chapter 13: properties of solutions sam white pd. 2
TRANSCRIPT
Chapter 13: Properties of Solutions
Sam White
Pd. 2
Introduction
A solution is any homogenous mixture, which means the components are uniformly intermingled on a molecular level
The Solvent is the most abundant component. It does the dissolving.
The Solute are any of the other components. They are the ones being dissolved
Formation of Solutions
With the exception of gas solutions, solutions form when the attractive forces between solute and solvent are comparable or greater than the intermolecular forces in either component
Formation of Solutions
Example: Salt Water- Attractive forces between Na+ or Cl- and the polar water molecules overcome the lattice energy of solid NaCl
Once separated, the Na+ and Cl- are surrounded by water. This interaction is known in all solutions as solvation
When the solvent is water, this interaction is known as hydration
Energy Change in Solution Formation
In order to form a solution, the solvent must form space to house the solute and the solute must be dissolved, both of which take energy
Enthalpy of Solution
Overall Enthalpy Change:solution=H1+H2+H3
Example with Salt Water:H1 accounts for the separation of NaCl to Na+
and Cl-
H2 accounts for the separation of solvent molecules to accommodate the solute
H3 accounts for the attractive interactions between solute and solvent
Overall Enthalpy Change
Saturated Solutions
As concentration of a solid solute increases, so does it’s chance of of colliding with the surface of the solid and becoming reattached to the solid
This is called crystallization
Solute + Solvent Solution
Saturated Solutions
When the rates of crystallization and dissolving become equal, no increase of solute in solution will occur
When a solution will not dissolve any more solute, it is a saturated solution
When a solution that can still dissolve solute into it is an unsaturated solution
Supersaturation
Under suitable conditions, it is sometimes possible to form a solution with more solute than that needed for a saturated solution
These solutions are supersaturated
Supersaturation
Supersaturation usually occurs because many solutes are more soluble at one temperature than another
Example: Sodium acetate, NaC2H3O2, will dissolve in water more readily at higher temperatures. When a saturated solution is made at higher temperatures then slowly cooled, all of the solute may remain dissolved even though the solubility decreases
Factors Affecting Solubility
The stronger the intermolecular attractive forces between solute and solvent, the greater the solubility
As a result of favorable dipole-dipole attractions, polar liquids tend to dissolve more readily in polar solvents
Water is not only polar, but has hydrogen bonds, making solutes that have hydrogen bonds able to dissolve in water as well
Factors Affecting Solubility
Pairs of liquids that mix in all proportions are miscible
Liquids that do not dissolve significantly in one another are immiscible
Hydrocarbons vs. Alcohols
Many hydrocarbons are immiscible in water because they are nonpolar moleculesAlcohols have an OH group, which are both polar and have hydrogen bonds, making them more readily soluble in waterAs the carbon chain become larger, the effect of the OH group becomes smaller, meaning that larger alcohol chains begin to become less soluble
Pressure Effects
Pressure only affects the solubility of gas in a solvent
As pressure increases, solubility of the gas increases
Henry’s Law
Cg = kPg
Cg is the solubility of the gas in solution (usually expressed in molarity)
Pg is the partial pressure of the gas over solutionk is the Henry’s Law Constant, which is unique for all solute-solvent pairs as well as the temperature
Temperature Effects
As temperature increases, the solubility of solid solutes (such as salts) normally increases
In contrast, as temperature increases, the solubility of gaseous solutes normally decreases
Solubility Charts
Gas Solubility Curve Solids Solubility Curve
Ways of Expressing Concentration
Mass percentage, ppm
Mole Fraction
Molarity
Molality
Mass Percentage and ppm
Mass % of component = (mc / mt) x 100mc = mass of component in solution
mt = total mass of solution
ppm of component = (mc / mt) x 106
mc and mt denote the same things for ppm as they denote for mass % of component
Mole Fraction
Mole Fraction of Component = (molc / molt)molc = moles of component
molt = total moles of all components
Molarity
Molarity = (mols / Ls)mols = moles solute
Ls = liters solution
Molality
Molality = (mols / kgs)mols = moles solute
kgs = kilograms solvent
Colligative Properties
Colligative properties depend on the quanity of solute, not the type of solute
The colligative properties are:Vapor-Pressure ReductionBoiling-Point ElevationFreezing-Point DepressionOsmotic Pressure
Vapor-Pressure Reduction
As the amount of solute increases, the vapor pressure of solution decreasesThis relationship can be expessed through Raoult’s Law:PA = XAPo
A
PA = Partial pressure exerted by solventXA = Mole fraction of solventPo
A = Vapor pressure of pure solvent
Boiling-Point Elevation
As amount of solute increase, boiling point increases
This relationship can be expressed as: Tb = dKbm Tb = total boiling point elevation d = dissociation factor of the solute Kb = molal boiling point elevation constant of the
solvent m = molality of solution
Freezing-Point Depression
As amount of solute increases, freezing point decreases
This relationship can be expressed as: Tf = dKfm Tf = total freezing point depression d = dissociation factor of the solute Kf = molal freezing point depression constant of
the solvent m = molality of solution
Osmotic Pressure
As amount of solute increases, osmotic pressure increasesThis relationship can be expressed as: = (n / V)RT = MRT = osmotic pressure n = number of moles solute V = volume of solution R = ideal gas constant temperature of solution molarity of solution