chapter 14.3 properties of solutions

Solutions

Chapter 14.3Properties of Solutions

Adapted by SA Green from:

John D. Bookstaver

St. Charles Community College

St. Peters, MO

2006, Prentice Hall, Inc.

Solutions

Solutions

• Solutions are homogeneous mixtures of two or more pure substances.

• In a solution, the solute is dispersed uniformly throughout the solvent.

Solutions

Solutions

How does a solid dissolve into a liquid?

What ‘drives’ the dissolution process?

What are the energetics of dissolution?

Solutions

How Does a Solution Form?1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.3. Enthalpy (H) changes with each interaction broken or

formed.

Ionic solid dissolving in water

Solutions

How Does a Solution Form?1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.3. Enthalpy (H) changes with each interaction broken or

formed.

Solutions

How Does a Solution Form

The ions are solvated (surrounded by solvent).

If the solvent is water, the ions are hydrated.

The intermolecular force here is ion-dipole.

Solutions

Energy Changes in SolutionTo determine the enthalpy

change, we divide the process into 3 steps. 1. Separation of solute

particles.

2. Separation of solvent particles to make ‘holes’.

3. Formation of new interactions between solute and solvent.

Solutions

Dissolution vs reaction

• Dissolution is a physical change—you can get back the original solute by evaporating the solvent.

• If you can’t, the substance didn’t dissolve, it reacted.

Ni(s) + HCl(aq) NiCl2(aq) + H2(g) NiCl2(s)dry

Solutions

Degree of saturation

• Saturated solutionSolvent holds as much

solute as is possible at that temperature.

Undissolved solid remains in flask.

Dissolved solute is in dynamic equilibrium with solid solute particles.

Solutions

Degree of saturation

• Unsaturated SolutionLess than the

maximum amount of solute for that temperature is dissolved in the solvent.

No solid remains in flask.

Solutions

Degree of saturation

• SupersaturatedSolvent holds more solute than is normally

possible at that temperature.These solutions are unstable; crystallization can

often be stimulated by adding a “seed crystal” or scratching the side of the flask.

Solutions

Degree of saturation

Unsaturated, Saturated or Supersaturated?

How much solute can be dissolved in a solution?

More on this in Chap 17(solubility products, p 739)

Solutions

Factors Affecting Solubility

• Chemists use the axiom “like dissolves like”:Polar substances tend to

dissolve in polar solvents.Nonpolar substances tend

to dissolve in nonpolar solvents.

Solutions

Factors Affecting Solubility

The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve.Example: ethanol in water

Ethanol = CH3CH2OH

Intermolecular forces = H-bonds; dipole-dipole; dispersion

Ions in water also have ion-dipole forces.

Solutions

Factors Affecting Solubility

Glucose (which has hydrogen bonding) is very soluble in water.

Cyclohexane (which only has dispersion forces) is not water-soluble.

Solutions

Factors Affecting Solubility• Vitamin A is soluble in nonpolar compounds

(like fats).• Vitamin C is soluble in water.

Solutions

Which vitamin is water-soluble and which is fat-soluble?

Solutions

Gases in Solution

• In general, the solubility of gases in water increases with increasing mass.

Why?• Larger molecules

have stronger dispersion forces.

Solutions

Gases in Solution

Solutions

Gases in Solution

• The solubility of liquids and solids does not change appreciably with pressure.

• But, the solubility of a gas in a liquid is directly proportional to its pressure.

Increasing pressure above solution forces more gas to dissolve.

Solutions

Henry’s Law

Sg = kPg

where

• Sg is the solubility of the gas;

• k is the Henry’s law constant for that gas in that solvent;

• Pg is the partial pressure of the gas above the liquid. So

• S1/P1 = S2/ P2

Solutions

Temperature

Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

Solutions

Temperature• The opposite is true of

gases. Higher temperature drives gases out of solution.

Carbonated soft drinks are more “bubbly” if stored in the refrigerator.

Warm lakes have less O2 dissolved in them than cool lakes.