Chemical Equilibrium Chapter 15

Practice Exercise bottom p 647 For the equilibrium PCl 5 (g) ⇌ PCl 3 (g) + Cl 2 (g) the equilibrium constant K p is at 500. K. A gas cylinder at 500. K is injected with PCl 5 (g) at 1.66 atm. Calculate the equilibrium pressures of all species at this temperature. Ans: P PCl5 = atmP PCl3 = P Cl2 = atm

Applications of Equilibrium Constants What does the equilibrium constant tell us? Calculating equilibrium concentrations Predicting the direction of a reaction

Reaction quotient (Q c ) - calculated by substituting the initial concentrations of the reactants and products into the equilibrium constant (K c ) expression. IF: Q c < K c system proceeds from left to right to reach equilibrium Q c = K c the system is at equilibrium Q c > K c system proceeds from right to left to reach equilibrium Fig 15.9

At 448 °C the equilibrium constant K c for the reaction is Predict in which direction the reaction will proceed to reach equilibrium at 448 °C if we start with 2.0 × 10 –2 mol of HI, 1.0 × 10 –2 mol of H 2, and 3.0 × 10 –2 mol of I 2 in a 2.00-L container. Sample Exercise Predicting the Direction of Approach to Equilibrium Solve: The initial concentrations: The reaction quotient is:

Factors that Affect Chemical Equilibrium Changes in Concentration Changes in Pressure/Volume Changes in Temperature Effect of Catalysts

If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position. Le Châtelier’s Principle Changes in Concentration N 2 (g) + 3H 2 (g) 2NH 3 (g) Add H 2 Equilibrium shifts right to offset stress Fig 15.11

Fig Schematic of Haber process to produce ammonia

Le Châtelier’s Principle Changes in Concentration (continued) ChangeShifts the Equilibrium Increase concentration of product(s)left Decrease concentration of product(s)right Decrease concentration of reactant(s) Increase concentration of reactant(s)right left aA + bB cC + dD Add Remove

Factors that Affect Chemical Equilibrium Changes in Concentration Changes in Pressure/Volume Changes in Temperature Effect of Catalysts

Le Châtelier’s Principle Changes in Pressure and Volume A (g) + B (g) C (g) ChangeShifts the Equilibrium Increase pressureSide with fewest moles of gas Decrease pressureSide with most moles of gas Decrease volume Increase volumeSide with most moles of gas Side with fewest moles of gas

Factors that Affect Chemical Equilibrium Changes in Concentration Changes in Pressure/Volume Changes in Temperature Effect of Catalysts

N2O4 (g)N2O4 (g) 2NO 2 (g) colorlessred-brown Shift in equilibrium with temperature Medium tempHighest tempLowest temp Fig 15.1

Le Châtelier’s Principle Changes in Temperature ⇌ CoCl 4 (aq) + 6 H 2 O (l) Co(H 2 O) 6 2+ (aq) + 4 Cl (aq) Fig Room T HeatedCooled

Le Châtelier’s Principle Changes in Temperature Change Exothermic Rx Increase temperatureK decreases Decrease temperatureK increases Endothermic Rx K increases K decreases colder hotter N2O4 (g)N2O4 (g) 2NO 2 (g) colorlessred-brown

Factors that Affect Chemical Equilibrium Changes in Concentration Changes in Pressure/Volume Changes in Temperature Effect of Catalysts

uncatalyzedcatalyzed Catalyst lowers E a for both forward and reverse reactions. Catalyst does not change equilibrium constant or shift equilibrium. Effect of Catalysts Does not change K Does not shift position of an equilibrium system System will reach equilibrium sooner Le Châtelier’s Principle Fig 15.15

Le Châtelier’s Principle ChangeShift Equilibrium Change Equilibrium Constant Concentrationyesno Pressure/volume yesno Temperature yes Catalystno

Chemistry In Action Life at High Altitudes and Hemoglobin Production K c = [HbO 2 ] [Hb][O 2 ] Hb (aq) + O 2 (aq) HbO 2 (aq)