Home > Chemistry > Chemistry 1A Part Deux: Lecture 18 Notes

Chemistry 1A Part Deux: Lecture 18 Notes

Overview

Today is a 48 minute lecture on solubility.  These lectures are very dense, as they are trying to cover almost all of general chemistry in one semester.

Details

Solubility equilibrium constant is a measure of the maximum amount of a solute that dissolves in a solvent.

Intermolecular attractions in a solution are similar to the intermolecular attractions in pure solutions.

The solubility of purple glucose, C6H12O6.  Glucose has many polar groups and is itself a polar molecule.  What is the most that can dissolve in 1 Liter of water?

C6H12O6 (solid) = C6H12O6(aqueous)

K solubility = 5.1 =  C6H12O6(aqueous)/C6H12O6 (solid)

Pure solids and liquids have K =1.

Physical Equilibria

Vapor Pressure

H2O liquid = H2O gas

K p = pH2o (gas)/ H2O (liquid) = pH2O (gas)

Solubility

How can you increase the concentration of glucose in a 7 moles of glucose added per liter, with a solubility of 5.1 molar?    Do not add glucose, do not add more water.

The Role of Water in Equilibria

This part begins at 18 minutes in.

K = H2O gas/H2O liquid

K = C6H12O6 aq/C6H12O6

K = [H+] [CH3COO-]/[CH3COOH]

Solubility Depends upon Temperature

A graph of solubility versus temp.  Q is an arbitrary concentration.  K is the concentration at equilibrium.

Q>K   Supersaturated

Q = K   Saturated

Q<K  Unsaturated

A very concentrated solution of sodium acetate.  The system will crystallized.  He also does a stalactite demo at 25 minutes in.

Like dissolves like.  The natural tendency is to spread out and mix.  Hexane is not miscible with water.

Hexane hexane interaction is just dipole dipole interactions.  Water water interaction are very strong with hydrogen bonding.

Micelles/Lipids/Surfactants

Polar and nonpolar part of amphillic molecules organize in water to form water soluble multimolecular structures like micelle, liposome, and bilayer sheets.

Detergent is used to move grease into water.  The micelle forms hydrophillic outside with hydrophobic insides.

Solubility of Gases in Liquids

CO2 g = CO2 aq

K h = gas/p gas

Henry’s Law: Solubility gas = Kh P gas

S gas = solubility of gas in a solvent in M/(L atm)

Equilibrium, pressurize Q<K, more dissolves, equilibrium restored.

He performs the menthos/coke experiment at 36 minutes.  The porous sugars in menthos act as nucleating sites for the CO2 in the soda.  You give it one crystal and then it knows how to equilibriate.

Henry’s Law

K h = [gas (aq_]/P gas

K h is solubility

Gas         K h (M/atm)

He           .00037
N2          .00070
O2         .0013
Ar          .0015
CO2        .023
NH3           .58

Larger molecules had larger dispersion forces.

Ammonia Fountain

A flask and a bubble are connected by a tube.

NH3 g = NH3 aq        K h = .58 M/atm

NH3 + H2O  = NH4+ + OH-   Kb = 1.8 x 10^-5

H2O with phenolphthalein acidic is colorless, basic is pink.

Diving

The solubility of nitrogen in the blood changes with depth.  The higher the depth, the higher the pressure, the more gas is dissolved.  The lower the depth, the lower the pressure, the less gas is dissolved.  Ascending too fast will cause N2 to come out of solution in the gas phase.

An aqueous solution has equal concentration of He aq and O2 aq.  Which is true: pO2 = < > pHe

pO2<pHe.

Review

Ideal Gas Law: low p, high t, point sources, elastic collisions, random motion, PV = nRT

van der Waals gas: (P+n^2/a^2)(V -nb) = nRT

n = 22.4 L

Electronegativity: NOF

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