Topic 5A: ions in solution and Solubility

Page 1

Ions are formed when the number of protons is not equivalent to number of electrons in an atom.

Cations are formed when the number of protons is greater, and anions are formed one number of electrons is greater.

Polyatomic ion: an ion made up of 2 or greater atoms

When an ionic compound is dissolved in solution, both cations and anions are formed

ionic compound nomenclature: the name of the cation comes first, and it is named as the element.

There are some complex cations, the most important of which is the ammonium ion (NH4+).

Electrolyte: any salt or ionizable molecule when dissolved in solution that gives the solution the ability to conduct electricity.

Strong electrolytes dissociate completely into their component ions, while weak electrolytes remain mostly undissociated.

Page 2

Hydrated ions: ions in an aqueous solution (ions surrounded or solvated by water molecules)

Water is a good solvent for many ionic and polar molecules. Often a sphere of hydration, or hydration shell, will form around the particles that dissolve in water as a result of the hydrogen bonds that these molecules form with water.

Hydration: the orientation of water molecules around ions in an ionic solid resulting in reduction of the localized charge on the ions forming dipole attractions, ultimately breaking up the solid and allowing it to dissolve in the solvent. This is a thermodynamically favorable process, and can release heat (heat of hydration).

Solubility is determined by the interaction of ions with the solvent.

In the case of water because water is polar, its properties allow ionic compounds and polar molecules to readily dissolve within it.

The disparity in electronegativity in the O-H bonds within the water molecule results in a dipole, which facilitates interactions with the ionic particles that dissolve within it.

The two lone pairs on the oxygen in water also contribute to the stabilization of any positively charged ions in solution.

Complex ion formation: when metal ions accept electron pairs from other molecules. In these cases, the metal ions are acting as Lewis acids.

Examples:

Ag+ (aq) + 2NH3 (aq) → Ag(NH3) 2+ (aq)

Cu2+ (aq) + 4CN− (aq) → Cu(CN) 2− 4 (aq)

Cd2+ (aq) + 4Br− (aq) → CdBr2− 4 (aq)

Formation constants (equilibrium constants of formation, Kf): an indicator of how stable the complex of ions will be in aqueous solution; The greater the value of Kf, the more stable the complex is.

The solubility of a less soluble salt can be increased by the addition of a Lewis base (donates electrons). The addition of an electron pair shifts equilibrium to increase the solubility of a compound by removing one of the ions through the formation of a complex iron.

AgCl (s) → Ag+ (aq) + Cl− (aq)

Addition of a pair of electrons to the above reaction in the form of cyanide ion would allow silver ions to form the complex ion Ag(CN)2-. This complex ion has a very large formation constant, meaning it is quite stable in solution. The cyanide ion remove silver ions from the solution, driving the reaction towards the right and increasing the solubility of AgCl.

Lowering the pH increases the solubility of basic ionic compounds.

Page 3

concentration units:

Molarity (M): mole solute/liter solution.

This definition can also be considered an equation to solve for any of the 3 variables.

1 mol/L = 1000 mol/m^3

Dilution impacts the concentration and volume of solution. Changes after dilution are calculated with the following formula --> M1V1=M2V2

M1= initial concentration in molarity

V1= initial volume in liters

M2= final concentration in molarity

V2= final volume in liters

Page 4

Solubility product constant (Ksp): allows you to determine the solubility of a compound using the equilibrium constant, K.

Ksp = [products]/[reactants]

Represents the equilibrium between an ionic solid and its ions in solution. Can also be indicative of the tendency of the reaction to reach equilibrium.

Concentration is measured using molarity in this expression.

When writing any equilibrium expression remember:

pure solids and liquids are not included in equilibrium expressions; Aqueous and gaseous species are included.

When the species has a coefficient, the concentration of that species should be raised to the power of the coefficient and also should be multiplied by the value of the coefficient.

If there are multiple products or reactants, they should all be included in the expression as appropriate.

If explicitly stated that the species fully dissociates in solution, you can assume that the concentration of the undissociated species is equivalent to the concentration of the individual dissociated components.

Page 5

When one of the ions in a solution is added at equilibrium, the equilibrium shifts to remove the added ion; This results in decreased solubility and increased precipitate formation.

Common ion effect: The decrease in solubility of a substance from the addition of another substance with a common ion. This effect is attributed to the shift in equilibrium.

Applications: the common ion effect can be used to separate out a desired compound from a solution. For example, if there is silver chloride and silver sulfate in solution, and we wanted to take out the silver chloride, we could add sodium chloride to decrease the solubility of the silver chloride so it could be separated from the solution.

The common ion effect also plays a role in pH regulation of buffer solutions. For example, in the case of an acidic buffer, the hydrogen ion concentration well be decreased in comparison to a solution containing the pure weak acid. The addition of a base will suppress the ionization of the acid and shift the equilibrium towards the undissociated acid, thereby decreasing the concentration of hydrogen ions and increasing the pH of the solution, or keeping the pH higher.