1. 13.2 Solubility and Factors Affecting Solubility
Dr. Fred Omega Garces
Chemistry 201
Miramar College

2. Properties of Solution
Characteristics:
• Distribution of particles is uniform
• Components in solution do not separate upon standing
• Components cannot be separated by filtration.
• Solute / Solvent mixes in ratios - up to the solubility limit.
• Solution is almost always transparent.
• Compounds of solution may be separated by other methods i.e., distillation or chromatography.

3. Suspended in Solution: Solubility Process
Solubility - The process in which substances dissolve at the molecular level.
Solubility - The maximum mass of solute capable of dissolving in a given amount of solvent at a given temperature.
Unsaturated Saturated supersaturated
A solution that contains the maximum solute it can dissolve. (There are no residue)
A solution that contains more solute (in dissolved form) than the solubility limit
A solution that has the capacity to dissolve more solute.
Immiscible - When two liquids are not soluble in each other
Miscible - When two liquids are soluble in all proportion.

4. 1) Nature of Solute and Solvent
Dissolving Process: Why is water soluble in alcohol yet water is insoluble in oil ?
Solubility Factor:
Solute and Solvent characteristic:
• In aqueous solution, water will form strong intermolecular forces with only other polar molecules (the dissolution process)
• Oil is a nonpolar substance and therefore will only form strong IMF with other nonpolar substances such as organic compounds.
• The result is the immisciblity of water and oil.
(Later the Energetics of this process will be discuss)
Organic Chemist saying
“Like Dissolves Like”
An oil layer floating on water. For a substance to dissolve, the water-water hydrogen bonds must be broken to make a “hole” for each solute particle. However, the water-water interactions will break only if they are replaced by similar strong interactions with the solute.

5. Dissolution of Solid Solute
What is the driving force which causes solutes to dissolve to form solutions?
Covalent versus Ionic solute
1. Covalent solutes dissolve by H-bonding to water or by LDF
2. Ionic solutes dissolve by dissociation into their ions.
Ionic
Covalent
Picture of Ethanol and NaCl dissolving

6. Dissolving at the molecular Level: Ionic Compounds
Ionic substances NaCl, MgCl2, AgCl
The salt; NaCl or the ionic compound is a 3-D lattice.
The Na+ and Cl- are arranged in 3-D alternating ion lattice
The negative Cl- attracts the (+) of hydrogen in H2O while the positive Na+ attracts the ( -) of oxygen in H2O. Occurs due to coulombic (opposite) attracts.
The solvent (H2O) interaction to the ion is the Hydration process
H2O literally pulls the lattice arrangement of the solid salt apart because of the attraction between the + or  - of water to the - or + ions.
Consider:
1) M+ & X- (i.e., Na+ and Cl - )
2) M+ to - oxygen of H2O & X- to + hydrogen of H2O
If (1) is favorable (lattice energy) then solute does not dissolve (insoluble) i.e., AgCl
if (2) is favorable then hydration, solute does dissolve (soluble). NaCl

7. Equilibrium: Dissolution = Crystallization
Observe: After some time, no change in amount of solid precipitate at the bottom of the beaker.
Concentration of the solution is maintained
At the molecular level:
Amount of salt dissolving into solution equals to amount of salt recrystalizing
[Solute]  [Solute]
Solid
Solution

8. Dynamic Equilibrium Equilibrium LeChatelier Principle Example:
Situation in which changes occur at equal rates so there is no apparent net change.
LeChatelier Principle
A change (stress) on a system at equilibrium will cause the system to self adjust itself to reduce the stress until a new equilibrium is re-established.
Example:
Traffic at a toll bridge
Vapor Pressure
Sugar dissolving

9. Ionic Vs Covalent Compounds: Electrolyte Vs. Nonelectrolyte
Substance when dissolve can break-up to ions or stay intact. i.e., NaCl and sugar.
Type: % ionization: Solubility (in H2O)
Electrolyte: conducts electricity.
Strong electrolyte 100 % ionization very soluble
Weak electrolyte less 100% ionization slightly to very soluble
Nonelectrolyte: Do not conduct electricity
no conduction 0 % ionization insoluble or soluble

10. Dissolution at the molecular level?
Spontaneity of dissolving process:
Consider the Spontaneity due to Gravity:
Object drops and impact floor spontaneously.
The driving force is gravity.
High Energy
When systems proceed to a lower energy state, the process is exothermic:
Energy is released.
(Tends to be spontaneous)
Objects spontaneously fall because of the tendency for systems to be at a lower energy state.
Low Energy

11. Driving Force for Dissolution
Reaction exothermic H (-)
Downhill  Spontaneous
Reaction exothermic H (+)
Uphill Spontaneous ???

12. Dissolution at the molecular level?
Consider the dissolution of NaOH in H2O

13. Exothermic: Dissolution Process
Solution process as indicated by the enthalpy heat of solution: Dissolution via Hsoln can be thought of as the sum of three enthalpy changes; Hsolvent, Hsolute and Hmix. The result is Hsolution.
Note, If Hsoln < O, Then it is a downhill, Spontaneous

14. Hmix or Hhydration Closer Look, Hmix:
Hhydr is always negative based on Columb’s Law.
E = k |Q1•Q2| d
Trends in ionic heats of hydration. Heats of hydration (DHhydr) are always negative because ions and water attract each other and release heat. Values for the Group 1A(1), 2A(2), and 7A(17) ions are shown as descending posts, with the ionic radius on top. The DHhydr values depend on charge density smaller down a group as ionic size increases and larger from Group 1A(1) to Group 2A(2) as ionic charge increases.

15. Consider the Dissolution of NH4Cl
In the dissolution of ammonium chloride,NH4Cl, the system feels cold, indicating an Endothermic process.
An uphill process that is spontaneous.
How is this possible ?

16. Endothermic, Dissolution Process Hmix small, Hsoln (+)
Down hill (Exothermic) vs. up hill (Endothermic)
Will an unfavorable process (uphill) proceed spontaneously ?
Will a substance dissolve if Hsoln(+)
When a solute and solvent mixes, the magnitude of three H processes will determine if the dissolution process is exothermic, or endothermic. Factors determining spontaneity will be determine by thermodynamic factors and not only by enthalpies value

17. Second Law of Thermodynamics
Why does ink spread or gas expand or your room gets chaotic?
Greater probability of disorder than order.
One of the Basic Law of Nature
2nd Law of Thermodynamics:
Process in which disorder increases occur spontaneously

18. Enthalpy Diagram for Series of Dissolution Processes
NaCl in H2O
NaOH in H2O
NH4NO3 in H2O
Enthalpy diagrams for dissolving three ionic compounds in water. The enthalpy diagram for dissolving an ionic compound in water includes the negative of the DHlattice (DHsolute; always positive) and the heat of hydration (DHhydr; always negative). A, For NaCl, the magnitude of DHlattice is slightly greater than that of DHhydr, so DHsoln is small and positive. B, For NaOH, DHhydr dominates, so DHsoln is large and negative. C, For NH4NO3, DHlattice dominates, so DHsoln is large and positive.
NaCl in Heptane
Hexane in Heptane
Enthalpy diagrams for dissolving NaCl and octane in hexane. A, Since attractions between ions and hexane molecules are weak, DHmix is much smaller than DHsolute. Thus DHsolute is so positive that NaCl does not dissolve in hexane. B, Intermolecular forces in octane and in hexane are so similar that DHsolute is very small. Octane dissolves in hexane because the solution has greater entropy (more disorder) than the pure components.

19. Factors Affecting Solubility
1. Nature of Solute / Solvent. - Like dissolves like (IMF)
2. Temperate Factor -
i) Solids/Liquids- Solubility increases with Temperature
Increase K.E. increases motion and collision between solute / solvent.
ii) gas - Solubility decreases with Temperature
Increase K.E. result in gas escaping to atmosphere.
3. Pressure Factor -
i) Solids/Liquids - Very little effect
Solids and Liquids are already lose together, extra pressure will not increase solubility.
ii) gas - Solubility increases with Pressure.
Increase pressure squeezes gas solute into solvent.

20. 2 i) Temperature on Solubility: Solids & Liquids
Temperature - (Solid and Liquid)
Consider the extent in which sugar or NaCl dissolves in water. What are the conditions which will increase the solubility of sugar or salt in water.
[Solute]  [Solute]
Solid
Solution
As the temperature increase, both solute and solvent will be moving faster, this will result in the mixing of both substance to be more effective.

21. Equilibrium Revisited
[Solute]  [Solute]
Solid
Solution
Observe: After some time, no change in amount of solid precipitate at the bottom of the beaker.
Concentration of the solution is maintained
Equilibrium
Situation in which changes occur at equal rates so there is no apparent net change.
LeChatelier Principle
A Change (i.e., stress) on a system at equilibrium will cause the system to self adjust itself to reduce the stress until a new equilibrium is re-establish.

22. LeChatelier Principle
Teeter•Tooter
At Equilibrium
Stress applied
Self Adjust
Re-establish Equilibrium

23. Equilibrium: Stress / Relief on Reactant
Stress on Reactant, Rxn shift right
Relief on Reactant, Rxn shift left

24. Equilibrium: Stress / Relief on Product
Stress on Product, Rxn shift left
Relief on Product, Rxn shift right

25. Exothermic Heats of Solution
Exothermic Process. Energy is a product
Heating a solution in which the Hsoln is exothermic (Energy is a product) results in a shift of the reaction to the left or more solute precipitating out of solution.

26. Endothermic Heats of Solution
Endothermic Process Energy is a reactant
Heating a solution in which the Hsoln is endothermic (Energy is a reactant) results in a Shift of the reaction to the right or more solute dissolving in solution.

27. Solubilities of Solids Vs Temperature
Solubilities of several ionic solid as a function of temperature.
Some salts have negative enthalpy of solution, (exothermic process) i.e., Ce2(SO4)3 and they become less soluble with increasing temperature.

28. 2ii) Temperature & Solubility: Gases
Temperature - (Gas)
Consider the extent in which O2 or CO2 dissolves in water. What are the conditions which will increase the solubility of gas in water.
[Solute]  [Solute]
gas
Solution
As the temperature increase, both solute and solvent will be moving faster, the gas solute however will now have enough energy to leave the liquid interface.

29. Gas solute; Exothermic Hsoln
As the temperature increase, both solute and solvent will be move faster. The gas solute however will now have enough energy to leave the liquid interface because IMF can be overcome
Gas above soln
Gas in solution

30. Disaster: (1700 dead) from Gas Solubility
In the African nation of Cameroon in 1986 a huge bubble of CO2 gas escaped from Lake Nyos and moved down a river valley at 20 m/s (about 45 mph). Because CO2 is denser than air, it hugged the ground and displaced the air in its path. More than 1700 people suffocated. The CO2 came from springs of carbonated groundwater at the bottom of the lake. Because the lake is so deep, the CO2 mixed little with the upper layers of water, and the bottom layer became supersaturated with CO2. When this delicate situation was changed, perhaps because of an earth-quake or landslide, the CO2 came out of the lake water just like it does when a can of soda is opened.
Lake Nyos in Cameroon, the site of a natural disaster. In 1986 a huge bubble of CO2 escaped from the lake and asphyxiated more than 1700 people.

31. 3 i) Pressure on Solubility: Solids / Liquid
Pressure - (Solid and Liquid)
The solubility of solids and liquids are hardly affected by pressure.
Solids and liquids are already very close to each other. An increase in pressure will not affect solubility

32. 3 ii) Pressure on Solubility: Gas
Pressure - (Gas)
Solubility of gas is greatly affected by pressure
Gas solute is very sensitive to pressure. Gas particles are separated by large void space, an increase in pressure will increase these particles to come closer together thereby increasing the solubility of the gas.
Divers must be careful when diving to great depth because the potential of dissolved N2 gas in blood. Clinical term is the Bends.

33. Pressure Affect: Teeter Totter Analogy
[Solute]  [Solute]
gas
Solution
Pressure Sensitive
In utilizing LeChatelier Principle to determine the direction of solubility for a gaseous solute with variation in pressure, the first thing that must be establish is which side is more sensitive to pressure. In our case the gas is more sensitive than the solution.

34. Henry’s Law
At pressure of few atmosphere or less, solubility of gas solute follows Henry Law which states that the amount of solute gas dissolved in solution is directly proportional to the amount of pressure above the solution.
c = k • P
where, c = solubility of the gas (usually molality)
k = Henry’s Law Constant, (solute-solvent pair)
P = partial pressure of gas over solution
Henry’s Law Constant (25°C)
Gas k M/mmHg
N •10-7
O •10-6
CO •10-5

35. Henry’s Law: Example
The solubility of ethylene (C2H4) in water at 20°C and a pressure of atm is 1.27•10-4 m.
a) calculate the Henry’s Law constant, k in (m/torr)
b) How many grams of ethylene are dissolved in one kilogram of water at 20°C at 500 torr. ?
Detm’ k from c = k•P, k= c / P
k = •10-4 m = 1.27•10-4 m
0.300 atm (760torr/1atm) torr
a) k = •10-7 m / torr
c = k•P k is constant which depends only on T
c = 5.57•10-7 m / torr • 500 torr
= •10-4 m = 2.79 •10-4 m /kg H2O
b) mass = 2.79•10-4 mol • g / mol
mass = 7.80•10-3 g

36. Summary of Pressure Temperature Affect on Solubility
H (s, l or g) Temp Direction Solubility
(+) Endothermic   Product  increase
(+)   React  decrease
(-) Exothermic   React  decrease
(-) Exothermic   Product  increase
Pressure Direction Solubility
Gas solute   Product  increase
Gas solute   React  decrease