SOLUTIONS and SOLUBILITY

SOLUTION may be described as a single phase system composed of two or more substances representing homogeneous molecular dispersion and is composed of solute and solvent and phase is established usually by the solvent because it is present in larger amount.

Solution is characterized by their capacity to dissolve a solute.

Saturated solution → contains the maximum amount of solute that will dissolve in a given solvent at a specific temperature.

Unsaturated solution → contains less solute than it has the capacity to dissolve.

Supersaturated solution → contains more solute than is present in a saturated solution.

SOLUTION PROCESS takes place in 3 distinct steps: Step 1 is the separation of solvent molecules. Step 2 entails the separation of solute molecules. These steps (1 and 2) require energy input to break attractive intermolecular forces; therefore, they are endothermic.bIn step 3 the solvent and solute molecules mix; this process can be exothermic or endothermic.

SOLUTION PROCESS: the heat of solution (ΔHsoln) is given by: ΔHsoln = ΔH1 + ΔH2 + ΔH3  If the solute–solvent attraction is stronger than the solvent–solvent attraction and solute–solute attraction, the solution process is favorable, or exothermic (ΔHsoln < 0).  If the solute–solvent interaction is weaker than the solvent–solvent and solute–solute interactions, then the solution process is endothermic (ΔHsoln > 0).

SOLUBILITY is the concentration (g/mL) of solute in a saturated solution at a given temperature; unless otherwise specified, the temperature is assumed to be 25 °C.

SOLUBILITY may be expressed in many ways; official compendia have adopted a system of stating the amount of a particular solvent necessary to dissolve 1 gram of a substance.

SOLUBILITY may also be expressed as the number of grams of solute that can be dissolved in 100 grams of solvent.

Descriptive term:

mL solvent for 1 g solute is

Very soluble - < 1

Freely soluble - from 1 to 10

Soluble - from 10 to 30

Sparingly soluble - from 30 to 100

Slightly soluble - from 100 to 1,000

Very slightly soluble - from 1,000 to 10,000

Practically insoluble - > 10,000

CONCENTRATION EXPRESSIONS:

Molarity - expresses the number of moles (gram– molecular weight) of solute contained in 1 liter of solution. 𝑀 = 𝑔 𝑠𝑜𝑙𝑢𝑡𝑒 𝑀𝑊 𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 2)

Normality - expresses the number of equivalents (gram–equivalent weight) of solute contained in 1 liter of solution. 𝑁 = 𝑔 𝑠𝑜𝑙𝑢𝑡𝑒 𝑀𝑊 𝑓 𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛

Molality - expresses the number of moles (gram– molecular weight) of solute contained in 1 kilogram of solvent; is used to express thermodynamic properties of solutions. 𝑚 = 𝑔 𝑠𝑜𝑙𝑢𝑡𝑒 𝑀𝑊 𝐾𝑔 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 4)

Percent - is an expression of “parts of solute per 100 parts of solution”. % 𝑤 𝑣 = 𝑔 𝑠𝑜𝑙𝑢𝑡𝑒 𝑚𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑥100 10

SOLUBILITY PRODUCT CONSTANT or solubility product, is the product of the molar concentrations of the constituent ions of a compound, each raised to the power of their stoichiometric coefficient in the equilibrium equation:

MgF2(s) ↔ Mg+2 (aq) + 2F –1 (aq) Ksp = [Mg+2 ][F–1 ] 2 Ag2CO3(s) ↔ 2Ag+1 (aq) + CO3 –1 (aq) Ksp = [Ag+1 ] 2 [CO3 –2 ]

SOLUBILITY PRODUCT CONSTANT or solubility product determines whether precipitation will occur or whether precipitates will dissolve under certain conditions.

REACTION QUOTIENT or ion product (Q), is used to predict whether a precipitate will form for concentrations of ions that do not correspond to equilibrium conditions.

Ksp possible relationships are:

Q < Ksp: unsaturated solution (no precipitation)

Q = Ksp: saturated solution (no precipitation)

Q > Ksp: supersaturated solution (precipitation occurs until the product of the ionic concentrations is equal to Ksp)

Prediction of Solubility:

SOLUBLE COMPOUNDS:

-compounds containing alakali metal ions (Li, Na, K, Rb, Cs) and the ammonium ion (NH4+)

-Nitrates (NO3-), acetates (CH3COO-), bicarbonates (HCO3-), chlorates (HCO3-), perchlorates (CLO4-)

-halides (Cl-, Br-, I-): excepts Halides of Ag+, Hg2²+, and Pb2

-Sulafates (SO4²-): excepts Sulfates of Ag+, Ca2+, Sr2+, Ba2+, Hg2²+, and Pb2+

INSOLUBLE COMPOUNDS:

-Carbonates (C03²-), phosphates (PO4²-), chromates (CrO4²), sulfides (S2-): excepts compounds containing alkali metal ions and the ammonium ion

-hydroxides (OH-):excepts compounds containing alkali metal ions and the Ba2+ ion.

COLLIGATIVE PROPERTIES are those which are totally dependent upon the number of solute entities in a solution, and are independent of the kinds of solute entities.

colligative properties involve vapor pressure, freezing point, boiling point, and osmotic pressure.

COLLIGATIVE PROPERTIES utilize molal concentration (m) and mole fraction (X) (ratio of the number of moles of any constituent in a system to the total number of moles of all constituents) in its quantitative aspects.

COLLIGATIVE PROPERTIES:

Vapor pressure lowering → follows Rauolt’s law, which states that the partial pressure of a component of a solution is directly proportional to its mole fraction in the solution; vapor pressure above a solution is lowered relative to that of the pure solvent. Pi = XiP° 2)

Freezing point depression → extent is expressed in terms of molal freezing point depression constant (Kf ); dissolved compounds depress the freezing point of a solution below that of the pure solvent. ΔTf = Kfm

Boiling point elevation → extent is expressed in terms of molal boiling point elevation constant (Kb ); dissolved compounds elevates the boiling point of a solution above that of the pure solvent. ΔTb = Kbm

Osmotic pressure → is the pressure that must be applied to stop the spontaneous flow of solvent in either direction of a semipermeable membrane. Π = 𝑀𝑅𝑇 16