Chemical EquilibriumBasic Skills: • Students should be able:

To explain the dynamic nature of equilibrium. • To be able to write the equilibrium constant expression

for a given equilibrium. • To explain the connection between an equilibrium and

the rates of the forward and reverse reaction. • To calculate the equilibrium constant for the reaction

reversed, multiplied, etc., given the equilibrium constant for a reaction

• To determine the equilibrium constant for a reaction that is some combination of these reactions, given the equilibrium constant for two or more reactions,.

The state of a chemical reaction in which its forward and reverse reactions occur at equal rates so that the concentration of the reactants and products does not change with time.

Chemical Equilibrium

Law of Mass ActionaA + bB + cC + ... <=> pP + qQ + rR + ...

Equilibrium Constant

[P]p [Q]q [R]r ...

K = --------------------- [A]a [B]b [C]c ...

Meaning of Equilibrium Constant• K>>1: reaction is product-favored; equilibrium

concentrations of products are greater than equilibrium concentrations of reactants.

• K<<1: reaction is reactant-favored; equilibrium concentrations of reactants are greater than equilibrium concentrations of products.

Acid-Base Equilibrium in Aqueous Solution

Acid Dissociation Constant

HC2H3O2 + H2O <=> H3O+ + C2H3O2-

[H3O+][C2H3O2-]

K = ---------------------- [H2O][HC2H3O2]

[H3O+][C2H3O2-]

Ka = K*[H2O] = ---------------------- [HC2H3O2]

Acid-Base Equilibrium in Aqueous Solution

Base Dissociation Constant

NH3 + H2O <=> NH4+ + OH-

[NH4+][OH-]K = -----------------

[H2O][NH3]

[NH4+][OH-]

Kb = K*[H2O] = ---------------- [NH3]

Chemical Equilibrium

• To calculate Kp from Kc and vice versa. • To be able to work with heterogeneous equilibria, including

knowing how liquids and solids are treated in equilibrium constant expressions.

• To know what the reaction quotient is and how it is used to predict the direction a  reaction must shift to achieve equilibrium.

• To use LeChâtelier's Principle to be able to predict shifts in equilibria for various disturbances.

• To calculate equilibrium quantities from Kp and Kc values, and determine the value of equilibrium constants from experimental data.  

Homework: 11, 13, 15, 17, 22, 33, 43, 51, 58, 64, 70, 77, 84, 90

Characteristics of Chemical Equilibrium ( Chem. Eq.)

• Chemical Equilibrium –finite concentrations of reactants and products and they remain constant– Products predominate- product-favored

– When mostly reactants –reactant-favored

[Etymology- “equi” = “eqal” (equal forward and reverse rates, not equal quantities or concentrations)

“librium” comes from “libra” = balance]

Chem. Eq. – An equal balance between two reaction rates

The Equilibrium Constant

• k forward/ k reverse = [trans-2-butene]/ [cis-2-butene]

– Kc = k forward/ k reverse = 1.65 ( at 500 K)

– Kc = 1.47 at 600 K and 1.36 at 700 K

• For a reaction at a given temperature, the quotient of equilibrium concentrations of reactant and product has a constant value and is called equilibrium constant, Kc .

– The mathematical expression is called an equilibrium constant expression.

Equilibrium Constant Expressions• The concentration of gases and of solutes in dilute

solutions appear in the eq. constant expression as they only can change as a reaction occurs.– Concentrations of pure solids, pure liquids and solvents in

dilute solutions do not appear in equilibrium constant expressions.

N 2 (g) + O2 (g) 2NO (g)

• Equilibria involving pure Liquids and Solids Number of molecules /unit volume is fixed

• Equilibria in Dilute Solutions Customary to omit the units

Eq. Constant Expressions

• The eq. constant for a reaction and that for its reverse are the reciprocals of one another.

• Eq. constant for a reaction that combines two or more other reactions ( p-642)– Kc3 = K c1 x Kc2

• Eq. constant in terms of pressure– Kp = K c (RT) n

Le Chatelier's PrincipleIf a stress, such as a change in concentration,

pressure, temperature, etc., is applied to a system at equilibrium, the equilibrium will shift in such a way as to lessen the effect of the stress.

Gas Phase Equilibrium

catalysis

N2(g) + 3 H2(g) <=====> 2 NH3(g) + heat

high pressure and temperature

The Principle of Le Chatelier

Changes in Concentration or Partial Pressure

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

an increase in N2 and/or H2 concentration or pressure, will cause the equilibrium to shift towards the production of NH3

The Principle of Le Chatelier

Changes in Temperature

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

for an exothermic reaction, an increase in temperature will cause the reaction to shift back towards reactants

The cobalt complexes participating in the equilibrium below comprise a humidity sensor. From Le Châtelier's principle, when the sensor is moist (excess H2O), what color is the cobalt complex?

Heterogeneous Equilibrium

CaCO3(s) + heat <===> CaO(s) + CO2(g)

Gibbs Free Energy and Equilibrium

G Reaction

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Negative Spontaneous

Positive Non-Spontaneous

Zero Equilibrium

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The Influence of Temperature on Free Energy

G, H, & S

G = H - T S

H S G

negative positive negative

spontaneous at all temperatures