Chapter 11Chapter 11Chapter 11Chapter 11

11

Solutions andTheir Properties

Solutions

• Homogeneous mixtures:

– Solutions – ions or molecules (small particles)

– Colloids – larger particles but still uniform (milk, fog)

The Solution Process

• During dissolution, some forces are broken and new forces are created

33

The Solution Process

• Solvent-solvent interactions

• Solute-solute interactions

• Solvent-solute interactions

� ∆∆∆∆Hsoln = ∆∆∆∆H1 + ∆∆∆∆H2 + ∆∆∆∆H3

44

Figure 12.2

The Solution Process

• Generally,

• if the solute is more strongly attracted to the solvent attraction (than to itself –weak IMF), then dissolution is favorable; exothermic (∆∆∆∆Hsoln < 0)

• if the solute is more strongly attracted to itself (than to the solvent – strong IMF), then dissolution is not favorable; endothermic (∆∆∆∆Hsoln > 0)

Hot and Cold Packs

• Hot packs – dissolving solid releases heat

– CaCl2 (s) ∆∆∆∆Hsoln = -81.3 kJ/mol

–MgSO4 (s) ∆∆∆∆Hsoln = -91.2 kJ/mol

• Cold packs – dissolving solid absorbs heat

– NH4NO3 (s) ∆∆∆∆Hsoln = +25.7 kJ/mol

Ways of Expressing Concentration

• Variety of units

– Most commonly used is M (molarity)

– Also ppm, mole fraction, molality, and Normality

• Qualitative terms relating to solubility

– insoluble, slightly soluble, soluble, very soluble

– <0.1 g/100g >2 g/100 g

• Other comparative terms:

– dilute, concentrated (solids in liquids)

– miscible, immiscible, partially miscible (liquids in liquids)

77

Concentration Units• Molarity

– Molarity = moles solute / liter solution = mol/L

– Depends on temperature; density of liquids changes with temperature (dH2O at 20oC = 0.9982 g/cm3)

– Molarity:

– Ex: 5.0 g NaCl in water that gives a volume of 251mL

– Ans: 0.34 M NaCl

• Mole fraction (X):

– Mole Fraction = mole A / (mole A + mole B)

– Ex: 5.0 g NaCl in 251 mL water

– Ans: 0.0061 NaCl; 0.994 water88

Concentration Units

• Mass Percent

– Mass Percent = (mass of solute / mass of solution) x 100%

– Ex: 5.0 g of NaCl in 251 mL water

– Ans: 2.0 % NaCl

• Molality

– molality = moles solute / kilograms solvent = mol/kg

– Independent of temperature because masses do not change with temperature

– Ex: 5.0 g NaCl in 251 mL water

– Ans: 0.34 m NaCl

Types of Solutions

• Unsaturated solution: contains less solute than it has the capacity to dissolve

• Saturated solution: contains the maximum amount of solute that will dissolve in a given solvent at a specific temperature

• Supersaturated solution: contains more solute than is present in a saturated solution

• Crystallization: process in which a dissolved solute comes out of solution and forms crystals

Supersaturated Solution Supersaturated Solution

Acetate

Solid Solubility and Temperature

• Solubility of ionic compounds in water generally increases with higher water temps

Figure 11.6

Effect of Temperature on Solubility

• The solubility of gases in water decreases with increasing temperature. Why?

• Solubility goes to 0 at boiling point of water.

1414

Effect of Pressure on Solubility

• Strong effect only for gases dissolved in liquids.

• What happens to the amount of dissolved solute when pressure is increased (2nd

picture)?

1515

More dilute solution More concentrated solution

Figure 11.8

Effect of Pressure on Solubility

• Henry’s Law: concentration (solubility) of a gas in a liquid is proportional to the pressure of the gas over the solution

• Why does a soft drink fizz when the

container is opened?

1616

Molarity to molality

Remember the distinction between molarity (M) and molality (m)

M = m =Moles of solute Moles of soluteL of solution kg of solvent

To do these calculations, you must have one extra

piece of information: density

What is the molality of a 0.123 M HCl(aq) solution? The density of the solution is 1.030 g/mL.

What is the molarity of a 3.21 m KOH(aq) solution? (density of solution = 1.163 g/mL)

Colligative Properties

• Colligative properties: depend only on the number of solute particles in solution and not on the nature of the solute particles. The particles may be covalent molecules or ionic compounds. (We’ll only deal with

covalent molecules.)

• Colligative means “depending on the collection”

– Examples: vapor pressureboiling pointfreezing pointosmosis / osmotic pressure

1919

Vapor Pressure Lowering

• Adding a solute to a solvent lowers the Pvap

• Pvap of a solution < Pvap of a pure solvent

2020

Vapor Pressure Lowering

• Pvap is lowered because fewer surface positions are occupied by solvent molecules and because intermolecular forces in solutions are usually greater than those in the separated substances.

• Raoult’s Law: partial pressure of a solvent vapor over a solution is the product of the mole fraction of the solvent (X1) and the vapor pressure of the solvent (Po1).

– P1 = X1Po1

B. P. Elevation/F. P. Depression

• Phase diagrams can be used to see how vapor pressure changes cause changes in boiling and freezing points.

2222

Freezing pure water

Figure 11.12

B. P. Elevation/F. P. Depression

• Lower vapor pressure (red lines)

Freezing solution

Freezing Point Depression

• Changes in freezing point are directly proportional to the number of solute particles

• Equations that govern these effects:

� ∆Tf = Kf m

� ∆Tf is the change in normal freezing point

– Kf is based on the identity of the solvent

– m is molality = moles solute/kg solvent

• Calculate the freezing point of a solution when 62.345 g of vinyl chloride (CH2CHCl) is added to 1264 g cyclohexane.

Boiling Point Elevation

• Changes in boiling point are directly proportional to the number of solute particles

• Equations that govern these effects:

� ∆Tb = Kb m

� ∆Tb is the change in normal boiling point

– Kb is based on the identity of the solvent

– m is molality = moles solute/kg solvent

• Calculate the boiling point of a solution when 62.345 g of vinyl chloride (CH2CHCl) is added to 1264 g cyclohexane.

2525

B.P. Elevation/F.P. Depression

• Kb and Kf depend only on the solvent being used

Group Work

• Calculate the molar mass of an unknown solute when the freezing point of water is lowered by 7.77°C after 651 g of the solute is dissolved in 2505 g of water.

Osmosis – pickling of cucumbers Osmotic Pressure

• Osmosis: selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one.

– Semipermeable membrane: allows the passage of solvent molecules but blocks the passage of solute molecules.

� ΠΠΠΠ = MRT

3030

Pos anim

Reverse Osmosis

• Uses high pressure to force water from a more concentrated solution to a less concentrated one through a semipermeable membrane. The membrane allows water molecules to pass through but not dissolved ions.