SCH 3U - Predicting Solubility & Factors Affecting Solubility

Predicting Solubility

 

Ionic Compounds 
Calculate the ΔEneg between the two atoms in the compound.  If the difference is small, the compound will be insoluble in water.

ex.  Between silver sulfide and sodium chloride, which compound is more likely water soluble?

AgS     ΔEneg (S, Ag) = 2.58 – 1.93 = 0.65 

NaCl    ΔEneg (Cl, Na) = 3.16 – 0.93 = 2.23  - soluble

Covalent Compounds

Covalent compounds do not have full positive & negative charges like ions, but some are still water soluble.  If the covalent compound has polar regions, it is more likely to be water soluble.

ex. Between ethanol (C₂H₅OH) and butane (C₄H₁₀), which compound is more likely water soluble?

Ethanol has an H directly bonded to an O, which creates a polar region in this compound.  Ethanol will be able to hydrogen bond to water and will be soluble.

Butane has only C-C and C-H bonds, which are non-polar.  Butane will not be attracted to water and will be insoluble.

Remember “like dissolves like.” 

Factors Affecting Solubility

1. Molecule Size

Small molecules are usually more soluble than large ones.

ex. Will water solubility increase, decrease or remain the same in going from methanol ↝ ethanol ↝ propanol ↝ butanol ↝ pentanol?

methanol ↝ ethanol         ↝ propanol             ↝ butanol                      ↝ pentanol

   CH₃OH ↝ CH₃CH₂OH ↝ CH₃CH₂CH₂OH ↝ CH₃CH₂CH₂CH₂OH ↝ CH₃CH₂CH₂CH₂CH₂OH

Notice that the amount of polar region (OH) remains the same going from molecule to molecule.  Conversely, the amount of non-polar region (CH_x) is getting larger as we proceed to the right.  Thus, as the length of the carbon chain increases, the molecule struggles to maintain its attraction to water.  So, as we move from left to right, water solubility decreases.

 

Check out this video to see this in action.

2. Temperature

Temperature affects both the rate of dissolution, as well as solubility.  Quantitatively, solubility is usually given as the mass of solute dissolved in 100 mL of water at a specific temp.

Recall that for dissolution to occur, energy is required to break apart the solid solute particles.  So, an increase in temperature can expedite this process.  Thus, an increase in temperature usually increases the solubility of a solid. 

However, since the forces of attraction between liquid molecules are weaker, temperature does not have a large affect on its solubility. 

Gases already have a lot of energy and when dissolved in liquid, this energy is lost – at higher temperatures, the gas regains some energy and comes out of solution, so gases are less soluble at higher temperatures

3. Pressure

Changes in pressure do not really affect solid/liquid solutions.   

However gas/liquid solutions are affected.  The solubility of the gas is directly proportional to the pressure of the gas above the solution - that is why carbonated drinks are homogeneous when closed and bubble when opened.

Homework: Learning Check p. 368 #7-10, 12

 

 

Student Questions:
 1.  Can you do p.368 #8?
Fluoride ions have a very different electronegativity compared to the other halogens, so fluoride compounds will have different solubility than other halogen compounds.

 

2.  Hey, lady!  Please do the rest of the questions.  I wish to become as smart as possible.

 

p. 368 # 7   Benzene is soluble in fats/oils, which are non-polar.  Thus, benzene is non-polar, so it should be water-insoluble.

 

p. 368 # 9   Liquid solutes dissolving in liquid water are least affected by temperature.   Thus, liquid methanol would be more water-soluble than solid potassium chlorate or gaseous nitrogen.

 

p. 368 # 10   Gases absorb kinetic energy at the warmer temperature.  This makes the gas leave the soda.  So, the flat soda tastes different than fizzy soda.

 

p. 368 # 12   Warm surface water would not contain as much dissolved gaseous oxygen, so fish are less likely to want to hang out there.