Ions in Aqueous Solution &

Colligative Properties

Chpt. 14

I. Compounds in aqueous solution

A. Dissociation- separation of an ionic compound that occurs when it dissolves1. Assume 100% dissociation2. indicate ions produced & # of moles

produced

NaCl(s) Na+(aq) + Cl–

(aq)

FeCl3 (s) Fe+3 (aq)+ 3Cl-1(aq)

Precipitation Reactions

Chart p.427 Solubility Guidelines! Chart – one of your 7 pages

3.Not all ionic compounds are soluble in water.

4. Precipitation Reactions occur when one or more ionic comounds are

insoluble. To determine which it is use a solubility chart .• a. precipitate – a solid produced from a

chem reaction in a solution– the solid settles out of the solution

5. Net ionic equation = an equation that includes only those cmpds & ions that undergo a chem change in a reaction in an aq solution• a. Spectator ions = ions that don’t take

part in a chem reaction & are found in solution both BEFORE & AFTER the reaction – these ions are NOT written in a net ionic equation

• b. LOOK FOR A PRECIPITATE! These ions are the ones involved in the reaction!

c. How to write net ionic equations

1) Start with a balanced equation (predict products if needed)

(NH4)2S (s) + Cd(NO3)2(s) water 2NH4NO3 +CdS

2) Write all strong electrolytes as ions (write them dissociated) : electrolytes conduct electricity in solution

Use your Ion Chart!!!• a) Strong Acids: HCl, HBr, HI, and any

oxygen containing acid with 2 or more oxygens (all organic acids --contain carbon– are weak electrolytes)

• b) Strong Bases: hydroxides of group I metals and barium and strontium are strong electrolytes

• c) Soluble salts: NaCl

(NH4)2S (s) → NH4+

(aq) + S-2

(aq)

  Cd(NO3)2(s) → Cd+2

(aq) + 2NO3

-1(aq)

soooooo…

NH4+

(aq) + S-2

(aq) + Cd+2 (aq)

+ 2NO3-1

(aq) double replacement reaction

Hints: Oxides, gases,and water are always written as molecular or undissociated

3) Check your solubility chart to see which new cmpd is insoluble.

NH4+

(aq) +S-2

(aq) + Cd+2 (aq)

+ 2NO3-1

(aq) NH4NO3(aq) + CdS (s)

4) Cross out all spectator ions (ions that are the same on the reactant and product side of the equation –if they stay dissolved then they are considered the same so, look for a precipitate…. something insoluble in water!)

NH4+

(aq) + S-2

(aq) + Cd+2 (aq)

+ 2NO3-1

(aq)

NH4+

(aq) + NO3

-1(aq) + CdS (s)

5) Write the Net Ionic equation using the substances NOT crossed

out!

Cd+2 (aq) + S-2

(aq) CdS (s)

electrolytes

B. Ionization- breaking apart of some polar molecular covalent cmpds into aqueous ions(it’s the creation of ions where none previously existed!)• 1. acids are polar molecules that ionize in

water HNO3(aq) + H2O(l) H3O+

(aq) + NO3–(aq)

• (H3O+ is the hydronium ion we’ll talk about it with acids)

Nonelectrolytes

C. Molecular Solvation• Molecules - the molecules dissolve

in water but they stay intact

• C6H12O6(s) C6H12O6(aq)

II. Colligative Properties

property that depends on the concentration of solute particles, not their identity----

Changes in the behavior of the solutions that are due primarily to the concentration of solute particles rather than the typical properties of those substances are known as colligative properties.

A. Types

1. Freezing Point Depression (tf)• a. f.p. of a solution is lower than f.p. of

the pure solvent

tf = kf · m · n

t: change in temperature (°C) kf: constant based on the solvent

(°C·kg/mol) page p.438 in the book

m: molality (m)

n: # of (ion) particles in solution (1 for non-electrolytes & you must write net ionic equations to find # for electrolytes)

 

Types

Freezing Point Depression

2. Boiling Point Elevation (tb)• b.p. of a solution is higher than b.p. of

the pure solvent

tb = kb · m · n

t: change in temperature (°C) kb: constant based on the solvent

(°C·kg/mol) page p.438 in the book

m: molality (m)

n: # of (ion) particles in solution (1 for non-electrolytes & you must write net ionic equations to find # for electrolytes)

 

Types

Solute particles weaken IMF in the solvent.

Boiling Point Elevation

3. Osmotic pressure - external pressure that must be applied to STOP osmosis– the greater

the # of particles the greater the osmotic pressure

a. osmosis – movement of water through a semipermeable membrane from the side of lower [SOLUTE] to the side of higher [SOLUTE] OR

the diffusion of water through a semipermeable membrane

b. eventually the concentrations of the 2 solutions become equal (equilibrium)

Osmosis

semipermeable membrane

4% starch 10% starch

H2O

Equilibrium is reached.

water flow becomes equal

7% starch

7% starch

H2OO

c. increasing the number of dissolved particles increases the osmotic

pressure

d. Osmotic Pressure of the Blood

1) Cell membranes are semipermeable

2) The osmotic pressure of blood cells cannot change or damage occurs.

3) The flow of water between a red blood cell and its surrounding environment must be equal

H2O

a. Medically 5% glucose and 0.9% NaCl are used (their solute concentrations provide an osmotic pressure equal to that of red blood cells)

b) Hypotonic Solutions

Lower osmotic pressure than red blood cells

Lower concentration of particles than RBCs

In a hypotonic solution, water flows in to the RBC

The RBC undergoes hemolysis; it swells and may

burst.

Greater concentration of water outside cell than

inside so water moves into the cell.

H2O

H2O

Hypertonic Solutions

Has higher osmotic pressure than RBCHas a higher particle concentration In hypertonic solutions, water flows

out of the RBCThe RBC shrinks in size (plasmolysis)Greater concentration of H2O inside cell

than outside so water moves out

B. Applications

1. salting icy roads – lowers the f.p. of water so it must be colder outside in order to form ice

2. making ice cream – lowers the f.p. of ice & causes it to melt – when something melts it is an endothermic process!! It pulls E (heat) from the milk mixture making it colder

3. antifreeze – lowers f.p. of water & increases b.p. of water in radiator

(-64°C to 136°C)

4. Dialysis

a. normal cleaning of the blood occurs when

plasma with small solute particles pass

through a semipermeable membrane into the

kidney

b. Small particles retained inside kidney &

then sent to bladder

c. Hemodialysis is used medically (artificial

kidney) to remove waste particles such as

urea from blood

t: change in temperature (°C)k: constant based on the solvent

(°C·kg/mol) page p.445 in the bookm: molality (m)n: # of (ion) particles in solution (1

for non-electrolytes & you must write net ionic equations to find # for electrolytes)

tf = kf · m · n tb = kb · m · n

Calculations Find the freezing point of a saturated solution

of NaCl containing 28 g NaCl in 100. mL water. (freezing point DEPRESSION)

m = 4.8m

n = 2

tf = kf · m · n

WORK:

m = 0.48mol ÷ 0.100kg

GIVEN:

f.p. = ?

tf = ?

kf = 1.86°C·kg/mol

tf = (1.86°C·kg/mol)(4.8m)(2)

tf = 18°C

f.p. = 0.00°C - 18°C

f.p. = -18°C

Calculations

At what temperature will a solution that is composed of 0.73 moles of glucose in 225 g of phenol boil?

m = 3.2mn = 1tb = kb · m · n

WORK:

m = 0.73mol ÷ 0.225kg

GIVEN:b.p. = ?tb = ?

kb = 3.60°C·kg/moltb = (3.60°C·kg/mol)(3.2m)(1)

tb = 12°C

b.p. = 181.8°C + 12°C

b.p. = 194°C