Chem Notes Ch 5 Flashcards
Solute
Substance present in a smaller proportion
Solvent
Substance present in a larger proportion
Aqueous solution
Solvent is water
Solubility
Amount of solute that will saturate a particular solvent; rate of dissolution is equal to the rate of precipitation; The solid form and the dissolved form are in dynamic equilibrium.
For solids in liquids, solubility increases with temperature. Gases are more volatile at higher temperatures, which causes them to be less soluble in liquids. A gas is more soluble in a liquid when the pressure is increased.
Phase solubility rules - solids in liquids
Increased temperature = increased solubility
Phase solubility rules - gases in liquids
Increased temperature = decreased solubility
Increased pressure = increased solubility
Crystallization
Process of crystal formation when a dissolved solute comes out of a solution
Supersaturated
Solutions contain more solute than found in a saturated solution. They are formed when manipulating temperature or pressure.
Salt solubility rules
- All salts of Group 1 metal cations (Li(+), Na(+), K(+), Rb(+), Cs(+)) and ammonium (NH4(+)) are
soluble. - All salts of nitrate (NO3(-)), perchlorate (CIO4(-)) and acetate (C2H3O2(-)) are soluble.
- All salts of silver (Ag(+)), lead (Pb(2+)), and mercury (Hg(2+)) are insoluble.
- All salts of hydroxide (OH(-)), carbonate (CO3(2-)), phosphate (PO4(3-)), and sulfide (S(2-)) are
insoluble. - When a salt consists of a “soluble ion” and an “insoluble ion”, it is SOLUBLE in water
(ex. NaOH = soluble).
Molarity (M)
Concentration of a solution expressed in moles of solute per volume (in liters) of solution. Dependent of both temperature and pressure.
Molality (m)
Concentration in terms of moles of solute per mass (in kilograms) of solvent. Independent of both temperature and pressure.
Mole fraction
Moles of one component of a solution divided by the total number of moles in solution. It is unit-less because it is a ratio
Normality (N)
The number of mole equivalents (n) per litre of solution. Used for acids and bases.
Formula: (Number of moles)*(Molarity)
The mole equivalents of an acid or base are calculated by determining the number of H+ or OH- ions per molecule. For instance, the mole equivalents of H2SO4 is two because dissolution of sulfuric acid releases two H+ ions into
solution.
Colligative Properties
Dependent on the number of solute particles in the solution.
- Vapor-pressure depression
- Boiling-point elevation
- Freezing-point depression
- Osmotic pressure
Vapor-Pressure Depression
A colligative property. The pressure exerted by the gaseous phase of a liquid that evaporated from the exposed surface of the liquid.
A higher vapor pressure corresponds to a lower boiling point and the more easily a substance will evaporate.
If a solution contains a dissolved solute, the solute molecules are in a state of high entropy, decreasing the vapour pressure, making it more difficult for the solution to boil and requires more energy.
Boiling point
Temperature at which the vapor pressure of the solution is equal to the atmospheric pressure.
At sea level, atmospheric pressure is 760 mmHg
Raoult’s Law
The vapor pressure (PA) of a solution with a non-volatile solute is equal to the pure vapor pressure of the solvent (P°A) multiplied by the mole fraction of the solvent (XA).
The mole fraction of the solvent is the moles of solvent divided by the total number of moles in solution.
Since the presence of solute causes XA to be smaller than 1, PA is lower than P°A
Boiling-Point Elevation
A colligative property. The increase in boiling point of solution.
Δ(Tb) = (kb)(i)(m)
kb is the solvent’s boiling-point elevation constant, i is the solute’s van ’t Hoff factor or the number of particles the solute breaks into when it dissolves, and m is the molal concentration of the solution.
Freezing-Point Depression
A colligative property. Liquid will be less able to achieve a solid state when a solute is present and the freezing point will decrease.
Δ(Tf) = -(kf)*(i)(m)
kf is the solvent’s freezing-point depression constant, i is the solute’s van ’t Hoff factor, and m is the molal concentration of the solution
Osmotic Pressure
A colligative property. The pressure it would take to stop osmosis (net movement of water across a semipermeable membrane from an area of low solute concentration to a region of higher solute concentration) from happening.
van’t Hoff equation can be used to compute osmotic pressure: Π= (M)(i)(R)*(T)
Π is osmotic pressure in atm, M is molarity of the solution, i is the van’t Hoff factor, R is
the universal gas constant of 0.082, and T is the temperature in Kelvin.
Non-Colligative Properties
Depend on the identity of the dissolved molecules and the solvent. Ex. colour, viscosity, surface tension, and solubility.
Dissolution
When the solute breaks up from a larger crystal of molecules into much smaller groups or individual molecules. Caused by interactions between the solute and solvent. Intermolecular forces that contribute to dissolution include dipole-dipole, Van der Waals, ion-dipole, and hydrogen bonding.
From weakest to strongest, the list of intermolecular forces
London dispersion, to dipole-dipole, to hydrogen bond, to ion-dipole.
Polarity
Solutes dissolve best in solvents with similar intermolecular forces. Solutes whose polarity matches that of the solvent will most likely be soluble.
The intermolecular forces being broken in the solute are being replaced by equal (or stronger) intermolecular forces between the solvent and the solute.
Electrolytes
Used to make conductive solutions. Pure water is an example of a solution that does not conduct an electrical current.
A strong electrolyte dissociates completely into its constituent ions. (Ex. NaCl and KI, which are both ionic compounds. Molecular compounds with highly polar covalent bonds that dissociate into ions when dissolved, such as HCl in water.)
A weak electrolyte would ionize or hydrolyze incompletely in aqueous solution, with only a portion of the solute present in ionic form. (Ex. Acetic acid, ammonia, and HgCl2. Nonpolar gases and organic compounds, such as sugar, do not ionize at all in aqueous solution.)
Dielectric constant
Determines polarity of solvent, higher value is more polar
