Transcript 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 reactions
B) Acid/ Base reactions
C) 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.
AB(aq) 
A+(aq) + B- (aq)
CD(aq) 
C+(aq) + D- (aq)
In order to appreciate what is happening, we must
examine all terms ...
“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
water SOLVENT
gas solute
liquid solute
aqueous SOLUTION
“Strong Electrolytes”
Double Replacement reactions in aqueous solutions occur
when both reactants are “100% ionized” in aqueous
solutions.
H2O
AB ----------> A+(aq)
+ B-(aq)
H2O
CD ----------> 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.
DEMONSTRATION!!!
Compounds which form ions in water solution are
considered “electrolytes” because their presence
allows the solution to conduct electric current.
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.
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 completely
ionized, the solution will readily conduct a current
and 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
Ammonia in water
HCl in water
Acetic Acid in Water
Sugar in Water
alcohol in water
NaOH 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 in
nature: water, alcohol (CH3CH2OH), sugar (C12H22O12).
STRONG ELECTROLYTES:
REACTANTS, DOUBLE REPLACEMENT REACTIONS
Salts: metal or ammonium cation , monatomic or
polyatomic anion:
NaCl K2SO4 AgNO3 NH4BrO3 CuI2
Strong Bases: Soluble Metal hydroxides:
NaOH, KOH
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)
Cu(NO3)2(s)
H2O
Cu2+(aq) + 2 (NO3)-(aq)
Al2(SO4)3(s)
H2O
2 Al3+(aq) + 3 (SO4)2-(aq)
K3PO4(s)
H2O
3 K+(aq) + PO43-(aq)
BASES IN WATER: Cation not H+; Anion: OH-
KOH(s)
NaOH (s)
H2O
K+(aq) + OH-(aq)
H2O
Na+(aq) + OH-(aq)
NH3(g) + H2O(l)
NH4+(aq) + OH-(aq)
ACIDS IN WATER: H+ Cation; Anion, not OH-
H2SO4(l)
-
HSO4 (aq)
H2O
100%
H2O
H+(aq) + HSO4-(aq)
H+(aq) + SO42-(aq)
50%
HCl(g)
HC2H3O2(aq)
H2O
H2O
99%
H+(aq) + Cl-(aq)
H+(aq) + C2H3O2-(aq)
Group Work 5.1: Electrolytes in
Solution
Compound
Type
#Ions in
Solution
Compoun Type
d
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
# Ions in
Solution
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 precipitate
when the solutions are mixed.
This type of reaction goes to completion because
ions are removed from solution as an insoluble
precipitate.
Reactions in Aqueous Solutions #1: Precipitation reactions
AB(aq) +
CD(aq)
 AD(s) + CB(aq)
[A+(aq) + B- (aq)] + [C+(aq) + D- (aq)]  AD(s) + [C+(aq) + B- (aq)]
“salt A(aq)
+
salt B (aq)  salt C (s) + salt D (aq)”
NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3 (aq)
+
Na+
Cl-
Cl
Na
+
+
NO3- NO3- Ag
Ag+
NO3+
NO3- Ag
Ag+
-
Cl
+
Na
Cl-
Na+
NO3Na+
NO3-
Na+
Na+
NO3-
NO3Na+
AgC l
AgC l
AgC l
AgC l
Net Ionic Equations
When we finish writing and balancing a double
replacement reaction, we go several steps further:
• 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 the
various types of double replacement reactions.
Total balanced equation:
AB(aq) +
CD(aq)
 AD(s) + CB(aq)
Total Ionic Equation:
[A+(aq) + B- (aq)] + [C+(aq) + D- (aq)]  AD(s) + [C+(aq) + B- (aq)]
Net Ionic Equation:
A+(aq) + D- (aq)  AD(s)
Total equation:
NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3 (aq)
Total Ionic Equation:
Na+ (aq) + Cl- (aq) + Ag+(aq) + NO3- (aq) 
AgCl(s)
+ Na+ (aq) + NO3- (aq)
Net Ionic Equation:
Ag+(aq) + Cl- (aq)  AgCl(s)
SOLUBILITY OF “Strong Electrolytes” IN WATER
To predict when a precipitate will form, we need to
know some solubility guidelines:
Acids: Mostly water soluble, commercially available
in water solution
Salts and Bases: if both cation and anion are large in
size and small in charge, ( +1,- 1), it is probably soluble
in H2O.
Checkout following tables ...
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 the
solubility guide on the last slide, principally the
ones noted below, which you should be aware of:
Insoluble in Water:
AgCl,
PbI2
BaSO4
Group Work 5.2: SOLUBILITY WORKSHEET
#1
Formula Name
FeCO3
K2CO3
(NH4)3PO4
Co3(PO4)2
Mg(NO3)2
AgCl
CuI2
Cr2(SO4)3
H2O Soluble?
GROUP WORK 5.3: SOLUBILITY WORKSHEET #2
Name
Bi(ClO4)3
Ag Br
CdSO4
Zn(NO3)2
Ni(NO3)2
KMnO4
BaSO4
Al2O3
Mg3N2
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 produces
an 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)
PO43- (ppt???)
Decision: No cation to precipitate with the
phosphate 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)
PO43- (ppt???)
Decision: The copper(II) ion will form an insoluble
precipitate with the phosphate ion and therefore
reaction will occur.
Step Two: since reaction will occur to form insoluble
copper(II) phosphate, determine the correct formula for
the products.
Cu2+
(NH4)+
(PO4)3- ---> Cu3(PO4)2
+ (SO4)2- ---> (NH4)2SO4
SMART ACTION: DOUBLE CHECK YOUR FORMULAS!
Charge per ion:
2+
3-
Cu3(PO4)2
Total Charge:
6+
6-
1+ 2(NH4)2SO4
2+
2-
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:
[3 Cu2+ (aq) + 3 SO42- (aq) ] + [6 NH4+ (aq) + 2 PO43- (aq) ]
---> Cu3(PO4)2 (s) + [6 NH4+ (aq) + 3 SO42- (aq) ]
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)
GROUP WORK 5.4: NET IONIC EQUATION
a) Na2CO3 (aq) + Al(NO3)3 ( aq) ---> ?
b) K2CO3 (aq) + NH4NO3 (aq) ---> ?
c) CrCl3 (aq) + K3PO4 (aq) --->?
1. Decide which of above is NR (“no reaction”)
by writing down all four ions involved
For reaction(s) which “go to completion”:
2. Complete Product Formulas
3. Balance Equation
4. Do Total Ionic Equation
5. Do Net Ionic Equation
Reactions in Aqueous Solutions:
#2 Acid/Base Reactions
Acid
+ Base
---> Salt + H2O
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...
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
Note
names:
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 as
substances 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- or
O2- anion.
The list of strong bases in water is severely limited
by the lack of solubility of most metal hydroxides
and oxides in water; all of these compounds
do 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 strong
electrolytes because they are the only ones
soluble in water. Other metal hydroxides
and oxides are basic but insoluble.
AMMONIA AS A WEAK BASE
As we have seen earlier:
NH3(g) + H2O
99%
--->
NH4+(aq)
<-------------
+ OH-(aq)
1%
This reaction with water produces a small number
of hydroxide ions in water solution, so aqueous
ammonia is considered a weak base.
SALTS IN AQUEOUS SOLUTION
All common salts which are water soluble are
strong electrolytes, 100% ionized in the aqueous
solution.
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 molecule
from the H+ of the acid and the OH- or O2- of the
base 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 Equation
4. Do a Total Ionic Equation
5. Do a Net Ionic Equation
Reaction #1 HCl(aq) + KOH(aq) ---> ? ---> 1. H2O + salt
2. Formulas of products: Combine the H and OH to form
water, and the leftover ions to make the salt:
Cl- + K+ ----> KCl
3. Balance the equation (ok as written):
HCl(aq) + KOH(aq) ---> HOH(l)
+
KCl (aq)
4. Total Ionic Equation:
H+(aq) + Cl-(aq) + K+ (aq) + OH- (aq)
-----> H2O(l) + K+ (aq) + Cl- (aq)
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)
Now, another:
1.
H3PO4 (aq) + NaOH(aq) ---> ? -----> H2O + salt
2. Predict the salt formula and complete equation:
( PO4)3H3PO4 (aq) +
+ Na+ ---> Na3PO4
NaOH(aq) ---> H2O(l) + Na3PO4(aq)
3. balance the equation:
H3PO4 (aq) + 3 NaOH(aq) --->3H2O(l) + Na3PO4(aq)
H3PO4 (aq) + 3 NaOH(aq) --->3H2O(l) + Na3PO4(aq)
4. Convert into total ionic equation, noting that the
phosphoric 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 Reactions
2. Acid/Base Reactions
3. 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
Sulfide salt + acid
K2S(aq)
----> new salt + H2S(g)
+ H2SO4 (aq) -----> K2SO4 (aq) + H2S(g)
2 K+(aq) + S2- (aq) + 2 H+2 (aq) + SO42- (aq) ----->
2 K+(aq) + SO42- (aq) + H2S(g)
2 H+2 (aq) + S2- (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)