Chem 122 Ch. 15.1-3

The magnitude of Ksp indicates how _______ a compound is.

soluble

A larger Ksp value is ____ soluble

more

Molar solubility units

mol/L (M)

Solubility units

g/L = g/100mL

Solubility equilibrium

the dynamic equilibrium that exists in a saturated solution when the rate of dissolution of a solid equals the rate of precipitation of that solid

Ksp is the equilibrium constant for a dissolution reaction. It tells you:

the concentrations of the dissolved ions when a solution has become saturated and dissolution and precipitation occur at equal rates.

Solubility product constant

Ksp

Why is the solid (precipitate) not included in a Ksp expression?

Because Ksp only describes the dissolved ions at equilibrium. The concentration of a pure solid is constant, so changing the amount of solid does not affect the equilibrium ion concentrations. Therefore, only the dissolved ions are included in the Ksp expression.

“Write the dissolution equation”

Solid dissolving into 2/3 ions equilibrium equation

“Write the solubility product”

Ksp equation []

s

molar solubility

What happens when a common ion is added to a solubility equilibrium?

Adding a common ion shifts the equilibrium toward the solid (left) according to Le Châtelier's Principle. More ions combine to form the solid, so less solid dissolves and the solubility decreases.

Qsp

solubility reaction quotient

What is Qsp (the solubility reaction quotient)?

Qsp is the reaction quotient for a dissolution reaction. It is calculated using the current ion concentrations immediately after solutions are mixed (before equilibrium is reached).

Qsp < Ksp

No precipitate

Qsp = Ksp

Saturated (at equilibrium)

Qsp > Ksp

Precipitate forms

Always soluble

Group 1 cations, NH₄⁺, NO₃⁻ (nitrate), CH₃COO⁻(acetate), ClO₃⁻, ClO₄⁻

Usually insoluble

CO₃²⁻, PO₄³⁻, S²⁻, OH⁻

Cl⁻, Br⁻, I⁻ are soluble except with

Ag⁺, Pb²⁺, Hg₂²⁺

SO₄²⁻ is soluble except with

Ba²⁺, Sr²⁺, Pb²⁺, Ca²⁺

Usually insoluble, unless paired with Na+, K+, or NH4+

CO₃²⁻, PO₄³⁻, C₂O₄²⁻ (oxalate), CrO₄²⁻, S²⁻, Most OH⁻ compounds

“Will a precipitate of __ form if x is mixed with y?” steps

1) Make equation 2) Calculate equilibrium M concentrations (LxM)/(L) 3)Multiply x and y equilibrium concentrations to get Qsp 4) Compare Qsp to Ksp to determine if a precipitate forms

The ______ Ksp value precipitates first.

smallest