Equilibrium Suroviec Spring 2014 Chapter 14 Equilibrium Suroviec Spring 2014

I. Equilibrium So far we have mostly considered just forward reactions aA + bB  cC + dD But all reactions can move in either direction

A. State of equilibrium Rate of equilibrium Equilibrium is dynamic

B. Positions of equilibrium

II. Equilibrium Constant Omit concentrations of pure solids and liquids because those concentrations will not change Look at elementary steps:

A. What does K mean? Large value of K means product favored 2. Small value of K means reactant favored

III. Reaction Quotient

Example Given the K of this reaction to be 170, when the conditions are changed what is the Q? Given the new Q, what is the direction of the reaction to re-establish equilibrium?

IV. Calculations with K We need the equilibrium concentrations to determine K, be we usually know initial concentrations, we can therefore get there from the balanced chemical equations!!! Butane interconverts to isobutane at 25oC with a K = 2.5. If 0.017 moel of butane is in 50 mL, what is the concentration of both species at equilibrium?

Example PCl5 (g) PCl3 (g) + Cl2 (g) K is 1.2 at 25oC. If you place 0.300 M PCl5 (g) in a flask what are the equilibrium concentrations of PCl3 (g), PCl3 (g) and Cl2 (g)?

Example H2 (g) + Br2(g) 2HBr (g) K is 2.18 × 106 at 730oC. If you place 0.267 M HBr (g) in a flask and allow it to come to equilibrium, what are the equilibrium concentrations of H2 (g), Br2 (g) and HBr (g)?

V. Manipulating Equilibrium Expressions N2(g) + 3H2 (g) 2NH3 (g) 2N2(g) + 6H2 (g) 4NH3 (g)

Rules for Manipulating When the stoichiometric coefficients in a balanced reaction are changed by a factor of n: When a chemical reaction is reversed in direction: When several balanced reactions (each with its own equilibrium constant) are added to obtain a net balanced equation:

Ex Calculation Knet for the following reaction: Fe (s) + H2O (g) FeO (s) + H2 (g) Given the following information: H2O (g) + CO (g) H2 (g) + CO2 (g) K1 = 1.6 FeO (s) + CO (g) Fe (g) + CO2 (g) K2 = 0.67

VI. LeChatelier’s Principle Any change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner to counteract the effect of the change Systems always want to be at equilibrium and will work to get back there.

2SO2 (g) + O2 (g) 2SO3(g) ΔHrxn = -197 kJ Example 2SO2 (g) + O2 (g) 2SO3(g) ΔHrxn = -197 kJ Disturbances: Add more SO2(g) Take away O2 (g) Decrease the volume by ½