1 HOW ARE EQUILIBRIUM EXPRESSIONS WRITTEN? Q VS. K

“Systems”: two reactions that differ only in direction Any reversible reaction H 2 + I 2 ↔ 2HI noted by the double arrow; ↔

Reversible Reactions H 2 + I 2 ↔ 2HI the products may react back to original reactants. “closed system”: ONLY if all reactant are present If one piece is completely gone it has ”gone to competition” and no longer reversible

Equilibrium The state in which a chemical reaction and its reverse reaction occur at the same rate.

Equilibrium = No change in amount over time

6 Properties of an Equilibrium Equilibrium systems are DYNAMIC (in constant motion) REVERSIBLE can be approached from either direction Equilibrium systems are DYNAMIC (in constant motion) REVERSIBLE can be approached from either direction 2 H 2 O E + Co(H 2 O) 6 Cl 2 Co(H 2 O) 4 Cl H 2 O

Le Chatelier’s Principle If a system at equilibrium is stressed, it will react to undo the stress. Can we explain WHY?

8 The Reacton Quotient, Q In general, all reacting chemical systems are characterized by their REACTION QUOTIENT at equilibrium, Q = K

9 Reaction Quotient At any point in the reaction ---> H 2 + I 2 ---> 2 HI At any point in the reaction ---> H 2 + I 2 ---> 2 HI

10 Equilibrium Constant Equilibrium achieved In the equilibrium region

12 THE EQUILIBRIUM CONSTANT For any type of chemical equilibrium of the type a A + b B ---> c C + d D the following is a CONSTANT (at a given T). If K is known, then we can predict concs. of products or reactants.

13 Writing and Manipulating K Expressions Solids NEVER appear in equilibrium expressions. S(s) + O 2 (g) ---> SO 2 (g)

14 Writing and Manipulating K Expressions Liquids NEVER appear in equilibrium expressions. NH 3 (aq) + H 2 O(liq) ---> NH 4 + (aq) + OH - (aq)

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16 Writing Equilibrium Expressions

17 The Determination of K K comes from thermodynamics. See Chapter 19, page ∆G˚ negative: product favored ∆G˚ positive: reactant-favored If K > 1, then ∆G˚ is negative If K < 1, then ∆G˚ is positive If K > 1, then ∆G˚ is negative If K < 1, then ∆G˚ is positive

18 Product- or Reactant Favored Product-favoredReactant-favored

19 For: N 2 (g) + 3 H 2 (g) ---> 2 NH 3 (g) Using K: Is the reaction product-favored or reactant-favored? When K is much greater than 1 the reaction is strongly product-favored.

20 For AgCl(s)  Ag + (aq) + Cl - (aq) K c = [Ag + ] [Cl - ] = 1.8 x If K is much less than 1 The reaction is strongly reactant-favored. Ag + (aq) + Cl - (aq) AgCl(s) Ag + (aq) + Cl - (aq)  AgCl(s) is product-favored. Ag + (aq) + Cl - (aq) AgCl(s) Ag + (aq) + Cl - (aq)  AgCl(s) is product-favored.

21 calculating Equilibrium constants: product or reactant favored?

22 Using K: Can determine if the reaction is at equilibrium.

23 If [iso] = 0.35 M and [n] = 0.15 M, are you at equilibrium? If not, which way does the reaction “shift” to approach equilibrium?

24 REACTION QUOTIENT, Q Characterize all chemical systems Q (2.33) < K (2.5) Reaction is NOT at equilibrium, _. [iso] must ________ and [n] must ____________.. If Q = K, then system is at equilibrium.

25 Experimental Determination of Equilibrium Constant, K 2 NOCl(g) ---> 2 NO(g) + Cl 2 (g) Place 2.00 mol of NOCl is a 1.00 L flask. At equilibrium you find 0.66 mol/L of NO. Calculate K. Set of an “ICE” table of concentrations [NOCl][NO][Cl 2 ] Initial Change Equilibrium0.66

26 Determining K 2 NOCl(g) ---> 2 NO(g) + Cl 2 (g) [NOCl][NO][Cl 2 ] Initial Change Equilibrium

27 2 NOCl(g) ---> 2 NO(g) + Cl 2 (g) [NOCl][NO][Cl 2 ] Initial Change Equilibrium