Chemical EquilibriumChapter 15

Practice Exercise bottom p 647

For the equilibrium

PCl5 (g) PCl⇌ 3 (g) + Cl2 (g)

the equilibrium constant Kp is 0.497 at 500. K. A gas cylinder at 500. K is injected with PCl5 (g) at 1.66 atm.

Calculate the equilibrium pressures of all species at this temperature.

Ans: PPCl5 = 0.967 atm PPCl3 = PCl2 = 0.693 atm

Applications of Equilibrium Constants

What does the equilibrium constant tell us?

• Calculating equilibrium concentrations

• Predicting the direction of a reaction

Reaction quotient (Qc) - calculated by substituting the initial concentrations of the reactants and products into the equilibrium constant (Kc) expression.

IF:

• Qc < Kc system proceeds from left to right to reach equilibrium

• Qc = Kc the system is at equilibrium

• Qc > Kc system proceeds from right to left to reach equilibrium

Fig 15.9

At 448 °C the equilibrium constant Kc for the reaction

is 50.5. 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 H2, and 3.0 × 10–2 mol of I2 in a 2.00-L container.

Sample Exercise 15.10 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

N2 (g) + 3H2 (g) 2NH3 (g)

AddH2

Equilibrium shifts right to offset stress

Fig 15.11

Fig 15.12 Schematic of Haber process to produce ammonia

Le Châtelier’s Principle

• Changes in Concentration (continued)

Change Shifts 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

AddAddRemove 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)

Change Shifts the Equilibrium

Increase pressure Side with fewest moles of gas

Decrease pressure Side with most moles of gas

Decrease volume

Increase volume Side 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) 2NO2 (g)

colorless red-brown

Shift in equilibrium with temperature

Medium temp Highest tempLowest temp

Fig 15.1

Le Châtelier’s Principle

• Changes in Temperature

⇌ CoCl4 (aq) + 6 H2O (l) Co(H2O)62+

(aq) + 4 Cl(aq)

Fig 15.14

Room T Heated Cooled

Le Châtelier’s Principle

• Changes in Temperature

Change Exothermic Rx

Increase temperature K decreases

Decrease temperature K increases

Endothermic Rx

K increases

K decreases

colder hotter

N2O4 (g) 2NO2 (g)

colorless red-brown

Factors that Affect Chemical Equilibrium

• Changes in Concentration

• Changes in Pressure/Volume

• Changes in Temperature

• Effect of Catalysts

uncatalyzed catalyzed

Catalyst lowers Ea 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

Change Shift EquilibriumChange Equilibrium

Constant

Concentration yes no

Pressure/volume yes no

Temperature yes yes

Catalyst no no

Chemistry In Action

Life at High Altitudes and Hemoglobin Production

Kc = [HbO2]

[Hb][O2]

Hb (aq) + O2 (aq) HbO2 (aq)