1. Chapter 20
Solutions

2. The Dissolving Process
Solution - a homogeneous mixture
Most consist of a solid dissolved in a liquid
Can’t be separated into components by filtration
Solvent - Subst. which is more abundant in soln.
Usually does the dissolving

3. The Dissolving Process
Solute - Less abundant subst.
said to be dissolved
Most common solvent is water
molecs are very polar & are attracted to other polar molecs. & ions

4. The Dissolving Process
NaCl - made of Na+ & Cl-
in water, polar molecs. are attracted to crystal surface
water molecs. surround & isolate surface ions
ions become hydrated

5. The Dissolving Process
The attraction betw. hydrated ions & remaining crystals decreases
no longer held by crystal
Gradually move away from crystal
Dissociation - the separation of ions from ea. other in soln.
Solvation - the surrounding of solute particles by solvent particles

6. The Dissolving Process
When ions dissociate, ea. ion type in the soln. acts as if it were present alone
a soln. of NaCl acts as a soln. of Na+ & Cl- ions
no characteristic behavior of NaCl bec. no NaCl is actually present
Now a soln. containing both ions uniformly mixed

7. Solvent - Solute Combinations
4 Simple soln. situations are considered
(Ionic substs. are included w/ polar substs.)

8. Solvent - Solute Combinations
Polar solvent - Polar solute
like NaCl in water
Polar solvent particles solvate polar solute particles
attach themselves due to polar attraction
intracrystalline forces are reduced & surface particles are carried away
In water, this is hydration

9. Solvent - Solute Combinations
Polar solvent - Nonpolar solute
Solvent particles are attracted to ea other, but not the solute
Solution is unlikely
Ex. trying to dissolve wax in water

10. Solvent - Solute Combinations
Nonpolar solvent - Polar solute
Similar to #2 - solute particles are attracted to ea. other & not to solvent
Solution is unlikely
Ex. dissolving NaCl in Hexane

11. Solvent - Solute Combinations
Nonpolar solvent - Nonpolar solute
Only van der Waals forces exist among solvent particles and solute particles
All particles are subject to same forces
Solution can occur
Random motion of solute molecs. cause them to leave the surface of solute crystal

12. Solvent - Solute Combinations
Nonpolar solvent - Nonpolar solute
Solvation occurs, but forces are very weak
nonpolar particles are randomly dispersed
Ex. Wax will dissolve in benzene

13. Solvent - Solute Combinations
Not all polar substs. are soluble in ea. other.
Consider: a solid dissolved in a liquid
Solubility of nonpolar substs. in nonpolar liquid depends on 2 factors
1. Melting point
2. Enthalpy of fusion

14. Solvent - Solute Combinations
When a solid dissolves , a liquid soln. results
change of phase
Solids w/ low melt. pts. & low DHfus are more soluble than those w/ high melt. pts. & DHfus
Diff. due to stronger attractive forces w/in crystals w/ high melt. pts.

15. Solids, Liquids, & Gases in Solutions
W/ 3 states of matter, there are 9 possible combinations of solute-solvent pairs
Miscibility - the property of mutual solubility of 2 liquids
2 liquids are completely miscible if they are mutually soluble in all proportions
Ex. antifreeze & water - both are polar

16. Solids, Liquids, & Gases in Solutions
Immiscible liquids do not dissolve in ea. other
some separate into layers on standing
Ex water and oil
2 liquids that don’t dissolve in ea. other completely are partially miscible
diethyl ether and water

17. Solids, Liquids, & Gases in Solutions
Some metals are mutually soluble & form solid-solid solns.
Alloys

18. Solution Equilibrium
When crystals are 1st put in solvent, many particles leave surface & go into soln.
As # of solute particles incr., some dissolved particles return to crystal
eventually, # of particles going into soln. = # of particles returning to crystal
Solution Equilibrium

19. Solution Equilibrium
Saturated Soln. - a soln. in which an undissolved subst. is in equilibrium w/ the dissolved subst.
Unsaturated Soln. - soln. containing less than the saturated amt. of solute for that temp.

20. Solution Equilibrium
Larger amts. of solute can be dissolved in a higher temps.
If a hot soln. is cooled, an unstable soln. can be formed
Supersaturated Soln. - soln. which contains more solute than a saturated soln. can normally hold

21. Supersaturated Solutions
Possible bec. solids won’t crystallize w/out a special surface to start crystallization
Can’t crystallize on smooth glass surface
If a crystal of solute is dropped into soln., it will crystallize almost immediately

22. Solution Equilibrium
@ any given temp., there is a limit to the amt. of solute that will dissolve in a given amt. of solvent
20oC, max of 64.2 g NiCl2 will dissolve in 100 ml of water
- saturated solution (solubility of 20oC)
Solubility can be changed when temp. is changed

23. Solution Equilibrium
Precipitation rxns. are important to analytical chemists
differing solubilities can be used to separate specific ions or comps. from mixtures

24. Solution Equilibrium
If a soln. contains 2 ionic comps., it may be separated by choosing a reactant that will precipitate only 1 ion
BaCl2(aq) + H2SO4(aq) BaSO4(cr) + 2HCl(aq)
NaCl(aq) + AgNO3(aq) AgCl(cr) + NaNO3(aq)

25. Dissolving Effects
Rate of soln. is affected by surface area of crystal exposed to fresh solvent
the more exposed area, the more solute particles are subjected to solvation
Surface area can be incr. by crushing crystals, \ increasing rate of soln.

26. Dissolving Effects Rate of soln. can also be incr. by stirring
saturated solvent moves away from solute & fresh solvent can come into contact w/ solid surface

27. Dissolving Effects
Soln. rate is also effected by kinetic energy of solute & solvent particles
higher kinetic energy, faster solute will dissolve

28. Dissolving Effects
Rxns. involving soln. of most solids in water are endothermic
DHsoln is (+)
Some solids and most gases have a (-) DHsoln.
exothermic

29. Dissolving Effects The effects of temp. on solubility:
Most solids w/ (+) DHsoln are more soluble in hot water than cold
Gases w/ (-) DHsoln are more soluble in cold water

30. Dissolving Effects
Pressure has little effect on solns. unless solute is a gas
more gas will high press. than low press.
Henry’s Law - the mass of a gas which will dissolve in a a given temp. varies directly w/ the partial press. of the gas

31. Concentration Molarity = moles of solute dm3 of soln.
If 1 mole of NaCl is dissolved in enough water to make 1L of soln., it’s a 1 M soln.
NaCl is in the form of dissociated ions in soln.
soln. can be said to be 1M Na+ & 1M Cl-

32. Concentration Molarity is the most common concentration unit in chem.
If soln. has a known molarity, a meas. of vol. will also give the # of particles
M x vol = moles of solute
Standard soln. - soln. in which the concentration is known w/ much certainty
used as a reference for comparison w/ other soln.s

33. Ion Concentration
If we know the conc. of a soln. in which an ionic subst. is dissolved, we can find the conc. of ea. ion
What is the conc. of each ion in a 1M soln. of Al(NO3)3?

34. Ion Concentration
What is the conc. of bromide if 193 g of MgBr2 is dissolved in ml of soln.?

35. Ion Concentration
Molality - conc. in terms of moles of solute per kg of solvent
m = moles of solute
kg of solvent
a 1 m soln. contains 1 mole of solute in 1 kg of solvent
If 52.0 g of K2CO3 are dissolved in 518 g of water, what is the molality of the solution?

36. Ion Concentration
Mole Fraction - shows comparison of moles of solute to moles of soln.
ratio of # of moles of 1 soln. component (either solute or solvent) to total # of moles of all components in soln.
What is the mole fraction of alcohol in a solution made of 2.00 moles of ethanol and 8.00 moles of water?

37. Ion Concentration
Sometimes concen. is expressed in Mass Percent = # grams of solute
total mass of soln (in g)
Change answer to a percent (X 100)
Find the mass percent of 142 g of H2O2 in 331 g of H2O.