HOW ARE EQUILIBRIUM EXPRESSIONS WRITTEN?

HOW ARE EQUILIBRIUMEXPRESSIONS WRITTEN? Q VS. K

“Systems”: two reactions that differ only in direction • Any reversible reaction H2 + I2↔ 2HI noted by the double arrow; ↔

Reversible Reactions H2 + I2 ↔ 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

Properties of an Equilibrium Equilibrium systems are • DYNAMIC (in constant motion) • REVERSIBLE • can be approached from either direction E + Co(H2O)6Cl2nCo(H2O)4Cl2 + 2 H2O

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

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

Reaction Quotient At anypointin the reaction H2 + I2 --->2 HI

Equilibrium achieved Equilibrium Constant In the equilibrium region

THE EQUILIBRIUM CONSTANT For any type of chemical equilibrium of the type aA + bB--->cC + dD the following is a CONSTANT (at a given T) If K is known, then we can predict concs. of products or reactants.

Writing and Manipulating K Expressions Solids NEVER appear in equilibrium expressions. S(s) + O2(g) ---> SO2(g)

Writing and Manipulating K Expressions Liquids NEVER appear in equilibrium expressions. NH3(aq) + H2O(liq) ---> NH4+(aq) + OH-(aq)

Writing Equilibrium Expressions

Product- or Reactant Favored Product-favored Reactant-favored

For: N2(g) + 3 H2(g) ---> 2 NH3(g) Using K: Is the reaction product-favored or reactant-favored? When K is much greater than 1 the reaction is strongly product-favored.

For AgCl(s) g Ag+(aq) + Cl-(aq) Kc = [Ag+] [Cl-] = 1.8 x 10-5 If K is much less than 1 The reaction is strongly reactant-favored. Ag+(aq) + Cl-(aq) gAgCl(s) is product-favored.

calculating Equilibrium constants:product or reactant favored?

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

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?

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

Experimental Determination of Equilibrium Constant, K 2 NOCl(g) --->2 NO(g) + Cl2(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] [Cl2] Initial 2.00 0 0 Change Equilibrium 0.66

Determining K 2 NOCl(g) ---> 2 NO(g) + Cl2(g) [NOCl] [NO] [Cl2] Initial 2.00 0 0 Change -0.66 +0.66 +0.33 Equilibrium 1.34 0.66 0.33

2 NOCl(g) ---> 2 NO(g) + Cl2(g) [NOCl] [NO] [Cl2] Initial 2.00 0 0 Change -0.66 +0.66 +0.33 Equilibrium 1.34 0.66 0.33

HOW ARE EQUILIBRIUM EXPRESSIONS WRITTEN?