Chapter 20
Solutions
The Dissolving Process n Solution - a homogeneous mixture
n Most consist of a solid dissolved in a liquid n Can’t be separated into components by
filtration n Solvent - Subst. which is more
abundant in soln. n Usually does the dissolving
The Dissolving Process n Solute - Less abundant subst.
n said to be dissolved n Most common solvent is water
n molecs are very polar & are attracted to other polar molecs. & ions
The Dissolving Process n NaCl - made of Na+ & Cl-
n in water, polar molecs. are attracted to crystal surface
n water molecs. surround & isolate surface ions
n ions become hydrated
The Dissolving Process n The attraction betw. hydrated ions &
remaining crystals decreases n no longer held by crystal n Gradually move away from crystal
n Dissociation - the separation of ions from ea. other in soln.
n Solvation - the surrounding of solute particles by solvent particles
The Dissolving Process n When ions dissociate, ea. ion type in
the soln. acts as if it were present alone n a soln. of NaCl acts as a soln. of Na+ & Cl-
ions n no characteristic behavior of NaCl bec. no
NaCl is actually present n Now a soln. containing both ions uniformly
mixed
Solvent - Solute Combinations n 4 Simple soln. situations are
considered n (Ionic substs. are included w/ polar
substs.)
Solvent - Solute Combinations 1. Polar solvent - Polar solute
n like NaCl in water Ø Polar solvent particles solvate polar
solute particles Ø attach themselves due to polar attraction Ø intracrystalline forces are reduced &
surface particles are carried away Ø In water, this is hydration
Solvent - Solute Combinations 2. Polar solvent - Nonpolar solute
Ø Solvent particles are attracted to ea other, but not the solute
Ø Solution is unlikely Ø Ex. trying to dissolve wax in water
Solvent - Solute Combinations 3. Nonpolar solvent - Polar solute
Ø Similar to #2 - solute particles are attracted to ea. other & not to solvent
Ø Solution is unlikely Ø Ex. dissolving NaCl in Hexane
Solvent - Solute Combinations 4. Nonpolar solvent - Nonpolar solute
Ø Only van der Waals forces exist among solvent particles and solute particles Ø All particles are subject to same forces Ø Solution can occur
Ø Random motion of solute molecs. cause them to leave the surface of solute crystal
Solvent - Solute Combinations 4. Nonpolar solvent - Nonpolar solute
Ø Solvation occurs, but forces are very weak Ø nonpolar particles are randomly dispersed Ø Ex. Wax will dissolve in benzene
Solvent - Solute Combinations n Not all polar substs. are soluble in ea.
other. n Consider: a solid dissolved in a liquid
n Solubility of nonpolar substs. in nonpolar liquid depends on 2 factors n 1. Melting point n 2. Enthalpy of fusion
Solvent - Solute Combinations n When a solid dissolves , a liquid soln.
results n change of phase
n Solids w/ low melt. pts. & low ΔΗfus are more soluble than those w/ high melt. pts. & ΔΗfus n Diff. due to stronger attractive forces w/in
crystals w/ high melt. pts.
Solids, Liquids, & Gases in Solutions n W/ 3 states of matter, there are 9 possible
combinations of solute-solvent pairs n Miscibility - the property of mutual
solubility of 2 liquids n 2 liquids are completely miscible if they are
mutually soluble in all proportions n Ex. antifreeze & water - both are polar
Solids, Liquids, & Gases in Solutions n Immiscible liquids do not dissolve in ea.
other n some separate into layers on standing n Ex water and oil
n 2 liquids that don’t dissolve in ea. other completely are partially miscible n diethyl ether and water
Solids, Liquids, & Gases in Solutions n Some metals are mutually soluble &
form solid-solid solns. n Alloys
Solution Equilibrium n When crystals are 1st put in solvent,
many particles leave surface & go into soln. n As # of solute particles incr., some
dissolved particles return to crystal n eventually, # of particles going into soln. =
# of particles returning to crystal n Solution Equilibrium
Solution Equilibrium n Saturated Soln. - a soln. in which an
undissolved subst. is in equilibrium w/ the dissolved subst.
n Unsaturated Soln. - soln. containing less than the saturated amt. of solute for that temp.
Solution Equilibrium n Larger amts. of solute can be dissolved
in a solvent @ higher temps. n If a hot soln. is cooled, an unstable soln.
can be formed n Supersaturated Soln. - soln. which
contains more solute than a saturated soln. can normally hold
Supersaturated Solutions n Possible bec. solids won’t crystallize w/
out a special surface to start crystallization
n Can’t crystallize on smooth glass surface
n If a crystal of solute is dropped into soln., it will crystallize almost immediately
Solution Equilibrium n @ any given temp., there is a limit to
the amt. of solute that will dissolve in a given amt. of solvent n Ex. @ 20oC, max of 64.2 g NiCl2 will
dissolve in 100 ml of water n - saturated solution (solubility of NiCl2 @ 20oC)
n Solubility can be changed when temp. is changed
Solution Equilibrium n Precipitation rxns. are important to
analytical chemists n differing solubilities can be used to
separate specific ions or comps. from mixtures
Solution Equilibrium n If a soln. contains 2 ionic comps., it may
be separated by choosing a reactant that will precipitate only 1 ion
n BaCl2(aq) + H2SO4(aq) BaSO4(cr) + 2HCl(aq)
n NaCl(aq) + AgNO3(aq) AgCl(cr) + NaNO3(aq)
Dissolving Effects n Rate of soln. is affected by surface area
of crystal exposed to fresh solvent n the more exposed area, the more solute
particles are subjected to solvation n Surface area can be incr. by crushing
crystals, ∴ increasing rate of soln.
Dissolving Effects n Rate of soln. can also be incr. by stirring
n saturated solvent moves away from solute & fresh solvent can come into contact w/ solid surface
Dissolving Effects n Soln. rate is also effected by kinetic
energy of solute & solvent particles n higher kinetic energy, faster solute will
dissolve
Dissolving Effects n Rxns. involving soln. of most solids in
water are endothermic n ΔHsoln is (+)
n Some solids and most gases have a (-) ΔHsoln. n exothermic
Dissolving Effects n The effects of temp. on solubility:
n Most solids w/ (+) ΔHsoln are more soluble in hot water than cold
n Gases w/ (-) ΔHsoln are more soluble in cold water
Dissolving Effects n Pressure has little effect on solns.
unless solute is a gas n more gas will dissolve @ high press. than
low press. n Henry’s Law - the mass of a gas which
will dissolve in a liquid @ a given temp. varies directly w/ the partial press. of the gas
Concentration n Molarity = moles of solute
dm3 of soln. n If 1 mole of NaCl is dissolved in enough
water to make 1L of soln., it’s a 1 M soln. n NaCl is in the form of dissociated ions in
soln. n soln. can be said to be 1M Na+ & 1M Cl-
Concentration n Molarity is the most common
concentration unit in chem. n If soln. has a known molarity, a meas. of
vol. will also give the # of particles n M x vol = moles of solute
n Standard soln. - soln. in which the concentration is known w/ much certainty
n used as a reference for comparison w/ other soln.s
Ion Concentration n If we know the conc. of a soln. in which
an ionic subst. is dissolved, we can find the conc. of ea. ion n What is the conc. of each ion in a 1M soln.
of Al(NO3)3?
Ion Concentration n What is the conc. of bromide if 193 g of
MgBr2 is dissolved in 500.0 ml of soln.?
Ion Concentration n Molality - conc. in terms of moles of
solute per kg of solvent n m = moles of solute kg of solvent
n a 1 m soln. contains 1 mole of solute in 1 kg of solvent
n If 52.0 g of K2CO3 are dissolved in 518 g of water, what is the molality of the solution?
Ion Concentration n Mole Fraction - shows comparison of
moles of solute to moles of soln. n ratio of # of moles of 1 soln. component
(either solute or solvent) to total # of moles of all components in soln.
n What is the mole fraction of alcohol in a solution made of 2.00 moles of ethanol and 8.00 moles of water?
Ion Concentration n Sometimes concen. is expressed in
Mass Percent = # grams of solute total mass of soln (in g)
n Change answer to a percent (X 100) n Find the mass percent of 142 g of H2O2 in
331 g of H2O.