Bowen_Chpt 13_Ions in Aqueous Solution and Colligative Properit

Ions in Aqueous

Solution and

Colligative

Properties

13-1 Compound in Aqueous 13-2 Colligative Properies

What is Dissolving ?

When an ionic compound (eg salt) dissolves in water, the compound disassociates. (breaks apart into cations and anions)

Ex: Ca(NO₃)_2(s) Ca2+

(aq)+ 2NO3-(aq)

When a covalent compound (eg. sugar) dissolves in water, the molecules simply

disperse due to attraction with polar water molecules. They do not disassociate. 

An Ionic Compound

Dissolves:

A Covalent Compound

Dissociation - is the separation of ions that occurs when an ionic compound is dissolved

CaCl_2(s)  Ca2+

(aq) + 2Cl-(aq)

1mol 1mol 2mol

“dissociated ions” or

Ionization- when the ions are formed from the molecular solute, by the action of the solvent. This is different from dissociation.

Spectator Ions- ions that do not take part in the rxn.

AgNO_{3 (aq)} + NaCl_(aq) AgCl_(s) + NaNO_3(aq)

What happens when you put AgNO₃ and NaCl in water?

“Complete Molecular Equation”

Ag+

NO₃

-Na+

-Ag+

NO₃

-Na+

-AgCl NO₃

AgCl NO₃

-Na+

NO₃

-Na+

NO₃

-Na+ NO3

-Na+

NO₃

-Na+

Na+ NO

3

-Na+

NO₃- Na+

NO₃ -Na+

NO₃

-Na+

AgNO_{3 (aq)} + NaCl_(aq) AgCl_(s) + NaNO_3(aq)

complete ionic equation

_{Complete ionic equation- more accurately shows}

the reacting species as ions and the products either as

ions or a precipitate

Ag+ _(aq) + NO₃- _(aq) + Na+ _(aq) + Cl- _(aq)

NO₃- Na+

AgCl

AgCl

NO₃- Na+

AgCl

NO₃- Na+

These ions do not participate in the reaction. They are called

The Net Ionic Equation is constructed from the complete ionic equation:

NO₃- and Na+ are not participating in the reaction

What are they called?????? Spectator Ions Ag+ (aq) + NO₃- (aq) + Na+ (aq) + Cl- (aq)

net ionic equation

Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq)

AgCl (s) + NO3- (aq) + Na+ (aq)

PREDICTING THE FORMATION OF A PRECIPITATE

• _{Consider the possibility that a precipitate}

may form.

• _{Must use the solubility table to decide}

• _{Possible outcomes}

– _{No visible reaction (nvr)} – _{Formation of one ppt.}

Complete Molecular equation

AgNO

+

KCl



AgCl

+

KNO

Complete ionic equation

Ag+

(aq)+NO3(aq)+K+(aq)+Cl-(aq)AgCl(s)+K+(aq)+NO3- (aq)

Net ionic equation

Ag+

(aq) + Cl-(aq)



(leave out spectator ions)

__Na₂CO_{3( )} + __FeCl_{2( )}  __FeCO_{3( )} + 2_NaCl_{( )}

Na+

(aq)+CO3

2-(aq)+Fe +2

(aq) + 2 Cl

-(aq) FeCO3(s)+2 Na

+ _{+ 2Cl} -(aq)

Fe+2

(aq) + CO3

Colligative

Properties

of

Solutions

Colligative Properties of Solutions

Colligative properties are those that depend on the concentration of particles in a solution, not upon the identity of those particles.

Vapor Pressure Lowering Boiling Point Elevation

Vapor Pressure Lowering

• _{A nonvolatile substance added to a solvent has a lower}

vapor pressure than a pure solution

• _{As the number of solute particles increase the}

proportion of solvent (water) particles decrease

• _{Solute particles disrupts solution}

• _{This lowers the vapor pressure over the solution}

• _{Lower vapor pressure results in solution staying in the}

Freezing Point Depression

 Solute added to solvent (water) disrupts the crystallization which takes place as water

freezes

 Each mole of solute particles lowers(-) the freezing point of 1 kilogram of water by 1.86o

Celsius.

K_f = -1.86 C  kilogram/mol

m = molality of the solution

i = # of particles

solute

f

m

K

i

T







Boiling Point Elevation

 Solute added to solvent (water) lowers vapor

pressure by occupying surface area and increased attraction with solvent

 Each mole of solute particles raises the boiling

point of 1 kilogram of water by 0.51oCelsius.

K_b = 0.51 C  kilogram/mol

m = molality of the solution

i = # of particles

solute

b

m

K

i

T







Freezing Point Depression and Boiling Point Elevation Constants, _C/m

DNW

Solvent K_f (_C/m) K_b (C/m)

Acetic acid 3.90 3.07

Benzene 5.12 2.53

Nitrobenzene 8.1 5.24

Phenol 7.27 3.56

Water 1.86 0.512

The van’t Hoff Factor,

i

Electrolytes may have two, three or more times the effect on boiling point,

Dissociation Equations and the Determination of i

NaCl(s) 

AgNO₃(s) 

MgCl₂(s) 

Na₂SO₄(s) 

AlCl₃(s) 

Na+_{(aq) + Cl}-_(aq)

Ag+_{(aq) + NO}

3-(aq)

Mg2+_{(aq) + 2 Cl}-_(aq)

2 Na+_{(aq) + SO}

42-(aq)

Al3+_{(aq) + 3 Cl}-_(aq)

i = 2

i = 2

i = 3

i = 3

Example

What is the freezing point depression of a sodium chloride solution when 74.8 grams of NaCl are

Example

What is the boiling point elevation of a solution

Osmotic Pressure

• A semipermeable membrane allows the

passage of some particles

• The movement of a solvent from lower to

higher solute concentration is osmosis

• Osmotic pressure is the pressure that must

be supplied to stop osmosis from occurring

• The greater the concentration of solution

Osmotic Pressure Calculations

 = Osmotic pressure

M = Molarity of the solution

R = Gas Constant = 0.08206 Latm/molK

i = # of particles

iMRT

I. Colligative Properties

a. properties that depend on the

concentration of solute particles but not on their identity.

A. Vapor Pressure Lowering

i. The vapor pressure of a liquid is the pressure of a vapor in equilibrium with the liquid phase.

ii. The vapor pressure of a solvent is

B. Freezing Point Depression

i. The freezing point (T_f) of a pure solvent is lowered by the addition of a solute which is

insoluble in the solid solvent, and the

measurement of this difference is called

C. Boiling Point Elevation

i. which means that it is dependent on the

presence of dissolved particles and their number, but not their identity.

C. Osmotic Pressure

i.The osmotic pressure of a solution is the difference in pressure between the solution and the pure liquid solvent when the two are in equilibrium across a semi permeable

membrane, which allows the passage of

ii. If the two phases are at the same initial pressure, there is a net transfer of solvent