Solubility Equilibria

Writing the Ksp Expression

The solubility product constant (Ksp) describes the equilibrium between a solid salt and its ions in solution. For a salt with formula Pb(IO3)2:

• Ksp Expression: For Pb(IO3)2, the equilibrium is:

• General Rule: The Ksp expression is the product of the concentrations of the ions, each raised to the power of its coefficient in the balanced equation.

Calculating Qsp and Predicting Precipitation

The ion product (Qsp) is calculated using initial ion concentrations. Compare Qsp to Ksp to predict precipitation:

• Qsp > Ksp: Precipitate forms.

• Qsp < Ksp: No precipitate; solution is unsaturated.

• Qsp = Ksp: Solution is at equilibrium; saturated.

• Example: If initial concentrations of Pb2+ and IO3- are known, calculate Qsp and compare to Ksp.

Finding Ksp from Molar Solubility

Molar solubility is the number of moles of solute that dissolve per liter. Use stoichiometry to relate solubility to Ksp:

• Example: For AgCl, if molar solubility is s:

Complex Ion Formation: KF and KD

Complex ions increase solubility. The formation constant (KF) and dissociation constant (KD) are related:

• Example: If KF for a complex ion is given, KD can be calculated as its reciprocal.

Common Ion Effect

The presence of a common ion decreases the solubility of a salt by shifting equilibrium toward the solid.

• Example: Solubility of MgF2 in 0.10 M NaF is reduced because F- is a common ion.

• Calculation: Set up the equilibrium with the initial concentration of the common ion and solve for solubility.

Solubility in Acidic Solution

Some salts dissolve more in acidic solutions if their anion reacts with H+.

• Example: AgCN dissolves more in acid because CN- reacts with H+ to form HCN.

Effect of Complex Ion Formation on Solubility

• Formation of complex ions increases solubility by removing ions from solution and shifting equilibrium.

Qsp vs Qc

• Qsp: Ion product for solubility equilibria.

• Qc: Reaction quotient for general chemical reactions.

• Both compare initial conditions to equilibrium constants.

F- Replaces OH- in Tooth Enamel

• Fluoride ions replace hydroxide ions in hydroxyapatite, forming fluorapatite, which is more resistant to acid.

Calculating Molar Solubility from Ksp

• Set up the equilibrium expression and solve for solubility.

• Example: For AgCl:

Calculating Molar Solubility from g/L

• Convert grams per liter to moles per liter using molar mass.

Thermodynamics

State Functions

• State functions: Properties that depend only on the state, not the path (e.g., enthalpy, entropy, free energy).

• Not state functions: Work and heat.

Enthalpy Sign for Exothermic Reactions

• Exothermic: Releases heat; is negative.

Spontaneous Processes

• Processes that occur without external intervention.

• Examples: Iron rusting, ice melting.

Why Study Thermodynamics?

• To predict whether reactions are spontaneous, understand energy changes, and determine equilibrium.

Gibbs Free Energy Equation

• Gibbs free energy determines spontaneity.

• Spontaneous:

Second Law of Thermodynamics

• Entropy of the universe increases in spontaneous processes.

Calculating Entropy Change

• Use standard entropy values or calculate from heat transfer:

Calculating DeltaG0 from DeltaH0 and DeltaS0

Equilibrium and DeltaG

• At equilibrium,

Thermodynamic Equilibrium Constant (K)

• R = gas constant (8.314 J/mol·K), T = temperature in Kelvin

Entropy Change in Chemical Reactions

• Determine if entropy increases or decreases by comparing number of particles, phase changes, etc.

Criterion for Spontaneity

• Best criterion:

What Free Energy Tells Us

• Indicates direction of reaction and whether it is spontaneous.

Three Laws of Thermodynamics

• First Law: Energy is conserved.

• Second Law: Entropy of the universe increases.

• Third Law: Entropy of a perfect crystal at absolute zero is zero.

Essay Topics

Common Ion Effect Example

• Adding NaCl to a saturated AgCl solution decreases AgCl solubility due to increased Cl- concentration.

Solubility Calculations

• Given solubility in g/L, convert to molar solubility using molar mass.

Calculating DeltaG0 from DeltaHf0 and DeltaS0

• Use the equation:

Summary Table: Solubility and Thermodynamics Concepts