Unit 2.6: Equilibrium

The Concept of Equilibrium Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.

The Concept of Equilibrium As a system approaches equilibrium, both the forward and reverse reactions are occurring. At equilibrium, the forward and reverse reactions are proceeding at the same rate.

A System at Equilibrium Once equilibrium is achieved, the amount of each reactant and product remains constant

Reactions are Reversible A + B  C + D ( forward) C + D  A + B (reverse) Initially there is only A and B so only the forward reaction is possible As C and D build up, the reverse reaction speeds up while the forward reaction slows down. Eventually the rates are equal

What is at Equilibrium? Rates are equal Concentrations are not. Rates are determined by concentrations and activation energy. The concentrations do not change at equilibrium. or if the reaction is verrrry slooooow.

Depicting Equilibrium Since, in a system at equilibrium, both the forward and reverse reactions are being carried out, we write its equation with a double arrow. N2O4 (g) 2 NO2 (g)

Equilibrium Constant Forward reaction: Rate Law: N2O4 (g)  2 NO2 (g) Rate = kf [N2O4]

Equilibrium Constant Reverse reaction: Rate Law: 2 NO2 (g)  N2O4 (g) Rate = kr [NO2]2

Equilibrium Constant kf kr [NO2]2 [N2O4] = Therefore, at equilibrium Ratef = Rater kf [N2O4] = kr [NO2]2 Rewriting this, it becomes kf kr [NO2]2 [N2O4] =

Equilibrium Constant Keq = kf kr [NO2]2 [N2O4] = The ratio of the rate constants is a constant at that temperature, and the expression becomes Keq = kf kr [NO2]2 [N2O4] =

aA + bB cC + dD The equilibrium expression becomes: Equilibrium Constant aA + bB cC + dD The equilibrium expression becomes: Kc = [C]c[D]d [A]a[B]b

Equilibrium Constant

Equilibrium Constant

Equilibrium Constant Expression In an equilibrium constant expression All concentrations are equilibrium values Product concentrations appear in the numerator, and reactant concentrations appear in the denominator Each concentration is raised to the power of its stoichiometric coefficient in the balanced chemical equation The value of the constant K depends on the particular reaction and on the temperature Units are never given with K

Writing Equilibrium Constant Expressions Reactions involving solids S(s) + O2(g) SO2(g) Concentration of a solid is determined by its density… and density is fixed Meaning the concentration of a solid is essentially a constant and not included in the expression The concentrations of any solid reactants and products are not included in the equilibrium constant expression

Writing Equilibrium Constant Expressions

Writing Equilibrium Constant Expressions Reactions in Aqueous solutions The real question here isn’t about aqueous… it’s about pure liquids Both the concentrations of liquids and solids can be obtained by multiplying the density of the substance by its molar mass — and both of these are constants at constant temperature.

Writing Equilibrium Constant Expressions

Writing Equilibrium Constant Expressions

Writing Equilibrium Constant Expressions Reactions involving Gases: Kc and Kp Since pressure is proportional to concentration for gases in a closed system, the equilibrium expression can also be written Kp = (PCc) (PDd) (PAa) (PBb)

Writing Equilibrium Constant Expressions Relationship between Kc and Kp From the Ideal Gas Law we know that PV = nRT Rearranging it, we get P = RT n V

Writing Equilibrium Constant Expressions Plugging this into the expression for Kp for each substance, the relationship between Kc and Kp becomes where Kp = Kc (RT)n n = (moles of gaseous product) - (moles of gaseous reactant)

Practice

Practice Write the equilibrium constant expressions for the following reactions: N2(g) + 3H2(g) 2NH3(g) H2CO3(g) + H2O(l) HCO3-(aq) + H3O+(aq)

Unit 2.6: Equilibrium Constants

Equilibrium Can Be Reached from Either Direction As you can see, the ratio of [NO2]2 to [N2O4] remains constant at this temperature no matter what the initial concentrations of NO2 and N2O4 are.

Equilibrium Can Be Reached from Either Direction This is the data from the last two trials from the table on the previous slide.

Equilibrium Can Be Reached from Either Direction It doesn’t matter whether we start with N2 and H2 or whether we start with NH3: we will have the same proportions of all three substances at equilibrium.

The Meaning of the Equilibrium Constant, K If K>>1 the reaction is product-favored; product predominates at equilibrium; system contains almost all product “reaction has gone to completion”

The Meaning of the Equilibrium Constant, K If K<<1 the reaction is reactant-favored; reactant predominates at equilibrium; system contains almost all reactant

Practice Are the following reactions product- or reactant-favored at equilibrium? [Cu(NH3)4]+2  Cu+2(aq) + 4NH3(aq) K = 1.5 x 10-13 [Cd(NH3)4]+2  Cd+2(aq) + 4NH3(aq) K = 1.0 x 10-7 If each reaction has a reactant concentration of 0.10M, in which solution is the NH3 concentration greater?

Equilibrium Constants The equilibrium constant denotes the progress of the reaction If the equilibrium constant is greater than 1,the rxn favors the formation of the products. More products are formed than reactants remain. If the equilibrium constant is less than 1, the rxn favors the formation of the reactants. Most of the original reactant remains as reactant, little product is formed. The equilibrium constant is temperature dependent, only The value of the equilibrium constant is not affected by changes in the pressure or concentration of any of the reactants or products.