LECTURE TenCHM 151 ©slg

TOPICS:

1. Ions in Aqueous Solutions: Double Replacement Reactions2. Electrolytes 3. Precipitation Reactions

Chapter Five Double Replacement Reactions in Aqueous

Solutions:AB(aq) + CD(aq) AD + CB

A) Precipitation reactionsB) Acid/ Base reactionsC) Gas Formation reactions

NOTE: All these reactions are often represented by a balanced “net ionic equation” which we will meet as we look at these types!

“The Game Players”

Generally, two aqueous solutions, each containing a 100% ionized solute, a “strong electrolyte,” are required to commence action in any of these three types of reactions.

In order to appreciate what is happening, we mustexamine all terms ...

AB(aq) A+(aq) + B- (aq)

CD(aq) C+(aq) + D- (aq)

“AQUEOUS SOLUTIONS”

“AQUEOUS SOLUTION” = homogeneous mixture of some solute in a specific solvent, water

HOMOGENEOUS: uniform composition throughout, one phase. Liquid solution: transparent, no boundaries, layers, bubbles or solid particles visible.

solid solute

liquid solute

gas solute

water SOLVENT

aqueous SOLUTION

+

“Strong Electrolytes”

Double Replacement reactions in aqueous solutions occur when both reactants are “100% ionized” in aqueous solutions.

H2OAB ----------> A+

(aq) + B-(aq)

H2OCD ----------> C+

(aq) + D-(aq)

The solutes described above are called “strong electrolytes”. To better understand this concept, let’s consider the following demonstration and video clip.

SOLUTES TO BE BE TESTED AS ELECTROLYTES:

salt (NaCl), sugar (C12H22O12), hydrochloric acid (HCl), acetic acid (vinegar, HC2H3O2), ammonia (NH3), alcohol (CH3CH2OH), sodium hydroxide (NaOH)

We are going to see what happens to the demo light bulb when the electrodes are immersed in first pure water, and then into water containing these solutes.

DEMONSTRATION!!!

Compounds which form ions in water solution are considered “electrolytes” because their presence allows the solution to conduct electric current.

If there are NO ions present in solution, the liquid or solution will NOT conduct a current and the light bulb will not “light up”

If the solute present in the solution is completelyionized, the solution will readily conduct a currentand the light bulb will “light up brightly”

If the solute is ionized to a small extent (“mostly molecular”), then the light bulb may “glow faintly”

Summary, Results:

pure water

salt water

HCl in water

Acetic Acid in Water

alcohol in water

NaOH in water

Ammonia in water Sugar in Water

We can sort out our results into three categories, based on our observations:

Strong electrolytes: allow current to flow through solution: NaCl, HCl, NaOH, all 100% ionized in solution

Weak Electrolytes: allow a small amount of current to flow through solution: HC2H3O2 and NH3 (aq) as“NH4OH”, small amount of ionic presence, mostly“molecular” in nature

Non Electrolytes: No ions, no current: molecular innature: water, alcohol (CH3CH2OH), sugar (C12H22O12).

Strong Bases: Soluble Metal hydroxides: NaOH, KOH

STRONG ELECTROLYTES: REACTANTS, DOUBLE REPLACEMENT REACTIONS

Salts: metal or ammonium cation , monatomic or polyatomic anion: NaCl K2SO4 AgNO3 NH4BrO3 CuI2

Strong Acids: H written first in the formula, Strong: HCl, HBr, HI, H2SO4, HNO3

Salts in water: Cation, not H+, Anion, not OH-

NaCl(s)H2O Na+

(aq) + Cl-(aq)

H2O

H2O

H2O

Cu(NO3)2(s) Cu2+(aq) + 2 (NO3)-(aq)

Al2(SO4)3(s) 2 Al3+(aq) + 3 (SO4)2-

(aq)

K3PO4(s) 3 K+(aq) + PO4

3-(aq)

BASES IN WATER: Cation not H+; Anion: OH-

KOH(s)H2O

K+(aq) + OH-

(aq)

NaOH (s)H2O Na+(aq) + OH-

(aq)

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

ACIDS IN WATER: H+ Cation; Anion, not OH-

H2SO4(l)H2O

H+(aq) + HSO4

-(aq)

100%

HCl(g)H2O

H+(aq) + Cl-(aq)

HSO4-(aq)

H2O

50%

H+(aq) + SO4

2-(aq)

HC2H3O2(aq) H2O H+(aq) + C2H3O2

-(aq)

99%

Double Replacement Reactions “go to completion”because collisions between some of the mixed anions and cations causes precipitates, molecules or gases to form, removing ions from solution. Our first type of reaction between aqueous solutions containing electrolytes involves forming a precipitatewhen the solutions are mixed.

This type of reaction goes to completion because ions are removed from solution as an insoluble precipitate.

AB(aq) + CD(aq) AD(s) + CB(aq)

[A+(aq) + B- (aq)] + [C+

(aq) + D- (aq)] AD(s) + [C+(aq) + B- (aq)]

Reactions in Aqueous Solutions #1: Precipitation reactions

“salt A(aq) + salt B (aq) salt C (s) + salt D (aq)”

NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3 (aq)

Na +Cl -

Na +Cl -

Na +

Na +

Cl -

Cl - Ag +

Ag +NO 3

-

NO 3-

NO 3-

NO 3-

Ag +

Ag +

AgCl AgClAgCl

AgCl

Na +

Na +

Na +

Na +

NO 3-

NO 3-

NO 3-

NO 3-

+

Net Ionic Equations

When we finish writing and balancing a double replacement reaction, we go several steps further:

After we have completed an equation representing all the ions present in water, we delete all ions present on both sides of the equation.

The final result is called the “net ionic equation”.

We will practice this procedure as we go through thevarious types of double replacement reactions.

AB(aq) + CD(aq) AD(s) + CB(aq)

[A+(aq) + B- (aq)] + [C+

(aq) + D- (aq)] AD(s) + [C+(aq) + B- (aq)]

A+(aq) + D- (aq) AD(s)

Total balanced equation:

Total Ionic Equation:

Net Ionic Equation:

NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3 (aq)

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

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

Ag+(aq) + Cl- (aq) AgCl(s)

Total equation:

Total Ionic Equation:

Net Ionic Equation:

Acids: Mostly water soluble, commercially available in water solution

Salts and Bases: if both cation and anion are large insize and small in charge, ( +1,- 1), it is probably soluble in H2O.

Checkout following tables ...

SOLUBILITY OF “Strong Electrolytes” IN WATER

To predict when a precipitate will form, we need toknow some solubility guidelines:

The Electrolyte is Usually Water Soluble if:

THE CATION IS:

• Na+

• K+

• NH4+

OR THE ANION IS*:

• Cl-, Br-, I-

• ClO4-, ClO3

-

• NO3-

• SO42-

• C2H3O2-

THE COMPOUND IS PROBABLY INSOLUBLE IF:

• THE CATION IS NOT:

• Na+

• K+

• NH4+

• THE ANION IS:

• CO32-

• PO43-

• S2-

• O2-

• OH-

There are a few notable exceptions to thesolubility guide on the last slide, principally theones noted below, which you should be aware of:

Insoluble in Water:

AgCl, AgBr, AgIPbCl2, PbBr2, PbI2

BaSO4, PbSO4

SOLUBILITY WORKSHEET #1

Formula Name H2O Soluble?FeCO3

K2CO3

(NH4)3PO4

Co3(PO4)2

Mg(NO3)2

AgCl

CuI2

Cr2(SO4)3

SOLUBILITY WORKSHEET #1

Formula Name H2O Soluble?

FeCO3 Iron(II) Carbonate NO, carbonate

K2CO3 Potassium Carbonate Yes, K+

(NH4)3PO4 Ammonium Phosphate Yes, NH 4+

Co3(PO4)2 Cobalt(II) Phosphate NO, phosphate

Mg(NO3)2 Magnesium Nitrate Yes, nitrate

AgCl Silver Chloride NO, exception

CuI2 Copper (II) Iodide Yes, iodide

Cr2(SO4)3 Chromium(III) Sulfate Yes, sulfate

SOLUBILITY WORKSHEET #2

Bi(ClO4)3Ag Br

CdSO4

Zn(NO3)2

Ni(NO3)2

KMnO4

BaSO4

Al2O3

Mg3N2

Bi(ClO4)3Ag Br

CdSO4

Zn(NO3)2

Bismuth(III) Perchlorate Yes, PerchlorateSilver Bromide No, ExceptionCadmium Sulfate Yes, SulfateZinc Nitrate, Yes, Nitrate

Ni(NO3)2

KMnO4

BaSO4

Al2O3

Mg3N2

Nickel(II) Nitrate Yes (nitrate)Potassium Permanganate Yes (K+)Barium Sulfate No (exception)Aluminum Oxide No (oxide)Magnesium Nitride No (Nitride)