Some reactions go to completion Some reactions go to completion A precipitate forms A precipitate forms A gas forms A gas forms CH 4 (g) + O 2 (g)  CO 2 (g) + 2H 2 O(g) Some are reversible Some are reversible The reactants can reform The reactants can reform 3Fe(s) + 4H 2 O(g)  Fe 3 O 4 (s) + 4H 2 (g)

Chemical equilibrium Rate of forward reaction = rate of reverse reaction. The concentration of reactants and products remain unchanged.

LeChatelier’s Principle If stress is applied to a system at equilibrium, the system will try to adjust to reduce the stress. If stress is applied to a system at equilibrium, the system will try to adjust to reduce the stress. Stresses could be Stresses could be Temperature Temperature Pressure (for gases) Pressure (for gases) Concentration Concentration

H 2 CO 3 (aq) CO 2 (aq) + H 2 O(l) 99% 99% If more CO 2 is added, equilibrium shifts to the left to minimize the stress. If CO 2 is removed, equilibrium shifts right.

Concentration Removing products shifts the equilibrium to create more product (right). Removing products shifts the equilibrium to create more product (right). Adding a product shifts equilibrium toward the reactants (left). Adding a product shifts equilibrium toward the reactants (left). “toward a take-away” “toward a take-away” “away from an addition” “away from an addition”

Temperature Increasing temperature causes equilibrium to shift in the direction that absorbs heat. Increasing temperature causes equilibrium to shift in the direction that absorbs heat. 2SO 2 (g) + O 2 (g) 2SO 3 (g) + heat add heat, shifts left add heat, shifts left remove heat, shifts right remove heat, shifts right Opposite shift for endothermic reactions exothermic

Pressure Only affects gases with unequal number of moles of reactants and products. Only affects gases with unequal number of moles of reactants and products. Increasing pressure, shifts equilibrium to side with fewer moles. Increasing pressure, shifts equilibrium to side with fewer moles. N 2 (g) + 3H 2 (g) 2NH 3 (g) 4 moles of reactants 2 moles of product Increase pressure, shift right Increase pressure, shift right decrease pressure, shift left decrease pressure, shift left

What effect do the following changes have on equilibrium? PCl 5 (g) + heat PCl 3 (g) + Cl 2 (g) a. Addition of Cl 2 b. Increase in pressure c. Increase temperature d. Removal of PCl 3 as it forms left right left right

Equilibrium constant H 2 (g) + I 2 (g) 2HI(g) H 2 (g) + I 2 (g) 2HI(g) Rate of forward reaction = k f [I 2 ][H 2 ] Rate of reverse reaction = k r [HI] 2 At equilibrium k f [I 2 ][H 2 ] = k r [HI] 2 or k f = [HI] 2 equilibrium constant K eq k r [I 2 ][H 2 ] k r [I 2 ][H 2 ]

mA + n B  s P + r Q K eq = [P] s [Q] r [A] m [B] n [A] m [B] n If K eq <1, equilibrium is established before much product is formed If K eq is large the reaction approaches completion.

Sample problems Write the equilibrium expression for 4HCl + O 2  2Cl 2 + 2H 2 O K eq = [Cl 2 ] 2 [H 2 O] 2 [HCl] 4 [O 2 ] [HCl] 4 [O 2 ] Determine the K eq if the concentrations are [HCl]=.200M, [O 2 ]=.500M, [Cl 2 ]=.32M, and [H 2 O]=.42M. K eq = 22.6

What is the equilibrium concentration of SO 3 in the following reaction if [SO 2 ] and [O 2 ] are M and K eq = 85.0? 2SO 2 + O 2  2SO 3 2SO 2 + O 2  2SO 3 K eq = [x] 2 [0.0500] 2 [0.0500] [0.0500] 2 [0.0500] X 2 = X=0.103M