TOPIC V: Ions in Aqueous Solutions

LECTURE SLIDES:

• Precipitation Reactions• Solubility : Acids, Bases, Salts• Net Ionic Equations• Acid/Base Reactions• Gas Forming Reactions

Kotz & Treichel, Chapter 5, Sections 5.1- 5.6

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%

Group Work 5.1: Electrolytes in Solution

Compound Type #Ions inSolution

Compound

Type # Ions inSolution

HClO4 ACID 1 H +

1 ClO4-

Cu(ClO4)2

Ca(OH)2 Ca (NO3) 2

KHSO4 NH4H2PO4

Fe2(SO4)3 HBrO3

Ni(CH3CO2)2 SnCl4

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

• we prepare a “total ionic equation” in which all soluble electrolytes are representing as separate ions

• we cancel out any ion 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.

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

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+

Always!

OR THE ANION IS*:• Cl-, Br-, I-

• ClO4-, ClO3

-

• NO3-

• SO42-

• CH3CO2-

*Mostly.....

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, PbI2

BaSO4

Group Work 5.2: SOLUBILITY WORKSHEET #1

Formula Name H2O Soluble?FeCO3

K2CO3

(NH4)3PO4

Co3(PO4)2

Mg(NO3)2

AgCl

CuI2

Cr2(SO4)3

GROUP WORK 5.3: SOLUBILITY WORKSHEET #2

Bi(ClO4)3Ag Br

CdSO4

Zn(NO3)2

Ni(NO3)2

KMnO4

BaSO4

Al2O3

Mg3N2

Name H2O Soluble?

Reactions in Aqueous Solutions #1 Precipitation Reactions

This type of reaction goes to completion if and only if any recombination of the reactant ions producesan insoluble precipitate.

Let’s Consider two possible reaction sequences:

K2SO4 (aq) + (NH4)3PO4 (aq)---> ?

CuSO4 (aq) + (NH4)3PO4 (aq)---> ?

K2SO4 (aq) + (NH4)3PO4 (aq)---> ?

Step One: Write out the four reactant ions,decide if any combination is insoluble:

Cations: Anions:

K+ (no ppt) SO42- (usually soluble)

NH4+ (no ppt) PO4

3- (ppt???)

Decision: No cation to precipitate with thephosphate ion, therefore, no reaction!

K2SO4 (aq) + (NH4)3PO4 (aq) ---> NR, no reaction

CuSO4 (aq) + (NH4)3PO4 (aq)---> ?

Step One. Write out each reactant ion. Decide if any combination of ions will produce a precipitate: Cations: Anions:

Cu2+(ppt???) SO42- (usually soluble)

NH4+ (no ppt) PO4

3- (ppt???)

Decision: The copper(II) ion will form an insolubleprecipitate with the phosphate ion and therefore reaction will occur.

Step Two: since reaction will occur to form insolublecopper(II) phosphate, determine the correct formula for the products.

Cu3(PO4)2(NH4)2SO4

Charge per ion: 2+ 3- 1+ 2-

Total Charge: 6+ 6- 2+ 2-

Cu2+ (PO4)3- ---> Cu3(PO4)2

(NH4)+ + (SO4)2- ---> (NH4)2SO4

SMART ACTION: DOUBLE CHECK YOUR FORMULAS!

[3 Cu2+

(aq) + 3 SO4

2- (aq) ] + [6 NH4

+ (aq) + 2 PO4

3- (aq) ]

---> Cu3(PO4)2 (s) + [6 NH4+

(aq) + 3 SO42-

(aq) ]

Step Three: Do a total balanced equation:

3CuSO4 (aq) + 2(NH4)3PO4 (aq)

---> Cu3(PO4)2 (s) + 3 (NH4)2SO4 (aq)

Step Four: Do a total ionic equation, showing the actual species involved in solution:

Step Five: Prepare a “net ionic equation” for this reaction, by eliminating any ion which appears on both sides of the equation,

A “SPECTATOR ION” :

3Cu2+ (aq)

+ 3SO42-

(aq) + 6NH4+

(aq) + 2 PO43-

(aq)

---> Cu3(PO4)2 (s) + 6NH4+

(aq) + 3SO42-

(aq)

NET IONIC EQUATION

3Cu2+ (aq)

+ 2 PO43-

(aq) ---> Cu3(PO4)2 (s)

a) Na2CO3 (aq) + Al(NO3)3 ( aq) ---> ?b) K2CO3 (aq) + NH4NO3 (aq) ---> ?c) CrCl3 (aq) + K3PO4 (aq) --->?

For reaction(s) which “go to completion”:

2. Complete Product Formulas 3. Balance Equation4. Do Total Ionic Equation5. Do Net Ionic Equation

1. Decide which of above is NR (“no reaction”) by writing down all four ions involved

GROUP WORK 5.4: NET IONIC EQUATION

Reactions in Aqueous Solutions: #2 Acid/Base Reactions

These reactions are double replacement, like the precipitation reactions we just studied. However,in this case, the reaction will ALWAYS go to completion because an un-ionized molecule, water, is formed. In both cases (precipitation; acid/base), the removal of ions from solution causes the reaction to go to completion...

Acid + Base ---> Salt + H2O

ACIDS are defined in the most common (“Arrhenius”) system as substances which increase the H+ ion concentration when dissolved in water.

ACIDS may be recognized by the convention of writing H first in the formula of the compound; in general the formula contains H plus some cation except OH-.

Strong acids are 100% ionized in aqueous solutions and therefore “strong electrolytes”;

Weak Acids are generally <5% ionized in aqueous solutions and therefore “weak electrolytes”.

Let’s consider names and formulas of common acids and bases which we meet in these reactions:

COMMON STRONG ACIDS:

HCl Hydrochloric acid

HBr Hydrobromic acid

HI Hydroiodic Acid

HNO3 Nitric Acid

HClO4 Perchloric Acid

H2SO4 Sulfuric Acid

Notenames:learn!

COMMON WEAK ACIDS:*

H3PO4 Phosphoric Acid

H2CO3 Carbonic Acid

H2SO3 Sulfurous Acid

H2S Hydrosulfuric Acid

CH3CO2H Acetic Acid

* and many, many more.....

Learn Names,Recognize!

BASES are defined in the Arrhenius system assubstances which increase the OH- ion concentration when dissolved in water.

BASES may be recognized as the combination of some metal or ammonium cation plus the OH- orO2- anion. The list of strong bases in water is severely limitedby the lack of solubility of most metal hydroxidesand oxides in water; all of these compoundsdo however react with acids to yield salts plus water.

COMMON STRONG BASES

(LiOH Lithium Hydroxide)

NaOH Sodium Hydroxide

KOH Potassium Hydroxide

These are the only bases classified as strongelectrolytes because they are the only onessoluble in water. Other metal hydroxides and oxides are basic but insoluble.

AMMONIA AS A WEAK BASE

As we have seen earlier:

NH3(g) + H2O ---> NH4+

(aq) + OH-(aq)

<-------------

99% 1%

This reaction with water produces a small numberof hydroxide ions in water solution, so aqueous ammonia is considered a weak base.

SALTS IN AQUEOUS SOLUTION

All common salts which are water soluble arestrong electrolytes, 100% ionized in the aqueoussolution.

Salts can be recognized by their formulas, in which a metal or ammonium cation is written first and some anion second.

NET IONIC EQUATIONS FOR ACID BASE REACTIONS

NOTE: If any acid and base are mixed together and at least one of them is in aqueous solution ,a reaction will always occur and go quickly to completion: the formation of the water moleculefrom the H+ of the acid and the OH- or O2- of thebase is very energy releasing and exothermic.

Acid + Base -----> Salt + H2O

Both strong and weak types react in this fashion!

Net Ionic Equations for Acid /Base Reactions

1. Recognize that all reactions between acids and bases go to completion if at least one of them is in aqueous solution.

2. Decide on formula of salt product to accompany the H2O formed.

3. Balance the Equation4. Do a Total Ionic Equation5. Do a Net Ionic Equation

2. Formulas of products: Combine the H and OH to form water, and the leftover ions to make the salt:

Cl- + K+ ----> KCl

4. Total Ionic Equation:

H+(aq) + Cl-

(aq) + K+ (aq) + OH-

(aq)

-----> H2O(l) + K+ (aq) + Cl-

(aq)

3. Balance the equation (ok as written):

HCl(aq) + KOH(aq) ---> HOH(l) + KCl (aq)

Reaction #1 HCl(aq) + KOH(aq) ---> ? ---> 1. H2O + salt

5. IDENTIFY THE SPECTATORS:

H+ (aq) + Cl- (aq) + K+ (aq) + OH- (aq) -----> H2O(l) + K+ (aq) + Cl- (aq)

NET IONIC EQUATION:

H+ (aq) + OH- (aq) -----> H2O(l)

1.H3PO4 (aq) + NaOH(aq) ---> ? -----> H2O + salt

2. Predict the salt formula and complete equation:

( PO4)3- + Na+ ---> Na3PO4

H3PO4 (aq) + NaOH(aq) ---> H2O(l) + Na3PO4(aq)

3. balance the equation:

H3PO4 (aq) + 3 NaOH(aq) --->3H2O(l) + Na3PO4(aq)

Now, another:

H3PO4 (aq) + 3 NaOH(aq) --->3H2O(l) + Na3PO4(aq)

4. Convert into total ionic equation, noting that thephosphoric acid is a weak electrolyte, about 99%molecular, so we do not show its ions in water:

H3PO4 (aq) + 3 Na+(aq) + 3 OH- (aq) --->

3H2O(l) + 3 Na+(aq) + PO43-(aq)

5. NET IONIC EQUATION:

H3PO4 (aq) + 3 OH- (aq) ---> 3H2O(l) + PO43-(aq)

Group Work 5.5

Do Net Ionic Equations for the following:

1. Fe2O3 (s) + HCl(aq) ---> ?

2. H2CO3 (aq) + KOH (aq) --->?

3. H2SO4 (aq) + Ba(OH)2 (s) -----> ?

5.5 Gas-Forming Reactions

Finish up on Net Ionic Equations: third type of double replacement reactions:

1. Precipitation Reactions2. Acid/Base Reactions3. Gas Formation Reactions

Two types considered:

Sulfide salt + acid -----> salt + H2S(g)

Carbonate salt + acid -----> salt + CO2(g) + H2O

Formation of gaseous product causes reaction completion

Sulfide Salt + Acid

2 K+(aq) + S2-

(aq) + 2 H+2 (aq) + SO4

2- (aq) ----->

2 K+(aq) + SO4

2- (aq) + H2S(g)

2 H+2 (aq) + S2-

(aq)-----> H2S(g)

Sulfide salt + acid ----> new salt + H2S(g)

K2S(aq) + H2SO4 (aq) -----> K2SO4 (aq) + H2S(g)

Carbonate Salt plus Acid

Carbonate salt + acid -----> new salt + CO2(g) + H2O

CuCO3(s) + 2 HCl(aq) -----> CuCl2(aq) + [ H2CO3(aq)]

[ H2CO3(aq)] -----> H2O(l) + CO2(g)

CuCO3(s) + 2 HCl(aq) -----> CuCl2(aq) + H2O(l) + CO2(g)

CuCO3(s) + 2 H+ (aq) + 2 Cl- (aq)

-----> Cu2+ (aq) + 2 Cl- (aq) + H2O(l) + CO2(g)

CuCO3(s) + 2 H+ (aq) -----> Cu2+ (aq) + H2O(l) + CO2(g)