8.8: Limits of Solubility

1. 8.8: Limits of Solubility • When we dissolve a solid in some liquid, what is actually happening? • Take glucose added to water • H-bonding between water molecules and glucose molecules pulls the individual molecules into solution • H-binding and Van der Waals interactions keep the glucose held together as a solid

2. Solubility: Saturated Solutions When we keep adding solid to a liquid, the solid will dissolve until… The solid is in equilibrium with the dissolved solute in a saturated solution

3. Solubility The molar solubility, s, of a substance is it molar concentration in a saturated solution A saturated solution is one in which the dissolved and undissolved solute are in equilibrium

4. 8.9: Like Dissolves Like • What is a good solvent to remove butter from your shirt? Soda from your carpet? • We want to use a solvent that has similar chemical properties because of the “Like Dissolves Like” rule

5. Like Dissolves Like • It all comes down to intermolecular forces… • When we dissolve something, we replace some of the specific intermolecular forces in the solution with the same kind of interactions with the solute. • Water (polar, hydrogen bonding solvent) will work well to dissolve polar or ionic solutes • Fatty stains like gravy or butter can be removed with organic solvents like hexane or dichloroethene

6. Soap • Soap is made by treating fats with strong hydroxide bases • In the resulting chemical reactions, the fatty acids are knocked off and have a polar head and an acyl tail • Soap molecules have hydrophobic parts (the tails) and hydrophilic parts (the head)

7. Summary: Solutions • For most ionic substances, dissolution increases the entropy of the system and they dissolve more readily at higher temperatures (since the free energy is dependent on TS) • For gas molecules, the entropy decreases when the temperature rises due to the contribution of Ssurr (solvent) • Dissolution depends on the balance between the change in entropy of the solution and the change in entropy of the surroundings.

8. Boiling Point Elevation and Freezing Point Depression • The presence of a solute will increase the boiling point of the solvent due to the decrease in Gibbs Free Energy This is one reason why people put salt into cooking water: It makes the temperature higher However: In reality, this elevation is extremely low.

9. Freezing Point Depression • The presence of the solute molecules in the solution decreases the freezing point of the solvent relative to its normal value It does so because of the increased solution entropy caused by the presence of the solute.

10. Boiling Point Elevation and Freezing Point Depression Freezing Point Depression = kf(molality) Boiling Point Elevation = kb(molality) 

11. 8.17: Osmosis • We’ve all heard of osmosis, but what is it really? Osmosis is the flow of solvent through a semi-permeable membrane from a region of low solute concentration to one of high solute concentration • In terms of entropy, this just makes sense, right?

12. Osmosis: Examples Red Blood Cells in • Normal Cells • Dilute Solution • Concentrated Solution

13. Osmosis in the Laboratory

14. Osmosis: The Science • If we set up an apparatus with a semi-permeable membrane on one end, filled it with a sugar solution, and then placed it into pure water, what would happen?

15. Osmosis: The Science • We can use this equation to determine the molar mass of a solute from osmotic pressure measurements i = van’t Hoff i factor (determined experimentally and related to ion charge) R = Gas constant T = Temperature C = Molar concentration  = iRTC 