Chapter 14 — Solutions 1

1 Chapter 14 — Solutions

•  A solution is a homogeneous mixture of two or more substances in a single phase.

•  By convention, the component present in largest amount is identified as the solvent and the other component(s) as the solute(s)

M = moles solute

L of solution

Molarity (M)

% = mass solute

Total mass of solution

Weight (mass) %

× 100

2 Chapter 14 — Solutions

Xi = moles solute (i)

Total moles in solution

Mole Fraction (X) Mole Fraction (X)

•  Where “i” is the moles of one component of the solute.

•  Total moles are all species: mols solute (i) + mols solvent.

•  The sum of all of the mole fractions for each component are 1 exactly.

m = moles solute

kg solvent

Molality (m)

m = mg solute

kg solution

Parts Per Million (ppm)

250. g H2O ×

1 mol H2O18.02 g

= 13.9 mol H2O

XC2H6O2

= 1.00 mol1.00 mol+13.9 mol

= 0.0671

62.1 g C2H6O2 ×

1 mol C2H6O2

62.1 g= 1.00 mol C2H6O2

3 Chapter 14 — Solutions

+

Left: careful layering of liquids put C8H18 on top, aqueous CuSO4 solution in the middle, CCl4 on the bottom. Right: after shaking!

4 Chapter 14 — Solutions

The many hydroxyl groups attached to the C-C bonded backbone of sugars makes sugars water soluble by virtue of their hydrogen bonding with H2O.

Glucose: most stable cyclic chair form.

•  Energy must be supplied to separate the ions from their attractive forces. (an endothermic process)

–∆latticeH •  Energy is evolved when the individual ions

dissolve in water where each ion is stabilized by solvation.

∆hydrationH •  This process, referred to as the Energy of

Hydration when water is the solvent, is strongly exothermic.

We can therefore represent the process of dissolving KF in terms of these chemical equations:

Step 1: KF(s) → K+(g) + F– (g) = – ∆latticeH Step 2: K+(g) + F–

(g) → K+(aq) +F–(aq) = ∆hydrationH

The overall reaction is the sum of these two steps. The enthalpy of the overall reaction, called the enthalpy of solution (∆solnH), is the sum of the two enthalpies.

Overall: KF(s)→ K+(aq) + F–(aq)

∆solnH = –∆latticeH + ∆hydrationH

5 Chapter 14 — Solutions

Gas solubility (mol/L)

Sg =kHPg

If the enthalpy of solution for a gas is exothermic:

Increasing the temperature of a solution (adding energy) will reduce the solubility of the gas:

gas + liquid solvent ΔsolnH < 0⎯ →⎯⎯⎯← ⎯⎯⎯⎯ saturated solution + energy

gas + liquid solvent Exothermic process

ΔsolnH < 0⎯ →⎯⎯⎯⎯⎯← ⎯⎯⎯⎯⎯⎯ saturated solution + energy

Adding energy shifts equilibrium to the left← ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

kH

T

•  Colloidal dispersions scatter light, a phenomenon known as the “The Tyndall effect”

6 Chapter 14 — Solutions

•  A hydrophobic colloid is stabilized by positive ions absorbed onto each particle and a secondary layer of negative ions.

•  Because the particles bear similar charges, they repel one another, and precipitation is prevented.

Soap molecules interact with water through the charged, hydrophilic end of the molecule. The long, hydrophobic end of the molecule binds through dispersion forces with nonpolar hydrocarbons and other non polar substances.

•  An emulsifier (also known as an emulgent) is a substance which stabilizes an emulsion by increasing its kinetic stability.

•  One class of emulsifiers is known as surface active substances, or surfactants.

•  Examples of food emulsifiers are egg yolk (where the main emulsifying agent is lecithin) and honey.

•  In some cases, particles can stabilize emulsions as well through a mechanism called Pickering stabilization.

•  Both mayonnaise and Hollandaise sauce are oil-in-water emulsions that are stabilized with egg yolk lecithin or other types of food additives such as Sodium stearoyl lactylate.