Chemical Equilibrium

Reversible Chemical Reaction A chemical reaction in which the products can regenerate the original reactants.

Forward Reaction: Reactants Products Reverse Reaction: Reactants Products

Reactants Products The two half-arrows show that the reaction can proceed in either direction. The two reactions are opposite processes.

Reversible Chemical Reactions                                                                                         2NO2(g) N2O4(g)

Water Cycle … Reversible Reactions

Reversible Chemical Reactions

CHEMICAL EQUILIBRIUM The rate of the forward reaction is equal to the rate of the reverse reaction.

CHEMICAL EQUILIBRIUM At Equilibrium concentrations of products and reactants remain constant.

CHEMICAL EQUILIBRIUM The reaction does not stop when it reaches equilibrium! Dynamic Equilibrium!

[ ] CHEMICAL EQUILIBRIUM Concentrations are expressed by the use of brackets. [ ] Moles Molarity = Liter

THE EQUILIBRIUM CONSTANT (keq) A ratio of the concentrations of the products over the concentrations of the reactants. cC + dD aA + bB [ C ] c [ D ] d Keq = [ A ] a [ B ] b

THE EQUILIBRIUM EXPRESSION aA + bB cC + dD Raised to the power of the coefficient PRODUCTS Keq = [ C ] c [ D ] d [ A ] a [ B ] b REACTANTS Concentrations in Moles per Liter

THE EQUILIBRIUM EXPRESSION ALWAYS WRITE PRODUCTS OVER REACTANTS. NEVER INCLUDE SOLIDS OR LIQUIDS IN THE EXPRESSIONS

[H20] Keq = [H2] [O2] Practice Problem #1 2 2 Write the equilibrium expression for the oxidation of hydrogen to form water vapor. (homogeneous equilibrium) 2H2(g) + O2(g) 2H2O (g) [H20] 2 Keq = [H2] [O2] 2

[CO2] Keq = [CO] *Practice Problem #10 2 2 Write the equilibrium expression for the following reaction. (heterogeneous equilibrium) SnO2(s) + 2CO(g) Sn(s) + 2CO2(g) [CO2] 2 Keq = [CO] 2

LAW OF CHEMICAL EQUILIBRIUM Every reversible reaction will move to the state of equilibrium where there is a specific ratio of products and reactants.

EQUILIBRIUM CONSTANT (Keq) Indicates the extent to which a reaction proceeds to completion “significantly greater than” If Keq > 1, equilibrium lies to the right. Favors the products! If Keq < 1, equilibrium lies to the left. Favors reactants, reaction barely even starts!

EQUILIBRIUM CONSTANT (Keq)

“approximately equal to” If Keq  1, you would find considerable concentrations of both reactants and products.

Q tells which direction the reaction will shift to reach equilibrium. REACTION QUOTIENT (Q) Q tells which direction the reaction will shift to reach equilibrium. Q is calculated the same way that Keq is; however the concentrations are measured at any point during the reaction.

Q < Keq What If . . . Reactant concentrations too LARGE Product concentrations too SMALL Reaction will proceed to the RIGHT

Q > Keq What If . . . Product concentrations too LARGE Reactant concentrations too SMALL Reaction will proceed to the LEFT

No shift in direction will occur. What If . . . Q = Keq The reaction is at equilibrium. No shift in direction will occur.

Practice Problem #20 For the reaction H2(g) + CO2(g) CO(g) + H2O(g) Keq = 5.10 at 527ºC. If [CO] = 0.15M, [H2O] = 0.25M, [H2] = 0.42M, and [CO2] = 0.37M, calculate Q and determine how the reaction will proceed.

[CO2] [H2] Q = [CO] [H2O] [0.37] [0.42] Q = [0.15] [0.25] Practice Problem #20 CO(g) + H2O(g) H2(g) + CO2(g) [CO2] [H2] Q = [CO] [H2O] [0.37] [0.42] Q = = 4.144 [0.15] [0.25]

The reaction will proceed direction of the products. Practice Problem #20 CO(g) + H2O(g) H2(g) + CO2(g) Q = 4.144 Compare Q to Keq Keq = 5.10 The reaction will proceed to the right in the direction of the products. Q < Keq