1. Solutions Part III: IMF, Energetics of Solutions, Henry’s Law Much of this is basic concepts that you should review on your own. Study Slides 1 thru 14 on your own. Slides 15 thru 26 are summarized in a handout. You should review these slides on your own. Lecture begins on Slide (Jespersen Chap. 13 Sec 1 – 4)
Dr. C. Yau
Fall 2014

2. What exactly does "solution" mean?
solute + solvent = solution
They can be in any physical state.
Most common is solid in liquid, such as NaCl in water to give salt water.
The only requirement is they form a HOMOGENEOUS MIXTURE, which means at the "particulate" level, particles are uniformly mixed.

3. You can have a “solution” of gas in gas:
e.g. Air is a homogeneous mixture of mainly N2 and O2.
You can have a “solution of gas in liquid:
e.g. O2 dissolved in water. This is how fish “breathes.”
You can have a solution of liquid in liquid:
e.g. Rubbing alcohol is a homogeneous mixture of isopropyl alcohol (liquid) in water.

4. Like Dissolves Like
KNOW THESE RULES WELL!
Nonpolar solutes prefer to dissolve in nonpolar solvents.
Polar solutes prefer to dissolve in polar solvents.
Ionic compounds are more like polar compounds than nonpolar.
They prefer to dissolve in polar solvents.

5. Solution at the Particulate Level
solid + solid solid solution
Cu + Zn brass (an alloy)
uniformly mixed at the particulate level
attracted by London forces

6. Solution at the Particulate Level
gas + gas gas solution
N O air
For ideal gases, there is no IMF.

7. Solution at the Particulate Level
solid + liquid solution
NaCl + water salt water
Na+ClNa+ClNa+Cl
ClNa+ClNa+ClNa+
ionic bonding
ion-dipole force
hydrogen bonding

9. Solution at the Particulate Level
liquid + liquid solution
alcohol + water alcohol soln
H-bonding
H-bonding
H-bonding

10. Dissolution Of A Polar Compound In Water
Dipole of the water interacts with the oppositely charged dipoles of the solid, extracting them from the crystal
Figure 12.4 Hydration of a polar molecule. A polar molecule of a molecular compound (such as the sugar glucose) can trade the forces of attraction it experiences for other molecules of its own kind for forces of attraction to molecules of water in an aqueous solution. Red areas indicate high electron density; blue areas have low electron density.

11. Why is oil and water immiscible?
oil water two separate layers
as a heterogeneous mixture
London forces
very strong H-bonding,
stronger than oil-water attraction

12. Miscibility of Liquids
Liquids that can dissolve in one another are miscible, while insoluble liquids are immiscible
Ethanol and water are miscible, while benzene and water are not. WHY NOT?

13. Which of the following are miscible in water?
carbon disulfide
ammonia
acetic acid
Acetic acid & ammonia can H-bond with water. CS2 is non polar and is too “unlike” water.

14. Which of the following are likely to be miscible with water?
CH3CH2CH2CH3
C6H6
CH3CO2H
All are expected to be miscible
A and B are hydrocarbons (nonpolar). C. is acetic acid and can H-bond with water.

15. Solvation is the process of solute particles becoming surrounded by solvent molecules.
Hydration is the process of solute particles becoming surrounded by water molecules
Hydration is solvation when the solvent is water, and the resultant solution...
is called an "aqueous solution."

16. Enthalpy (Heat) Of Solution
Heat of solution (Ηsoln ) is the energy exchanged when a solute dissolves in a solvent at constant pressure.
Enthalpy is a state function: the energy change depends only on the initial and final states of the system and not on the pathway one takes to go from the initial to final state.
When Ηsoln=0, solution is called an ideal solution. (It is neither endothermic nor exothermic)
Chem FAQs: What is a heat of solution?
What is an ideal solution?

17. What is molar ΔHsoln ? Molar ΔHsoln is the molar enthalpy of solution.
the heat of solution of one mole of solute.
(Note: 1 mol solute not 1 mol solution. We cannot talk about a “mole of solution” because a solution is not a compound. It is a homogeneous mixture, with a variable ratio of solute to solvent.)
It may be positive OR negative.
Heat may be absorbed OR evolved (released).
Can we predict which will happen?

18. Enthalpy of Solution There are 3 steps in the process of solution:
1) Solute separates from each other (endothermic)
2) Solvent separates from each other to make room for solute (endothermic)
3) Solute fits into the space between solvent molecules (exothermic)
If the net change in heat is negative, the solution process will give off heat.
If the net change is positive, the soln will absorb heat (the solution feels cooler).

19. Hess’ Law REMEMBER! Just like balancing a bank account....
Need to ADD heat to separate particles of solute and solvent.
Get heat back from solvation. Balance sheet?

20. Dissolution: Liquid in Liquid (Ideal)
Figure 12.7 Enthalpy changes in the formation of an ideal solution. The three-step and the direct-formation paths both start and end at the same place with the same enthalpy outcome. The sum of the positive ∆H values for the two endothermic steps, 1 and 2, numerically equals the negative ∆H value for the exothermic step, 3. The net ∆H for the formation of an ideal solution is therefore zero.
Shown here is an ideal solution where H = 0.
H  0 if H1 + H2  H3 . (Examples on next slide.)

21. Liquid into Another Liquid
e.g. Acetone in water is exothermic
but ethanol in hexane is endothermic.
WHY?
Hydration energy is large because the acetone can form H-bonding with water, making up for breaking H-bonding in water.
CH3CH2CH2CH2CH2CH2CH2CH3
Solvation energy is small because there is no H-bonding between ethanol and hexane; whereas, there is very strong H-bonding between the ethanol molecules.

22. Heat of Solution of Ionic Compound in Water
Step 1) Ionic compound breaks up into gaseous cations and gaseous anions
This is called "lattice energy."
It is a positive value.
Step 2) Gaseous ions become surrounded by water molecules.
This is called "hydration energy."
It is a negative value.
Overall energy change: sum of the two.

23. KI in Water KI (s) K+ (g) + I- (g) ΔH = + 632 KJ
K+ (g) I- (g) K+ (aq) + I- (aq)
ΔH = KJ
Overall:
KI (s) K+ (aq) + I- (aq)
ΔH = KJ
ΔH = + 13 KJ
What does this mean?
Heat is absorbed. Solution will feel cooler during the process because it is absorbing heat from the surroundings.

24. Water already has “pockets” for ions to fit in and therefore does not need to “expand.”

25. Dissolution: Gas In Liquid: 2 scenarios
Step1: Expansion of solvent
Step2: Mixing
Ηsoln = Η1 + Η2
Why missing one step?
Solute does not have to separate.
Figure 12.8 A molecular model of gas solubility. (a) A gas dissolves in an organic solvent. Energy is absorbed to open “pockets” in the solvent that can hold the gas molecules. In the second step, energy is released when the gas molecules enter the pockets where they are attracted to the solvent molecules. Here the solution process is shown to be endothermic. (b) At room temperature, water’s loose network of hydrogen bonds already contain pockets that can accommodate gas molecules, so little energy is needed to prepare the solvent to accept the gas. In the second step, energy is released as the gas molecules take their places in the pockets where they experience attractions to the water molecules. In this case, the solution process is exothermic.
Dissolution is endothermic.
Dissolution is exothermic.

26. Gas Solubility
Heats of solution for gases in organic solvents are often endothermic.
This is because it takes more energy to open a "pocket" of space in the organic solvents than the energy released when the gases go into the "pocket."
Heats of solution for gases in water are often exothermic.
This is because water already has pockets to hold the molecules so it does not take any energy, but energy is released in the hydration step.

27. Effect of Temp on Solubility
Solubility refers to the maximum amount of solute that will dissolve in a given volume of solvent.
It is usually given as #g solute /100g solvent.
This is the "saturation point" when no more solute can go into solution.
The solution is then referred to as the "saturated solution," which is an equilibrium state.

28. Saturated Solution solute (undissolved) solute (dissolved)
Dissolution of solids in water is usually endothermic.
heat + solute(undiss) solute (dissolved)
Thus, adding heat would shift the equilibrium to the right, making the substance more soluble (as is the case with sugar in water).

29. Gas in Solution
In the case of a gas in an aqueous solution, it is usually exothermic.
solute (undissolved) solute (dissolved) + heat
If heat is added, the equilibrium will shift to the LEFT, making the gas LESS soluble.
This is what you observe when you open a soda bottle in hot weather. The gas will come out much faster than in cold weather.
Sodas go flat faster at rm temp than in the fridge.

30. Examine the curve for table salt!
Fig p.594
Solubility in water vs. temp for several substances.
What does this graph tell you?
Examine the curve for table salt!
What is unusual?
...and Ce2(SO4)3?

31. Effect of Pressure on Solubility
gas + solvent solution
If pressure is increased, the equilibrium will shift to the right to relieve the pressure. This is because a solution occupies less space than a gas.
Henry's Law
Concentration of a gas (C) in a liquid at any given temperature is directly proportional to the partial pressure (P) of the gas over the solution.
C(gas)  P(gas)
C(gas) = kH P(gas)
You can increase the concentration of a gas in a solution by increasing the pressure above it.
That is how sodas are carbonated.

32. Application of Henry's Law
C(gas) = kH P(gas)
Ex (p.596)
At 20oC the solubility of N2 in water is g L-1 when the partial pressure of nitrogen is 585. torr. What will be the solubility of N2 in water at 20oC when its partial pressure is 823 torr?
Ans g/L
Ans g/L

33. Solubility of Gases in Water
Some gases are more soluble than others:
Polar gases are more soluble because they form stronger attraction to the solvent. (e.g. SO2, NH3)
Gases that react with the solvent are more soluble because the rxn removes the product & thus the equilibrium will shift to replenish the lost product.
e.g. SO2 (g) SO2 (aq) H2SO3 (aq)
H2O
H2O
The product SO2 (aq) is funneled off as H2SO3, so the equilibrium shifts to the right to replenish the lost SO2 (aq).