Chemistry chapter 14 1

Ions in Aqueous Solutions and Colligative Properties

Chapter 14

Chemistry chapter 14 2

Dissociation• The separation of ions that occurs when an

ionic compound dissolves.

Chemistry chapter 14 3

Dissociation examples

ClNaNaCl

ClCaCaCl 222

Chemistry chapter 14 4

You try• Write the equation for the dissolution of

NH4NO3 in water. If 1 mol of ammonium nitrate is dissolved, how many moles of each type of ion are produced?

• 1 mol of each type of ion

3434 NONHNONH

Chemistry chapter 14 5

Precipitation Reactions• When two solutions are mixed, a double

replacement reaction may occur.• If one of the products is insoluble, it will

form a precipitate.• See table 14-1 on page 427

Chemistry chapter 14 6

Example

• Solutions of (NH4)2S and Cd(NO3)2 are mixed. Will a precipitate form?

2424 2)( SNHSNH

32

23 2)( NOCdNOCd

Chemistry chapter 14 7

Example continued

CdSNONHNOCdSNH 342324 2)(

)()(2

)()()(

34

2324

sCdSaqNONHaqNOCdaqSNH

The cadmium sulfide is the precipitate.

Chemistry chapter 14 8

Net Ionic Equations• Includes only those compounds and ions

that undergo a chemical change in a reaction in an aqueous solution.

Chemistry chapter 14 9

Spectator Ions• Ions that do not take part in a chemical

reaction and are found in solution both before and after the reaction.

Chemistry chapter 14 10

Example

)(2)(2)(

)()(2)(2)(

43

243

2

aqNHaqNOsCdS

aqSaqNHaqNOaqCd

)()()( 22 sCdSaqSaqCd

Chemistry chapter 14 11

You try• A solution of sodium sulfide is combined

with a solution of iron(II) nitrate. Does a precipitate form?

• Iron(II) sulfide is the precipitate.

)()(2)()()(

3

232

sFeSaqNaNOaqNOFeaqSNa

Chemistry chapter 14 12

You try continued• Write the net ionic equation for the previous

reaction.

)()(2)(2

)(2)()()(2

3

322

sFeSaqNOaqNa

aqNOaqFeaqSaqNa

)()()( 22 sFeSaqFeaqS

Chemistry chapter 14 13

Ionization• The process that forms ions from solute molecules by the action of the solvent.

• The attraction between the solvent and the solute is strong enough to break the covalent bonds.

)()( aqClaqHHCl

Chemistry chapter 14 14

Hydronium• H3O+

• Formed when an H+ ion is combined with a water molecule (hydrated).

• Happens instantly when H+ ions are in water.

• Highly exothermic• Formed by many molecular compounds that

ionize

Chemistry chapter 14 15

A more accurate picture

)()( aqClaqHHCl

)()()()( 32 aqClaqOHgHCllOH

Chemistry chapter 14 16

Strong electrolytes• Any compound whose dilute aqueous

solutions conduct electricity well.• All or almost all dissolved compound is in

the form of ions– Not all compound has to dissolve, but the part

that does must be ions

Chemistry chapter 14 17

Weak electrolytes• Any compound whose dilute aqueous

solutions conduct electricity poorly.• A small amount of the dissolved compound

is in the form of ions.

Chemistry chapter 14 18

Be careful!• Strong electrolytes have a high degree of

ionization or dissociation, regardless of their concentration.

• Weak electrolytes have a low degree of ionization or dissociation, regardless of their concentration.

Chemistry chapter 14 19

Colligative Properties• Properties of solutions that depend on the

concentration of solute particles, but not the identity of solute particles.

Chemistry chapter 14 20

Nonvolatile substance• Has little tendency to become a gas under

existing conditions.

Chemistry chapter 14 21

Vapor-pressure lowering• The vapor pressure of a solvent containing a

nonvolatile solute is lower than the vapor pressure of the pure solvent at the same temperature.

• The solute lowers the concentration of solvent molecules at the surface.

• Fewer molecules enter the vapor phase.

Chemistry chapter 14 22

Effects• See figure 14-6 on page 436• The solution remains liquid over a wider

temperature range.• The freezing point is lowered and the

boiling point is raised.

Chemistry chapter 14 23

Molal freezing-point constant

• Kf

• The freezing point depression of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute.

• = -1.86 °C/m for water• 2 molal decreases 3.72 °C

Chemistry chapter 14 24

Freezing-point depression• The difference between the freezing points

of the pure solvent and a solution of a nonelectrolyte in that solvent.

• It is directly proportional to the molal concentration of the solution.

mKt ff

Chemistry chapter 14 25

Example• Determine the freezing point of a water

solution of fructose, C6H12O6 made by dissolving 58.0 g of fructose in 185 g of water.

• -3.24 °C

Chemistry chapter 14 26

You try• Determine the molal concentration of a

solution of ethylene glycol, HOCH2CH2OH, if the solution’s freezing point is -6.40 °C.

• 3.44 m

Chemistry chapter 14 27

Molal boiling-point constant• The boiling-point elevation of the solvent in

a 1-molal solution of a nonvolatile, nonelectrolyte solute.

• Kb = 0.51 °C/m for water

Chemistry chapter 14 28

Boiling-point elevation• The difference between the boiling points of

the pure solvent and a nonelectrolyte solution of that solvent.

• Directly proportional to the molal concentration of the solution

mKt bb

Chemistry chapter 14 29

Example• What is the boiling point of a solution of

25.0 g of 2-butoxyethanol, HOCH2CH2OC4H9, in 68.7 g of ether?

• 40.8 °C

Chemistry chapter 14 30

You try• What mass of glycerol,

CH2OHCHOHCH2OH, must be dissolved in 1.00 kg of water in order to have a boiling point of 104.5 °C?

• 810 g

Chemistry chapter 14 31

Electrolytes• 1 mole of an electrolyte produces more than

one mole of particles in solution.– The ions separate

Chemistry chapter 14 32

Example• A water solution contains 42.9 g of calcium

nitrate dissolved in 500. g of water. Calculate the freezing point of the solution.

• -2.92 °C

Chemistry chapter 14 33

You try• What is the expected boiling point of a

1.70 m solution of sodium sulfate in water?• 102.6 °C

Chemistry chapter 14 34

Actual values• Our expected values are not always what is

observed.• See table 14-3 on page 445• Differences are caused by attractive forces

between ions in solution.

Chemistry chapter 14 35

Semipermeable membrane• Allows the movement of some particles

while blocking the movement of others.• Example: allows water molecules through,

but not sucrose molecules

Chemistry chapter 14 36

Chemistry chapter 14 37

Osmosis• The movement of solvent through a

semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration.

Chemistry chapter 14 38

Osmotic pressure• The external pressure that must be applied

to stop osmosis.• The greater the concentration of a solution,

the greater the osmotic pressure.