O) 6 +2 + 4Cl - -> CoCl 4 + 6 H 2 O). Reversible Reactions. Reversible Reaction Reaction that can proceed in either direction ↔

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chapter 18

Chapter 18

Chemical Equilibrium

http://www.learner.org/resources/series61.html?pop=yes&pid=806# - Molecules in Action Starting at 15:30-23:52

(FYI: Co(H2O)6+2 + 4Cl- -> CoCl4 + 6 H2O)

reversible reactions
Reversible Reactions
  • Reversible Reaction
    • Reaction that can proceed in either direction
  • Chemical Equilibrium
    • Rate of forward rxn = rate of reverse direction
  • The Equilibrium Constant (K)
    • Compares concentrations of products to reactants at equilibrium
reversible reactions1
Reversible Reactions
  • For the reaction:

aA + bB  cC + dD

K = [C]c[D]d

[A]a[B]b

Interpretation  K = [products]coefficients

[reactants]coefficients

reversible reactions2
Reversible Reactions
  • Example:

H2 + I2 ↔ 2HI

Given data:

Calculations of K:

reversible reactions3
Reversible Reactions
  • Example:
    • A mixture of N2, O2, & NO at equilibrium has [N2] = 6.4x10-3M, [O2] = 1.7x10-3M, and [NO] = 1.1x10-5M. Find K for the reaction:

N2(g) + O2(g) ↔ 2NO(g)

reversible reactions4
Reversible Reactions
  • FYI – SOLIDS AND LIQUIDS HAVE CONCENTRATIONS SO LARGE THEY ARE ESSENTIALLY NOT GOING TO CHANGE (a intensive property), SO YOU CAN almost always IGNORE THEM IN THE K EXPRESSION!!
shifting equilibrium
Shifting Equilibrium
  • Remember LeChatlier????
  • Changes in pressure
    • Affects systems with GASES involved
    • Move mixture to a smaller container…
      • Creates increased pressure
      • Rxn will shift to side with FEWER mole of gas to help alleviate that pressure
      • Example
        • N2 (g) + 3H2 (g) ↔ 2NH3 (g)
        • In a smaller container – will shift RIGHT, K stays the same!!
shifting equilibrium1
Shifting Equilibrium
  • Changes in concentration
    • Add more of a substance, shifts to use it up (shifts away from an added substance), K stays the same
  • Changes in temperature
    • A reversible rxn is ENDO in one direction and EXO in the other
    • An increase in temp causes a shift so the ENDO rxn occurs more
    • K does change!!
shifting equilibrium2
Shifting Equilibrium
  • Common-ion effect
    • Adding a substance with an ion which is also in the rxn shifts equilibrium
    • Example
      • CH3COOH + H2O ↔ H3O+ + CH3COO-1
      • Adding NaCH3COO gives CH3COO-1, so causes a shift LEFT
calculations involving shifts in equilibrium
Calculations involving shifts in equilibrium
  • Reaction Quotient
    • Same form as K, but can be used at any point in a rxn, NOT just at equilibrium
    • Q = [C]c[D]d

[A]a[B]b

  • If Q = K, then
    • The system is at equilibrium
  • If Q > K, then
    • Need more reactants (less product), so shift LEFT
  • If Q < K, then
    • Need more product, so shift RIGHT
calculations involving shifts in equilibrium1
Calculations involving shifts in equilibrium
  • Example:

K for the rxn N2 (g) + 3H2 (g) ↔ 2NH3 (g) is 2.37x10-3. At a given point in the rxn, the concentrations are: [N2] = 0.683M, [H2] = 8.80M & [NH3] = 3.65M. Calculate the value of Q and determine the direction of the rxn.

equilibria of acids bases and salts
Equilibria of Acids, Bases, and Salts
  • For weak acids: An acid losing its hydrogen ion has an equilibrium constant, Ka
  • Example:

CH3COOH + H2O ↔ H3O+ + CH3COO-1

Ka = [H3O+][CH3COO-1]

[CH3COOH]

** H2O is not in expression, because it is a liquid so it has a concentration which essentially does not change **

equilibria of acids bases and salts1
Equilibria of Acids, Bases, and Salts
  • Buffers
    • Has both the acid and its conjugate base in the solution (or base and it conjugate acid).
    • Example
      • CH3COOH & CH3COO-1
      • NH3 & NH4+1
equilibria of acids bases and salts2
Equilibria of Acids, Bases, and Salts
  • Hydrolysis (adding water)
    • Anion hydrolysis
      • Weak acid anions (F-1, CH3COO-1) can react with H2O to remove a proton
      • F-1 + H2O ↔ HF + OH-1
        • A BASIC SOLUTION IS FORMED
    • Cation hydrolysis
      • Weak base cations (NH4+1) can react with H2O to add a proton
      • NH4+1 + H2O ↔ H3O+1 + NH3
        • AN ACIDIC SOLUTION IS FORMED
solubility equilibrium
Solubility Equilibrium
  • Solubility Product = Ksp
  • Used for slightly soluble or insoluble substance dissolving
  • Example
    • AgCl (s) ↔ Ag+1(aq) + Cl-1(aq) {AgCl is insoluble, but still a LITTLE will dissolve!!}
    • K = [Ag+1][Cl-1]
      • AgCl not included because it is a solid. So concentration does NOT change.
precipitation calculations
Precipitation Calculations
  • KBaSO4 = 1.1x10-10. If [Ba+2] = 5.0x10-3M & [SO4-2] = 2.5x10-3M, will a precipitate form?

BaSO4(s) ↔ Ba+2(aq) + SO4-2(aq)