Chemical EquilibriumChemical Equilibrium

● Not all reactions proceed to completion● Chemical Equilibrium – a state in which the ratio

of concentrations of reactants and products is constant

Chemical Equilibrium: Some RulesChemical Equilibrium: Some Rules

In evaluating an equilibrium constant, always remember the following:

● The concentrations of solutes should be expressed in molarities.

● The concentration of gases should be expressed in atmospheres.

● The concentrations of pure solids, liquids or solvents are unity (1).

Chemical Equilibrium: Le Chemical Equilibrium: Le ChChââtalier's Principletalier's Principle● The position of a chemical equilibrium is

independent of the route by which equilibrium is reached.

● Apply “stress” to a chemical system = disruption of equilibrium– “Stress”: change in either temperature, pressure or

total concentration of a reactant or product● To relieve stress, the equilibrium “shifts” to off-

set the sudden change.

Equilibrium and ThermodynamicsEquilibrium and Thermodynamics

● Enthalpy (ΔH) – amount of heat absorbed or released

● Entropy (S) – amount of order / disorder in the system

● Gibbs' Free Energy (ΔG) – determines if the reaction is favored, not favored or in equilibrium based on ΔH and S.

In Depth: Le ChIn Depth: Le Chââtalier's Principletalier's Principle

● The equilibrium constant Keq vs. the reaction quotient Q.

Q=[C ' ]p[D ' ]q[ A' ]m[B ' ]n

Chemical Equilibrium: The Chemical Equilibrium: The Solubility ProductSolubility ProductSolubility Product – a specific type of chemical

equilibrium in which a solid salt dissolves into its ions in a solution

Example:Hg2Cl2 (s) ↔ Hg2

+2 (aq) + 2Cl- (aq)

Ksp = [Hg2+2][Cl-]2 = 1.2 x 10-18

The Common Ion EffectThe Common Ion Effect

Given the example in the previous slide, what would happen if we add sodium chloride into the solution?

● The Common Ion Effect – a salt will be less soluble if one of its constituent ions is already present in the solution.

Application: Selective Application: Selective PrecipitationPrecipitation

Given the following:PbI2 (s) ↔ Pb+2 (aq) + 2I- (aq)

Hg2Cl2 (s) ↔ Hg2+2 (aq) + 2Cl- (aq)

Is it possible to separate the two metal ions in the solution? How?

Chemical Equilibria – Acids and Chemical Equilibria – Acids and BasesBases● Three (3) definitions of acids/bases

– Arrhenius– Bronsted-Lowry– Lewis

The Nature of HThe Nature of H++ and OH and OH--

● A proton cannot exist by itself!

pH – Power of HpH – Power of H++

● Autoprotolysis of water

● What is the equilibrium constant for this reaction?

Acid/Base StrengthAcid/Base Strength

● Depends on how much H+ is produced.– Strong = complete dissociation– Weak = partial dissociation

● Write down the equilibrium equation for the weak acid HA (A is an anion).

● Write down the equilibrium equation for the weak base B.

● Brain teaser: how is the equilibrium constant for autoprotolysis related to acid/base strength?

Chemical Equilibrium In ActionChemical Equilibrium In Action

Calculate for the [Ba+2] concentration from Ba(IO3)2 in a solution prepared by mixing 200 mL of 0.0100 M Ba(NO3)2 and 100 mL of 0.100 M NaIO3.

Ksp (Ba(IO3)2) = 1.57 x 10-9

Systematic Treatment of Systematic Treatment of EquilibriumEquilibrium

Systematic Treatment of Systematic Treatment of EquilibriumEquilibriumMost of the time, chemical equilibria are rarely

straightforward:BaSO4 (s) ↔ Ba+2 + SO4

-2

SO4-2 + H3O

+ ↔ HSO4- + H2O

2H2O ↔ H3O+ + OH-

Systematic Treatment: Charge-Systematic Treatment: Charge-Balance EquationsBalance Equations● Possible since the species in aqueous solutions

are ions.● Premise: all electrolyte solutions are neutral.

no. mol/L (+) = no. mol/L (-)● Remember: a charge-balance equation is based

on the equality in molar charge concentrations.no. mol/L (+) = [A] x no. of (+) charge

Systematic Treatment: Charge-Systematic Treatment: Charge-Balance EquationsBalance EquationsExample: Give the molar concentrations of

positive/negative charge for the following ions:● Na+

● Ba+2

● OH-

● SO4-2

● CH3COO-

Systematic Treatment: Charge-Systematic Treatment: Charge-Balance EquationsBalance EquationsExample: Given the system of equilibria in the

previous slide, construct its charge-balance equation.

BaSO4 (s) ↔ Ba+2 + SO4-2

SO4-2 + H3O

+ ↔ HSO4- + H2O

2H2O ↔ H3O+ + OH-

Systematic Treatment: Mass-Systematic Treatment: Mass-Balance EquationsBalance Equations● Restatement of the law of conservation of matter● The quantity of all species in a solution

containing a particular atom or group of atoms must equal the amount of that atom/group delivered to the solution.

Systematic Treatment: Mass-Systematic Treatment: Mass-Balance EquationsBalance EquationsExample. 0.05 M acetic acid in water--

CH3COOH ↔ CH3COO- + H+

cacetic acid = 0.05 M = [CH3COOH] + [CH3COO-]

Question: Why is [H+] NOT included in defining the MB expression of cacetic acid?

Exercise. Given the following equilibrium reactions for 0.05 M sulfuric acid:

H2SO4 → HSO4- + H+

HSO4- ↔ SO4

-2 + H+

Give the MB expression for the acid's formal concentration csulfuric acid.

Another Mass-balance ExpressionAnother Mass-balance Expression

Example. La(IO3)3 partially dissolves in water, as shown in the equation--

La(IO3)3 ↔ La+3 + 3 IO3-

La+3's relationship with iodate can be defined in the following MB expression:

[IO3-] = 3 x [La+3]

WHY?

MB Equations in a System of MB Equations in a System of EquilibriaEquilibriaChallenge! In the previous set of equilibria, write

down all the possible MB expressions present in the system.

BaSO4 (s) ↔ Ba+2 + SO4-2

SO4-2 + H3O

+ ↔ HSO4- + H2O

2H2O ↔ H3O+ + OH-

Systematic Treatment of Systematic Treatment of Equilibria: A 6-Step ApproachEquilibria: A 6-Step Approach(1) Write the pertinent reactions.(2) Write the charge-balance equation.(3) Write mass-balance equations. There may be

more than one.(4) Write the equilibrium constant for each

reaction.(5) # of equations ≥ # of unknowns. If not, find

more equilibria or fix some concentrations at known values.

(6) Solve for all unknowns, by hook or by crook.