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.