Solubility Equilibria, Precipitation, and Complex Ion Formation

Vocabulary flashcards covering solubility product constants, common ion effects, reaction quotients for precipitation, acid-induced solubility shifts, and complex ion equilibria from the lecture.

The solubility-product constant is also known as ___ value

Ksp value

what is molar solubility?

AI Overview

Molar solubility is the maximum number of moles of a solute that can dissolve in one liter of a saturated solution (expressed in units of mol/L or M). It represents the highest possible molar concentration of a specific compound in a solution at equilibrium with an undissolved solid at a given temperature

Solubility Product Constant ($K_{sp}$)

The equilibrium constant describing the dissociation of a slightly soluble ionic solid into its component ions in aqueous solution; a smaller $K_{sp}$ indicates lower solubility and fewer dissociated ions.

slide #12

Write the solubility-product expression for the following salts:

  a. Hg₂Cl₂

  b. HgCl₂

a.  Hg₂Cl₂

Hg₂Cl₂(s)          Hg₂²⁺(aq) + 2Cl^-(aq)

K_sp = [Hg₂²⁺][Cl^-]²

b.  HgCl₂

HgCl₂(s)          Hg²⁺(aq) + 2Cl^-(aq)

K_sp = [Hg²⁺][Cl^-]²

true or false

the smaller the ksp value the less the ion dissociation, and the less soluble the substance must be

True; a smaller $K_{sp}$ indicates lower solubility.

what are the units for ksp?

The units for $K_{sp}$ are typically expressed in molarity raised to the power of the total number of ions produced in solution. For example, if a salt dissociates into two ions, the units would be $mol^2 \times L^{-2}$.

Slide #21

An experimenter finds that the solubility of barium fluoride is 1.1 g in 1.00 L of water at 25°C. What is the value of K_sp for barium fluoride, BaF₂, at this temperature?

To find the value of $K_{sp}$ for barium fluoride, BaF₂, first convert the solubility from grams to moles by using the molar mass of BaF₂. The dissociation of BaF₂ in water is represented as:

$$BaF_2(s) \rightleftharpoons Ba^{2+}(aq) + 2F^{-}(aq)$$

Thus, if the solubility is 1.1 g/L, the concentration of Ba²⁺ will equal $s$ and that of F^- will equal $2s$. Calculating the $K_{sp}$ is done using the expression:

$K_{sp} = [Ba^{2+}][F^{-}]^2$.

The next step is to determine the molar mass of BaF2, which is approximately 175 g/mol. Therefore, the solubility in terms of moles is: $s = \frac{1.1\,g}{175\,g/mol} \approx 0.00629\,mol/L$. Then the concentration of $F^{-}$ ions is $2s = 2 \times 0.00629\,mol/L \approx 0.01258\,mol/L$. Now substituting these concentrations into the Ksp expression:

$K_{sp} = [Ba^{2+}][F^{-}]^2 = (0.00629)(0.01258)^2 \approx 9.94 \times 10^{-10}$.

Calomel, whose chemical name is mercury(I) chloride, Hg₂Cl₂, was once used in medicine (as a laxative and diuretic). It has a K_sp equal to 1.3 ´ 10-¹⁸. What is the solubility of Hg₂Cl₂ in grams per liter?

Look at chapter 17 slides 25-27 for explanation

what is the common ion effect?

The common ion effect refers to the decrease in solubility of an ionic compound when another source of one of its constituent ions is added to the solution. This shift occurs due to Le Chatelier's principle, which states that the equilibrium will shift to counteract the addition of the common ion.

Common Ion Effect

The reduction in the solubility of a sparingly soluble ionic compound caused by the addition of a soluble compound that shares an identical cation or anion, shifting the equilibrium toward the solid reactant side.

what is the reaction quotient used for ?

The reaction quotient, $Q$, is used to compare the ratio of products to reactants at any point in a reaction to the equilibrium constant, $K$. It helps determine whether a system is at equilibrium, and if not, in which direction the reaction will proceed to reach equilibrium.

Reaction Quotient ($Q_c$)

An expression with the same mathematical formulation as the $K_{sp}$ expression, evaluated using non-equilibrium or initial ion concentrations to predict whether a system will form a precipitate.

when does precipitation occur in these experiments?

Ksp is less than Qc

Condition for Precipitation ($Q_c > K_{sp}$)

The state wherein the concentration product of the dissolved ions exceeds the equilibrium solubility product, causing the equilibrium to shift to the left and force excess ions to precipitate out of solution.

What is Fractional Precipitation

A laboratory separation technique in which two or more dissolved ions are separated individually from a mixture by gradually adding a precipitating reagent that selectively precipitates the ions in sequence based on their relative $K_{sp}$ values.

true or false

the solubility of insoluble salts can be manipulated by adding species that react with either cation or anion

True, adding species that react with cations or anions affects solubility by shifting equilibrium.

Effect of pH on Solubility

When a salt contains the conjugate base of a weak acid, the pH will affect the solubility of the salt.As the pH increases, the solubility of that salt typically increases due to the decreased concentration of its conjugate base.

read

We will qualitatively explore the situation involving a generic salt, MX, where X is the conjugate base of a weak acid.

MX(s)          M⁺(aq) + X^-(aq)

As the acid concentration increases, X^- reacts with the H₃O⁺, forming HX and reducing the X^- concentration. As a result, more MX dissolves, increasing the solubility.

the smaller the ksp value’s exponent the more adding acid will increase the solubility

What is the Complex Ion formation?

Some cations form soluble complex ions. Their formation increases the solubility of a salt containing those cations

name metal ions that form complex ions?

include Ag⁺, Cd²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Ni²⁺, and Zn²⁺.

complex agents are ligands, lewis bases

A Lewis base (such as $NH_3$, $CN^-$, S₂O₃^2- or $OH^-$) that possesses lone pairs of electrons and donates them to an electron-deficient metal ion to form a complex ion.

hint for complex ion if you see a positive 10^ values they are Kf constants

why are kf values large?

This indicates that the formation of the complex ion is highly favored, meaning the reaction strongly prefers to form products over reactants.

Empty $d$ Orbitals in Complex Formation

The valence orbital feature characteristic of transition metal cations (such as $Ag^+$, $Cd^{2+}$, $Cu^{2+}$, $Fe^{2+}$, $Fe^{3+}$, $Ni^{2+}$, and $Zn^{2+}$) that enables them to accept electron pairs from ligands during complex ion synthesis.

Formation Constant ($K_f$)

The equilibrium constant associated with the assembly of a complex ion from a central metal cation and surrounding ligands, typically characterized by large positive-exponent values indicating strong product favoritism.

Dissociation Constant ($K_d$)

The equilibrium constant representing the breakdown of a complex ion into its constituent free metal cation and ligands, mathematically equivalent to the reciprocal of the formation constant ($K_d = \frac{1}{K_f}$).

Qualitative Analysis

A systematic analytical procedure used to identify and separate a mixture of cations (e.g., cobalt, iron, manganese, nickel, zinc) by manipulating precipitation equilibria using reagents, pH changes, and relative $K_{sp}$ thresholds.