TOPIC V: Ions in Aqueous Solutions
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Transcript of TOPIC V: Ions in Aqueous Solutions
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)