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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(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

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  1. 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)

  2. 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

  3. Reversible Reactions • For the reaction: aA + bB  cC + dD K = [C]c[D]d [A]a[B]b Interpretation  K = [products]coefficients [reactants]coefficients

  4. Reversible Reactions • Example: H2 + I2 ↔ 2HI Given data: Calculations of K:

  5. 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)

  6. 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!!

  7. 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!!

  8. 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!!

  9. 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

  10. 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

  11. 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.

  12. 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 **

  13. 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

  14. 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

  15. 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.

  16. 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)

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