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Chapter 15 Solutions

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					  Chapter 15
   Solutions
        15.1
Lots of definitions!
     Solutions:
1. Are Homogeneous (same composition throughout)
2. Are a physical combination of substances (mixture)
3. Are made up of ions or molecules as particles
4. Can have many proportions (not fixed)
5. Have two parts:
                  solvent and solute
     solvent: dissolves the solute
     solute: the substance that dissolves
See Table 15.1 (text) for the 8 types of solutions
The three most common are:
 Solid solute in liquid solvent (ie. Sea water)
 Liquid solute in liquid solvent (ie. alcohol in water)
 Gas solute in liquid solvent (ie. carbonated drinks)
Smoke and fog are NOT solutions…
…but “ suspensions ”!
  particle size >>> ions or molecules!
  (this causes cloudy mixtures - scatters light)
        The “Tyndall Effect”
   (Solutions do not scatter light)

                              But… the light
                              beam is NOT
Light beam is                 visible here!
visible here!




This contains a        This contains a
suspension!            solution!
5/30
       “Soluble” = will dissolve
       “Insoluble” = won’t dissolve
       “Miscible” liquids are liquid solutes and
       solvents that are soluble in ANY ratio!
       Example: alcohol and water
       “Immiscible liquids” are liquid solutes
       and solvents that are insoluble (don’t
       dissolve) in all ratios!
       Example: gasoline and water
   Dissolving Ionic Solutes in Water
Remember these facts to understand dissolving ions :

 1. Ionic compounds have (+) cations and (-) anions

 2. The ions are arranged in a crystal lattice.

 3. The crystal is in a low energy state

 4. Water molecules are in constant motion

 5. Water molecules are polar!
The ions attract polar water molecules (“ion/dipole
attraction”).
This is “hydration” (in other solvents, “solvation”)


     (-)


     (+)

           +
 -
           +
    The Energy given off (when the water bonds to
    the ions) breaks the lattice, releasing ions from
    the crystal…




    (-)


    (+)

          +
-
          +
 The polar water molecules form “ion/dipole” bonds
 with the ions, providing the energy required to
 separate them from the crystal.
 The ions, when separated from the crystal, are
 then “hydrated” - or surrounded by water
 molecules!


               (+)                   (-)


This 3-D “sheathe” of water molecules prevents the
ions from re-combining!!
10/30




        The separation of particles from a crystal is
                  called… “dissociation”
          Ionic substances dissolve by dissociation!
        (If they can’t dissociate, they don’t dissolve!)
    Water Also Dissolves Some
     Molecular Compounds!
• To dissolve molecules, water must be
  attracted to them.
• These molecules usually have O-H bonds
  in them, allowing hydrogen bonds to
  form between the water and the
  molecules!
                            A glucose
                            molecule…
                            a sugar…
                            no ions, but a
                            molecule!


OH---OH hydrogen bonding allows
water to dissolve sugar!
The sugar molecule doesn’t break apart-
it just separates (dissociates) from the
other sugar molecules!
Just What CAN Water Dissolve?
• Anything that is “polar”, like water!
• The rule to remember is…
• “LIKE DISSOLVES LIKE”
• If it is polar like water, it WILL dissolve
  in water, but WON’T dissolve in a non-
  polar solvent like oil or gasoline.
• If it is NON-polar, it WILL dissolve in
  non-polar solvents, like oil or gasoline, but
  WON’T dissolve in water!
For example, water will NOT dissolve
styrofoam…(this is why we can make
drinking cups out of styrofoam)
But gasoline dissolves styrofoam very
well!
If water is polar, then styrofoam
must be…
Non-polar!
And so is gasoline!
15/30



                O.K. - A Riddle…
    Question:Two bears - one from Alaska, and
    one from Florida - decide to go fishing
    together. When they entered the water,
    which one dissolved?


        Answer: The one from Alaska! He was…
                      …a polar bear!
  Factors That Influence the Rate
           of Solvation
Three things affect how fast a solute dissolves:
• Stirring – this brings fresh, unsaturated
  solvent to the solute.
• Crushing/powdering the solute – this
  increases the surface area of the solute.
  (solutes dissolve on their surface).
• Heating the solvent – increases the motion of
  the solvent molecules. (The only one of the
  three that also influences the amount that
  dissolves!)
                Concentration

If your iced tea is too sweet, it has too much
sugar dissolved in the water.
We call this condition “concentrated”.
If your iced tea is not sweet enough, there is too
little sugar dissolved.
We call this condition “dilute”.
However, the terms “concentrated” and
 “dilute” don’t tell chemists anything about
 amounts! (We like numbers!)
“Concentration” = how much solute is dissolved
  in 100 grams of solvent (g/100 g solvent).
This is a ratio that can be quantified.


Now, use the definition of “concentration” to
 solve these two problems…
Example 1: What is the concentration of salt in a
solution made with 6.00g of salt and 200.g water?
Answer:
          6.00g salt x 100 = 3.00g
          200.g H2O 100       100.g

Example 2: What is the concentration of salt in a
solution made with 0.300g of salt and 20.0g water?
Answer:    0.300g salt x 100 = 1.50g
           20.0g H2O     100   100.g
20/30              Solubility
“Solubility” = the highest concentration possible!
The MAXIMUM amount of solute that will dissolve!
Written as a RATIO! (same units as concentration)
               grams of solute .
             100 grams of solvent

Solubility is affected by Temperature! (Table 15-2)
USUALLY, more solute will dissolve at higher temps.
                 In other words…
…solubility USUALLY increases with higher temps!
        Temperature and Solubility

• MOST solid solutes dissolve better in HOT water!

• For these, the dissolving process is “endothermic” –
  it requires heat.

• However, some solids AND ALL GASES dissolve
  better in COLD water!

• Table 15.2 and Figure 15.7 (text) show the
  solubility (maximum amount dissolved) of several
  substances at different temperatures.
         Saturated Solutions
“Saturated” means the concentration has reached
its solubility limit at the temperature given!
           No more solute will dissolve!
In a saturated solution, either
1. the solid can be seen on the bottom, or…
2. added crystals sink to the bottom, undissolved.
TO TEST: Add more solute crystals…
In an unsaturated solution, the crystals dissolve.
In an saturated solution, the crystals settle out.
• There is still another “weird” condition
  called “supersaturated”…
• If the solute is dissolved, and you add
  just one more tiny crystal to the water,
  suddenly, a whole bunch of solute
  crystals form!
  A “Saturated solution”
An added crystal sits on the bottom…the
solvent is “full” and can’t hold any more…
25/30

        An “unsaturated solution”
  An added crystal dissolves…there is still
  “room” for more solute in the solvent…
  A “Supersaturated solution”
An added crystal “grows”…too much solute
had been dissolved!
           Solubility Problems
Example 3a: Is a solution made with 1.50g of KNO3
and 9.70g of water saturated at 20C? (the
solubility of KNO3 at 20C is 31.6g/100g H2O).
 Answer:
31.6g KNO3 x 9.70g H2O = 3.07g KNO3 will dissolve
100.g H2O                in 9.70g H2O at 20C.
             The solution is unsaturated!

 3b: How much more KNO3 can you dissolve?
            3.07g KNO3 will dissolve
           -1.50g KNO3 is dissolved
            1.57g more KNO3 can dissolve
   Factors Influencing Solubility

1. Like dissolves like… Polar/Polar, or
                        Nonpolar/Nonpolar.
2. Temperature (see Fig 15.7) (Note: most
   solids dissolve better in hot water, all gases
   dissolve poorly in hot water!
3. Pressure: No effect for solids/liquids. Gas
   solubility increases with higher pressure!
         Pressure and Solubility
•Pressure only affects the solubility of gases!
•At higher pressures, more gas dissolves
(solubility increases) in a direct proportion!


•This relationship is quantified by “Henry’s Law”:
                           …Where
   S1 S2
                          S1 = beginning solubility,
                           P1 = beginning pressure,
   P1 P2                   S2 = final solubility, and
                           P2 = final pressure
Example Problem:
The solubility of a gas at 1.0 atm pressure is
0.21g/L. What is pressure is needed to
increase the solubility of this gas to 1.05g/L?

 S1 S2
             Solve for P2…
 P1 P2

           S2P1 (1.05g/L)( 1.0 atm)
  so, P2                           5.0 atm
            S1       0.21g/L

				
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