Solutions
- Solutions
- Homogeneous mixtures
ARE solutions. You can't see the different parts (salt water looks
just like water), they are usually transparent (clear) and they may or
may not have color. Remember heterogeneous mixtures are those where
you can see the different ingredients like granite.
- A solution is a solute
dissolved in a solvent. Refer to
Solute
|
Solvent
|
Solution (example)
|
gas
|
gas
|
air
|
gas
|
liquid
|
soda
pop, HCl(aq)
|
liquid
|
liquid
|
martini
|
solid
|
liquid
|
saline,
Koolaid
|
solid
|
solid
|
brass,
steel, white gold
|
- Energy of Solutions
- IMF (intermolecular
forces) are real important in solutions (except gas-gas solutions which
have very, very little IMF)
- There are three main
IMF involved for a solid to dissolve in water:
- IMF between solute
molecules
(if ionic solids - then ion-ion forces which are ionic bonds, if
covalent solids - then H bonding forces or dipole dipole
forces likely) These must be broken as the solute must be pulled
apart to mix with the solvent. So heat is needed to break them apart and
DH is positive = endothermic.
Breaking an ionic solid apart is the same of overcoming the lattice
energy LE.
- IMF between solvent
molecules
(if water then H bonding forces) These must be broken
also as we need room between solvent molecules to insert the solute.
Breaking these apart requires heat so DH is positive = endothermic.
- IMF between solute and
solvent
are formed once the solution mixes (for ionic solid dissolved in
water - ion-dipole forces are formed) Forming IMF gives off heat so DH is negative = exothermic
- The total heat of
solution (DHsoln) is the sum of the three above. DHsoln= DHsolute + DHsolvent + DHsolute-solvent. Because the third
step is exothermic while the others are endothermic the total DHsoln may add up to be a
positive or negative number.
- Energy changes when
dissolving salts in water.
- Some
solids dissolving in water are exothermic (heat released or given off so
DH is negative or
<0) so they are used in heat packs. Magnesium sulfate is one
example.
- Some
solids dissolving in water are endothermic (heat required or taken in so
DH is positive or >0)
so they are used in cold packs. Ammonium nitrate is one example.
- Here
you can dissolve many different salts in water and see if they are
exothermic or endothermic.
- Remember "like
dissolves like" which means polar compounds dissolve polar compounds
and nonpolar dissolves nonpolar, etc...
- For NaCl(aq) the ion-ion forces (ionic bonds) of NaCl(s) are broken and the H bond forces of water are
broken so that we make salt water NaCl(aq) and ion-dipole forces are formed. Remember NaCl(aq) really means Na+(aq)
and Cl-(aq). Here
is a great movie. So for salt water NaCl(aq) the ion-dipole forces formed must be greater than
the ion ion and H bond forces broken.
- Solvated ions
(definition) = ions that are surrounded by solvent. If water is the
solvent we call them hydrated ions. Here is a picture of hydrated
ions.
- Skip the part about DG and DS of solution
- Concentration Units
- Molarity M =
moles solute / liters solution (mol/L)
- Mole Fraction c = moles solute / moles
total (no units as mol cancel)
- mass% = grams solute / total
grams x 100 (unit is %)
- ppm = grams solute / total
grams x 106 (unit is ppm)
- ppb = grams solute / total
grams x 109 (unit is ppb)
- molality m = moles solute / kg
solvent (mol/kg) (this is the only one where
the denominator is not a total solution number, it's just the solvent)
- Practice these!!!
- Solubility Factors
- Definitions
- unsaturated
= solution where more solute can dissolve
- saturated
= solution where no more solute can dissolve, if more solid solute is
added, it will simply sink to the bottom and remain solid
- supersaturated = solution where more solute at the temperature is
dissolved that should be. What??? Well if you take really
hot water and dissolve the maximum amount of salt possible, then COOL
the solution down then more is dissolved than could be if you started
with cold water. The solute can solidify easily if you add just
one tiny speck of solid to it. Then it will all crystallize quickly.
- solubility
= the maximum grams of a solute than can dissolve in one liter of water
- Affecting solubility
- As
temperature increases the solubility of solids usually increases
also. Why? As KE increases the solid can more easily break
apart and mix with the solvent.
- As
temperature increases the solubility of gases decreases.
Why? As KE increases the gas can more easily reach the surface and
break away from the solution and float away. This is why you put pop in
the refrigerator - the cooler temperature keeps the carbonation.
Another example is rivers warming - the oxygen gas is less soluble so
there is less oxygen for the fish and they may die.
- As
pressure increases the solubility of solids is unaffected.
Why? Solids are not compressible.
- As
pressure increases the solubility of gases increases also. Click
on activities then Henry's Law activity. See the pressure
increasing holds the gas molecules in the solvent. This is why you put a
lid on soda pop - it holds the carbonation in.
- Colligative Properties
- They depend on the
molality of the ions
- boiling
point elevation
- freezing
point depressions
- vapor
pressure lowering
- osmosis
- Solutions are very
different from their pure solvent. Salt water is very different
from plain water.
- Vapor Pressure lowering
- Consider solid in
liquid solutions (don't worry about others or the van Hoff crap)
- In the first picture
above the solvent is pure water and because it is a closed container a
vapor pressure has set up. Then in the second picture we have a
solution. Notice the vapor pressure is LOWER. It is because
the solid can't evaporate and the solution is no longer pure
solvent. So the vapor pressure depends on how much of the solution
is solvent and not solute. The more solute added, the lower the
VP. Psoln = XsolventPsolvent
- As solute goes up, the
solvent fraction X goes down, sot the VP goes down. Click
on activites then vapor pressure activity to
see this in action.
- Boiling point elevation and Freezing point
(melting point, same thing) depression
- A solution of a solid
solute in water is harder to boil since we have additional ion-dipole IMF
and becasue the VP is lower as we just
learned. So it takes MORE energy to get the liquid to boil and turn
into gas.
- A solution of a solid solute
in water is also harder to freeze since we have solute particles
interfering in the crystal lattice of the ice. So is must be COLDER
to get the water to freeze into ice.
- How large is this bp and mp change in
temperature? It depends on the molality of ions in
solution. Equations: DTb = Kbm
and DTf = Kf
m where
the Kb or f is a constant for boiling or freezing
- Calculate the bp and mp of a 33.33 grams
calcium chloride in 477 mg of water. First we need the moles of
ions. 33.33 g (1 mol / 110.98 g) (3
ions / mol) = 0.90097 mol
ions. (CaCl2 ionizes into 3 ions: one Ca2+
and two Cl- ions) Now we need molality. m = 0.90097 mol ions / 0.477 kg water = 1.8888 mol/kg. Now plug into the
equations.
- DTb = (0.51oC
kg / mol)(1.8888 mol
/ kg) = 0.96 so the new bp = 100.96oC
since water's original bp is 100
- DTf = (1.86oC
kg / mol)(1.8888 mol
/ kg) = 3.5 so the new fp = -3.5oC
since water's original fp is 0
- Osmosis
- Consider two solutions
are separated by a semipermeable (allows solvent not solute to pass)
membrane with different concentrations. The goal when we have two
solutions with different concentrations is to equalize the
concentrations. We do this by diluting the higher concentration
solution by having solvent molecules pass through the membrane.
Thus solvent flows from lower to higher solute concentrations. The volume
on the higher conc. side will increase as it is diluted, and the volume
on the lower conc. side will decrease as it is concentrated.
Eventually the conc will be equal unless pressure build up stops the flow.
- Example: We have
a solution on the left that is 1 M and a solution on the right that is 5
M. They are connected by a semipermeable membrane. Solvent
will flow from the 1 M solution to the 5 M solution thus increasing the
volume of the 5 M solution (diluting it) and the volume of the 1 M
solution will decrease (thus concentrating it). So the
concentrations get closer to each other - 1 M increases and 5 M
decreases.
Practice Problems on
Concentrations
- How many moles are in 234 mL of a 1.29 M solution?
- What is the molarity if 9.25 moles of sodium sulfide is
dissolved to make a 787 mL solution?
- How many grams of magnesium fluoride do you need to
prepare 838 mL of a 2.42 M solution?
- The salt is blood serum is 0.14 M. How many grams
of sodium chloride are in 5.00 mL of blood serum?
- I used 54.7 grams of sodium phosphate to make a 0.250 M
solution. What is the volume in mL?
- What is the ppm if I have 45.0 mg of lead in 2500 mL of
water?
- What is the ppm of dioxin if there is 0.0223 mg in 1550
mL of water?
- I had 25.0 mL of a 5.00 M solution. How much
water did I add if I diluted the solution to 2.30 M?
- Concentrated HCl is 6.00
M. In lab we used 1.00 M HCl. If we
need 500.0 mL in lab, how much of the concentrated do I need to dilute?
Answers
- 1.29 mol / L = x moles /
0.234 L, x = (1.29 mol/L)(0.234 L) = 0.302 moles
- M = (9.25 moles / 0.787 L) = 11.8 M
- (0.838 L)(2.42 mol/L) = 2.03
moles MgF2 needed, 2.03 mol MgF2
(62.3 g/mol) = 126 grams
- (0.14 mol/L)(0.00500 L) =
7.00 x 10-4 moles, 7.00 x 10-4 moles NaCl (58.5 g/mol) = 0.0410 g
- 54.7 g Na3PO4 (mol / 164 g) = 0.334 moles, 0.334 mol ( 1 L / 0.250 mol) = 1.34
L = 1340 mL
- Remember for water 1 g = 1 mL.
So we have 2500 grams of water. ppm = (0.0450 g / 2500 g)(106)
= 18.0 ppm
- ppm = (2.23 x 10-5 g / 1550 g)(106)
= 0.0144 ppm
- (5.00 M)(25.0 mL) = (2.30 M) V2, V2
= 54.3 mL so I added 29.3 mL
- (6.00 M) V1 = (1.00 M)(500.0
mL), V1 = 83.3 mL