Solutions and Their Properties

Chapter 14

Solutions and Their Properties

• A solution is a homogenous mixture of two or more substances in a single phase.– solute vs. solvent

• The changes that occur when a substance is dissolved in a pure liquid are called colligative properties.

14.1 Units of Concentration

• Molarity (mole solute/liter solution) does not work when dealing with colligative properties!– solute molecules take up some volume

• Four units that can be used to calculate concentration of solute per solvent are…– molality, mole fraction, weight percent,

parts per million

Mole Fraction

• The mole fraction, X, of a solution is defined as the amount of a given component of a mixture divided by the total amount of all the components of the mixture.

XA = nA / (nA + nB +…)

Practice Problem

• If you dissolved 10.0 g of sugar, C12H22O11 in a cup of water, what is the mole fraction of sugar?

14.2 Solution Process

• Unsaturated• Saturated• Supersaturated• Solubility is the

concentration of solute in equilibrium with undissolved solute in a saturated solution.

Liquids Dissolving in Liquids

• If two liquids mix to form a solution, they are miscible. Immiscible liquids do not mix and exist as separate layers. – Like dissolves like!

Solids Dissolving in Liquids

• Once an ion has entered a solution and is surrounded by water molecules, then they are hydrated.

• Heat of solution may be endothermic or exothermic depending upon how much energy is absorbed or lost by the reaction.

Heat of Solution

• Heat of solution can be calculated by taking the sum of the standard heat of formation of the products minus the sum of the heat of formation of the reactions.

Practice Problem

• Use the data in Table 14.2 (p. 625) to calculated the enthalpy of solution for NaOH.

Homework

• After reading sections 14.1-14.2, you should be able to do the following…

• P. 648 (2-4, 6-18 even)

14.3 Factors Affecting Solubility

• Solubility of a gas in a liquid is directly proportional to the gas pressure.

Sg = kHPg

where S is the gas solubility, P is the partial pressure of the gaseous solute, and k is the Henry’s Law constant

– carbonated soft drinks; SCUBA tanks

Temperature Effects

• Solubility of solids in water increases with increasing temperature

• Solubility of gases in water decreases with increasing temperature– Fish seek lower depths in summer– Thermal pollution

Temperature Effects

• Le Chatelier’s Principle– States that a change in any of the factors

determining an equilibrium causes the system to adjust to reduce or counteract the effect of the change.

Gas + liquid solvent saturated solution + heat

increasing temperature shifts reaction left

Homework

• After reading sections 14.3-14.4, you should be able to do the following…

• P.649 (20-32 even, 34-36)

14.5 Colloids

• Colloids are intermediate between a solution and a suspension. (jello, milk, fog, styrofoam)

• Tyndall effect: colloids scatter visible light when dispersed in a solvent

Colloids

• Hydrophilic and hydrophobic• Emulsions are colloidal dispersions of

liquid in liquid (mayo, milk)– emulsifying agents keep colloids from settling

out

Surfactants

• Soaps and detergents are emulsifiers.• These substances affect the properties of

surfaces are called surfactants; ones used for cleaning are called detergents.

Homework

• After reading Section 14.5, you should be able to do the following…

• P. 650 (41-49 odd)