acids bases and salts
Post on 03-Oct-2015
26 Views
Preview:
DESCRIPTION
TRANSCRIPT
-
26/02/2012
1
Acids,BasesandSaltsAcids,BasesandSalts
(H bd U it 4 109 182)(Hebden Unit4 page109182)
Acids,BasesandSaltsHebden Unit4(page109182)
We will cover the following topics:
1. Definition of Acids and Bases2. Bronsted-Lowry Acids and Bases
2CHEM0012LectureNotes
-
26/02/2012
2
Acids,BasesandSaltsHebden Unit4(page109182)
Arrhenius Definition of Acids and BasesAn acid is a substance that reacts with water to producephydronium ions, H3O+.
A base is a substance that produces hydroxide ions, OH-,in water.
3CHEM0012LectureNotes
When an acid and base reacts, an ionic compound is produced as one of the products. We call ionic compounds salts.
Hydroxide ionOH-
Acids,BasesandSaltsHebden Unit4(page109182)
Arrhenius Theory of Acid and BasesSupport for the Arrhenius concept comes from measurements ofSupport for the Arrhenius concept comes from measurements of H of neutralization of any strong acid and base. Consider the following reactions:
NaOH(aq) + HCl(aq) NaCl(aq) + H2O() H = -56 kJ/molLiOH(aq) + HBr(aq) LiBr(aq) + H2O() H = -56 kJ/molIf we write the net ionic equation for both of these reactions we
4CHEM0012LectureNotes
If we write the net ionic equation for both of these reactions we see that they are the same
H+(aq) + OH-(aq) H2O()
-
26/02/2012
3
Acids,BasesandSaltsHebden Unit4(page109182)
Monoprotic and Polyprotic acids
f1. Monoprotic acids capable of ionizing 1 H+ ion per molecule of acid.
Example:
HCl (aq) + H2O () H3O+ (aq) + Cl- (aq)
Strong acid = 100% dissociation into ions
5CHEM0012LectureNotes
CH3COOH (aq) + H2O () H3O+ (aq) + CH3COO- (aq)
Weak acid = partial dissociation into ions
Acids,BasesandSaltsHebden Unit4(page109182)
Monoprotic and Polyprotic acids2. Polyprotic acids capable of ionizing more than one H+ ion per yp p g p
molecule of acid.
Example of a Diprotic acid:
H2SO4 (aq) + H2O () H3O+ (aq) + HSO4- (aq)HSO4- (aq) + H2O () H3O+ (aq) + SO42- (aq)
Example of a Triprotic acid:
Youshouldbeabletowrite
aciddissociationreactions.
Payattentionto
6CHEM0012LectureNotes
Example of a Triprotic acid:
H3PO4 (aq) + H2O () H3O+ (aq) + H2PO4 - (aq)H2PO4 - (aq) + H2O () H3O+ (aq) + HPO42- (aq)HPO4 2- (aq) + H2O () H3O+ (aq) + PO43- (aq)
thechargesonalltheions,andbalancethereactions.
-
26/02/2012
4
Acids,BasesandSaltsHebden Unit4(page109182)
Monoprotic and Polyprotic bases
f O1. Monoprotic bases capable of ionizing 1 OH- ion per moleculeof base.
Example:
NaOH (aq) Na+ (aq) + OH- (aq)Nitrogen compounds are bases because they dissolve in water
Strong base = 100% dissociation into ions
7CHEM0012LectureNotes
g p yto produce OH- ions.
NH3 (aq) + H2O () NH4+ (aq) + OH- (aq)CH3NH2 (aq) + H2O () CH3NH3+ (aq) + OH- (aq)
Weak base = partial dissociation into ions
Acids,BasesandSaltsHebden Unit4(page109182)
Monoprotic and Polyprotic bases
f O2. Polyprotic bases capable of ionizing more than one OH- ionper molecule of base.
Example:
Ca(OH)2 (aq) Ca2+ (aq) + 2 OH- (aq)Al(OH)3 (aq) Al3+ (aq) + 3 OH- (aq)
Youshouldbeabletowrite
basedissociationreactions.
8CHEM0012LectureNotes
Payattentiontothechargesonalltheions,andbalancethereactions.
-
26/02/2012
5
Acids,BasesandSaltsHebden Unit4(page109182)
In general,
1 Ioni ation of an acid in ater1. Ionization of an acid in water
HA (aq) + H2O () H3O+ (aq) + A- (aq)2. Ionization of a base in water
B ( ) + H O () BH+ ( ) + OH ( )
9CHEM0012LectureNotes
B (aq) + H2O () BH+ (aq) + OH- (aq)or
B(OH)x (aq) Bx+ (aq) + x OH- (aq)
Acids,BasesandSaltsHebden Unit4(page109182)
Brnsted-Lowry Acids and Bases
When ne trali ation reaction occ rs in aq eo sRecallthat
ArrheniusDefinitionofWhen neutralization reaction occurs in aqueoussolution, Arrhenius definition of acid and baseworks well.
HCl (aq) + NH3 (aq) NH4Cl (aq) + H2O()Hydrochloric ammonia ammonium
acid (base) chloride
AcidandBaseAnacid asasubstancethatreactswithwatertoproduceH3O+.Abase thatreactswithwatertoproduce
10CHEM0012LectureNotes
acid (base) chloride OH.
-
26/02/2012
6
Acids,BasesandSaltsHebden Unit4(page109182)
Brnsted-Lowry Acids and Bases
This same ne trali ation reaction can occ r hen it is not in anThis same neutralization reaction can occur when it is not in an aqueous medium. Therefore, Arrheniuss definition of an acid and a base is too restrictive.
Example: Neutralization reaction of reactants in gas phase
HCl (g) + NH3 (g) NH4Cl (s)
11CHEM0012LectureNotes
Hydrogen ammonia smoke containingchloride gas tiny crystals
gas of ammonium chloride
We need a broader definition!
Acids,BasesandSaltsHebden Unit4(page109182)
Brnsted-Lowry Acids and Bases
What is in common bet een the gaseo s and aq eo s reactions?What is in common between the gaseous and aqueous reactions?
1. Both involve a transfer of a H+ from one reactant to another.
In solution, HCl is completely ionized and the H+ ion is transferred from HCl to NH3.
12CHEM0012LectureNotes
NH3 (aq) + H3O+ (aq) + Cl- (aq) NH4+ (aq) + Cl- (aq) + H2O ()In gas phase,
NH3 (g) + HCl (g) NH4+Cl- (s)
-
26/02/2012
7
Acids,BasesandSaltsHebden Unit4(page109182)
Brnsted-Lowry Acids and Bases
Vie acid base reactions as simpl the transfer of H+ (protons)View acid-base reactions as simply the transfer of H+ (protons) from one species to another.
Brnsted-Lowry Acids and Bases Definitions
Acid is a proton donor.Base is a proton acceptor
13CHEM0012LectureNotes
Base is a proton acceptor.
HCl (g) + NH3 (g) NH4+ Cl- (s)This species donatesa proton to NH3,therefore, it is an acid.
This species acceptsa proton, therefore, it is a base.
Acids,BasesandSaltsHebden Unit4(page109182)
Brnsted-Lowry Acids and Bases
In generalIn general,
HA + B BH+ + A-H+ donor
ACIDH+ donor
ACIDH+ acceptor
BASEH+ acceptor
BASE
14CHEM0012LectureNotes
CONJUGATE ACID-BASE PAIRS
Two species that differ by H+ are called a conjugate pair.
There is a whole table of Brnsted-Lowry Acids and Bases.
-
26/02/2012
8
Acids,BasesandSaltsHebden Unit4(page109182)
Brnsted-Lowry Acids and Bases
G Wng
th o
f aci
d
STR
ON
G WEA
K S
trength of
15CHEM0012LectureNotes
WE
AK
S
tren base S
TRO
NG
Acids,BasesandSaltsHebden Unit4(page109182)
Brnsted-Lowry Conjugate Acids-Bases Pairs
Acid BaseAcid BaseH2ONH4+
HCO3-
H3O+
N H
OH-
NH3CO32-
H2ON2H3-
-H+
16CHEM0012LectureNotes
N2H4H2ONH2-
HCO3-
PO43-
N2H3H3O+
NH3H2CO3HPO42-
+H+
-
26/02/2012
9
Acids,BasesandSaltsHebden Unit4(page109182)
We will cover the following topics:
1. Amphiprotic Substance2. Amphoteric Compounds
17CHEM0012LectureNotes
Acids,BasesandSaltsHebden Unit4(page109182)
1. Amphiprotic SubstanceA substance that can act either as a proton acceptor or a proton p p pdonor.
All amphiprotic substances contain a hydrogen atom.
Example 1 Water, H2O
18CHEM0012LectureNotes
H2O () + HCl (aq) H3O+ (aq) + Cl- (aq)base acid
H2O () + NH3 (aq) NH4+ (aq) + OH- (aq)acid base
-
26/02/2012
10
Acids,BasesandSaltsHebden Unit4(page109182)
1. Amphiprotic SubstanceA substance that can act either as a proton acceptor or a proton p p pdonor.
All amphiprotic substances contain a hydrogen atom.
Example 2 Bicarbonate, HCO3-
19CHEM0012LectureNotes
HCO3- (aq) + H3O+ (aq) H2CO3 (aq) + H2O ()base acid
HCO3- (aq) + OH- (aq) CO32- (aq) + H2O ()acid base
Youshouldbeabletowritetheseequations.
Acids,BasesandSaltsHebden Unit4(page109182)
1. Amphiprotic SubstanceA substance that can act either as a proton acceptor or a proton p p pdonor.
All amphiprotic substances contain a hydrogen atom.
Example 3 hydrogen sulfate ion, HSO4-
20CHEM0012LectureNotes
HSO4- (aq) + H3O+ (aq) H2SO4 (aq) + H2O ()base acid
HSO4- (aq) + OH- (aq) SO42- (aq) + H2O ()acid base
Youshouldbeabletowritetheseequations.
-
26/02/2012
11
Acids,BasesandSaltsHebden Unit4(page109182)
2. Amphoteric Substance A compound that can react as either an acid or a base.p Ampho- is Greek, it means both. Metals have amphoteric oxides. (eg ZnO, Al(OH)3)
Example 1 Zinc oxide, ZnO
Reacts with acid: ZnO(aq) + 2H3O+(aq) Zn2+(aq) + 3H2O()
21CHEM0012LectureNotes
acts as a base
Reacts with base: ZnO (aq) + 2 OH- (aq) + H2O () [Zn(OH)4]2- (aq)
acts as an acid zincate ion
Acids,BasesandSaltsHebden Unit4(page109182)
2. Amphoteric Substance A compound that can react as either an acid or a base.p Ampho- is Greek, it means both. Metals have amphoteric oxides. (eg ZnO, Al(OH)3)
Example 2 Water, H2O
Reacts with acid: H2O () + HCl (aq) H3O+ (aq) + Cl- (aq)
22CHEM0012LectureNotes
acts as a base
Reacts with base: H2O () + NH3 (aq) NH4+ (aq) + OH- (aq)acts as an acid
-
26/02/2012
12
Acids,BasesandSaltsHebden Unit4(page109182)
2. Amphoteric Substance A compound that can react as either an acid or a base.p Ampho- is Greek, it means both. Metals have amphoteric oxides. (eg ZnO, Al(OH)3)
Example 3 Aluminum hydroxide, Al(OH)3
Reacts with acid: Al(OH) (aq) + 3HCl (aq) AlCl (aq) + 3 H O ()
23CHEM0012LectureNotes
Al(OH)3 (aq) + 3HCl (aq) AlCl3 (aq) + 3 H2O ()acts as a base
Reacts with base: Al(OH)3 (aq) + OH- (aq) [Al(OH)4] - (aq)
acts as an acid aluminate ion
Acids,BasesandSaltsHebden Unit4(page109182)
Strong acidsAn acid which dissociates 100% into its ions in a water solution.An acid which dissociates 100% into its ions in a water solution.
HCl (aq) + H2O () H3O+ (aq) + Cl- (aq)The [H+] (or [H3O+]) concentration of a 0.10 M HCl solution is 0.10 M.
For monoprotic strong acids [acid] = [H+]
Strong acid = 100% dissociation into ions
24CHEM0012LectureNotes
A common notation to express [H+] is pH, where
pH = -log [H+] = -log 0.10 = 1.00
-
26/02/2012
13
Acids,BasesandSaltsHebden Unit4(page109182)
Recall the pH scalepH = - log [H+]
25CHEM0012LectureNotes
0.1 M 0.01 M
For an increase in 1 pH unit, the solutions [H+] decreases 10x.
0.001 M 10-9 M 10-11 M
10-10 M
Acids,BasesandSaltsHebden Unit4(page109182)
Strong acids Ka
These are strong acids
26CHEM0012LectureNotes
These are strong acids.
Dissociation is a single arrow to produce ions.
-
26/02/2012
14
Acids,BasesandSaltsHebden Unit4(page109182)
Weak acidsAn acid which dissociates less than100% into its ions in a watersolution Weak acid = < 100% dissociation into ionssolution.
CH3COOH (aq) + H2O () H3O+ (aq) + CH3COO- (aq)The [H+] (or [H3O+]) concentration of a 0.10 M CH3COOHsolution is LESS THAN 0.10 M.
Weak acid < 100% dissociation into ions.Use double arrows to indicate this.
27CHEM0012LectureNotes
Therefore, pH >1.00
In weak acids [acid] [H+]
Acids,BasesandSaltsHebden Unit4(page109182)
Weak acidsAn acid which dissociates less than100% into its ions in a watersolutionsolution.
HA (aq) + H2O (l) H3O+ (aq) + A- (aq)
28CHEM0012LectureNotes
-
26/02/2012
15
Acids,BasesandSaltsHebden Unit4(page109182)
Weak acids Ka
.
.
.These are weak acids exceptthe bottom two.
29CHEM0012LectureNotes
These last two are too weak to act as acids in water.
Note the single arrows pointing backward because the forward dissociation never occurs.
Acids,BasesandSaltsHebden Unit4(page109182)
Illustration of Strong and Weak Acids
Graphic representation of the behavior of acids of different strengths in aqueous solution. (a) A strong acid. (b) A k id
30CHEM0012LectureNotes
(b) A weak acid.
-
26/02/2012
16
Acids,BasesandSaltsHebden Unit4(page109182)
Illustration of Strong and Weak Acids
31CHEM0012LectureNotes
(a) A strong acid HA is completely ionized in water.
(b) A weak acid HB exists mostly as undissociated HB molecules in water.
Note that the water molecules are not shown in this figure.
(a) (b)
Acids,BasesandSaltsHebden Unit4(page109182)
Strong basesA base which dissociates 100% into its ions in a water solution.A base which dissociates 100% into its ions in a water solution.
NaOH (aq) Na+ (aq) + OH- (aq)The [OH-] concentration of a 0.10 M NaOH solution is 0.10 M.
For NaOH, [base] = [OH-]
Strong base = 100% dissociation into ions
32CHEM0012LectureNotes
A common notation to express [OH-] is pOH, where
pOH = -log [OH-] = -log 0.10 = 1.00
-
26/02/2012
17
Acids,BasesandSaltsHebden Unit4(page109182)
pH and pOH scales[H+] > [OH-] [OH-] > [H+]
NEUTRAL
[H+] = [OH-]
33CHEM0012LectureNotes
basicacidic
Convert between pH and pOH: pH + pOH = 14 at 25C
Acids,BasesandSaltsHebden Unit4(page109182)
Strong bases
These are strong bases.
Any substance which dissociates completely to produce OH-, O2-, or NH - is a strong base
34CHEM0012LectureNotes
or NH2 is a strong base.Examples: Group 1 hydroxides: LiOH, NaOH, KOH, RbOH, CsOH
Group 2 hydroxides: Mg(OH)2, Ba(OH)2, Sr(OH)2
-
26/02/2012
18
Acids,BasesandSaltsHebden Unit4(page109182)
Strong basesFor NaOH, [OH-] = [base]For NaOH, [OH ] [base]For Ba(OH)2, [OH-] = 2 x [base]
Salts which produce O2- are strong bases.
Example: CaO (s) Ca+2 (aq) + O2- (aq)
35CHEM0012LectureNotes
O2- (aq) + H2O () 2 OH- (aq)
Acids,BasesandSaltsHebden Unit4(page109182)
Weak basesA base which dissociates less than100% into its ions in a watersolution Weak bases = < 100% dissociation into ionssolution.
NH3 (aq) + H2O () NH4+ (aq) + OH- (aq)The [OH-] concentration of a 0.10 M NH3 solution is LESS THAN 0.10 M.
Weak bases < 100% dissociation into ions.Use double arrows to indicate this.
36CHEM0012LectureNotes
Therefore, pOH >1.00
In weak bases [base] [OH-]
-
26/02/2012
19
Acids,BasesandSaltsHebden Unit4(page109182)
Weak basesA base which dissociates less than100% into its ions in a watersolutionsolution.
B (aq) + H2O () BH+ (aq) + OH- (aq)
37CHEM0012LectureNotes
Acids,BasesandSaltsHebden Unit4(page109182)
Weak bases Ka
.These are weak bases found
Conjugate bases of strong acids are VERY weak bases.
38CHEM0012LectureNotes
.
.These are weak bases, foundon the right side of the table.
-
26/02/2012
20
Acids,BasesandSaltsHebden Unit4(page109182)
Amphiprotic Species
f ( SO ) Found on both sides of the table (eg HSO4-) Can act as acids or as bases When found on the left, it is an acid.
Ka
39CHEM0012LectureNotes
Acid Strengthincreases
HCO3- is a weaker acid than phenol and a stronger acid than hydrogen peroxide.
Acids,BasesandSaltsHebden Unit4(page109182)
Amphiprotic Species
f ( SO ) Found on both sides of the table (eg HSO4-) Can act as acids or as bases When found on the right, it is a base.
Ka
40CHEM0012LectureNotes
Base Strengthincreases
HCO3- is a weaker base than citrate ion and a stronger base than Al(H2O)5(OH)2+.
-
26/02/2012
21
Acids,BasesandSaltsHebden Unit4(page109182)
Brnsted-Lowry Acids and Bases
G Wng
th o
f aci
d
STR
ON
G WEA
K S
trength of
41CHEM0012LectureNotes
WE
AK
S
tren base S
TRO
NG
Acids,BasesandSaltsHebden Unit4(page109182)
We will work with the reference table
RELATIVE STRENGTHS OF BRNSTED-LOWRY ACIDS AND BASES
to determine the Acid-Base Equilibria and Relative Strengths of Acids and Bases.
By examining the relative strengths of the acids on both sides of
42CHEM0012LectureNotes
By examining the relative strengths of the acids on both sides of the equilibrium, we can determine which side of the equilibrium is favoured.
-
26/02/2012
22
Acids,BasesandSaltsHebden Unit4(page109182)
1. When H2CO3 and SO32- are mixed, an equilibrium is set up. Which side of the equilibrium is favoured?
H CO ( ) SO 2 ( ) HCO ( ) HSO ( )
ThiscanonlydonateaH+ion.Itisan
ACID.
ThisioncanonlyactasaBASE,acceptaH+ ion.
H2CO3 (aq) + SO32- (aq) HCO3- (aq) + HSO3- (aq)
HCO i th
Asaresult
43CHEM0012LectureNotes
HCO3 istheconjugatebaseofH2CO3.
HSO3 istheconjugateacidofSO3.
Acids,BasesandSaltsHebden Unit4(page109182)
1. When H2CO3 and SO32- are mixed, an equilibrium is set up. Which side of the equilibrium is favoured?
H CO (aq) + SO 2- (aq) HCO - (aq) + HSO - (aq)H2CO3 (aq) + SO32 (aq) HCO3 (aq) + HSO3 (aq)
acid base conjugate base conjugate acid
Compare the Ka of the two acids
44CHEM0012LectureNotes
H2CO3 is a slightly stronger acid than HSO3-. The equilibrium will favour the products.
-
26/02/2012
23
Acids,BasesandSaltsHebden Unit4(page109182)
1. When H2CO3 and SO32- are mixed, an equilibrium is set up. Which side of the equilibrium is favoured?
H CO ( ) SO 2 ( ) HCO ( ) HSO ( )H2CO3 (aq) + SO32- (aq) HCO3- (aq) + HSO3- (aq)acid base conjugate base conjugate acid
What is the value of K for this equilibrium?
45CHEM0012LectureNotes
Since K >1, equilibrium favors products
Acids,BasesandSaltsHebden Unit4(page109182)
2. When H2PO4- and CO32- are mixed, an equilibrium is set up. Which side of the equilibrium is favoured?
H PO ( ) CO 2 ( ) HCO ( ) HPO 2 ( )
Thisisanamphiprotic ion.ItcandonateoracceptaH+ ion.
ThisioncanonlyactasaBASE,acceptaH+ ion.
H2PO4- (aq) + CO32- (aq) HCO3- (aq) + HPO42- (aq)
CO i h O 2 i h
Asaresult
46CHEM0012LectureNotes
HCO3 istheconjugateacidofCO32.
HPO42 istheconjugatebaseofH2PO4.
Afterconsidering,inthismixture,H2PO4 willactas
anacid.
-
26/02/2012
24
Acids,BasesandSaltsHebden Unit4(page109182)
2. When H2PO4- and CO32- are mixed, an equilibrium is set up. Which side of the equilibrium is favoured?
H PO - (aq) + CO 2- (aq) HCO - (aq) + HPO 2- (aq)H2PO4 (aq) + CO32 (aq) HCO3 (aq) + HPO42 (aq)acid base conjugate acid conjugate base
Compare the Ka of the two acids
47CHEM0012LectureNotes
H2PO4- is a stronger acid than HCO3-. The equilibrium will favour the products.
Acids,BasesandSaltsHebden Unit4(page109182)
3. When HSO4- and NH3 are mixed, an equilibrium is set up.Which side of the equilibrium is favoured?
HSO - (aq) + NH (aq) NH + (aq) + SO 2- (aq)Thisisan
amphiprotic ion.ItcandonateoracceptaH+ ion.
Thiscanonlyactasa
BASE,acceptaH+ ion.
HSO4- (aq) + NH3 (aq) NH4+ (aq) + SO42- (aq) Asaresult
48CHEM0012LectureNotes
SO42 istheconjugatebaseofHSO4.
NH4+ istheconjugateacidofNH3.
Afterconsidering,inthismixture,HSO4 willactas
anacid.
-
26/02/2012
25
Acids,BasesandSaltsHebden Unit4(page109182)
3. When HSO4- and NH3 are mixed, an equilibrium is set up.Which side of the equilibrium is favoured?
HSO ( ) NH ( ) NH + ( ) SO 2 ( )HSO4- (aq) + NH3 (aq) NH4+ (aq) + SO42- (aq)acid base conjugate acid conjugate base
Compare the Ka of the two acids
49CHEM0012LectureNotes
HSO4- is a stronger acid than NH4+ . The equilibrium will favour the products.
Acids,BasesandSaltsHebden Unit4(page109182)
4. When HSO4- and H2PO4- are mixed, an equilibrium is set up.Which side of the equilibrium is favoured?
Both HSO4- and H2PO4- are amphiprotic ionsBoth HSO4 and H2PO4 are amphiprotic ions. Which will play the role of an acid? Lets compare their Ka values.
HSO4- is a stronger acid than H2PO4-. HSO4- will act as an acid donating protons to H2PO4-.
50CHEM0012LectureNotes
Afterconsidering,inthismixture,HSO4 willactasanacid.H2PO42 willactasthebase.
-
26/02/2012
26
Acids,BasesandSaltsHebden Unit4(page109182)
4. When HSO4- and H2PO4- are mixed, an equilibrium is set up.Which side of the equilibrium is favoured?
HSO - (aq) + H PO - (aq) H PO (aq) + SO 2- (aq)
Ka ofHSO4 isbigger.Ithasthehighertendencytod t H+
Ka ofH2PO4 issmaller.
Comparatively,ithaslesstendencyto
HSO4 (aq) + H2PO4 (aq) H3PO4 (aq) + SO42 (aq)
Asaresult
51CHEM0012LectureNotes
donateH+. ydonateH+. SO42 isthe
conjugatebaseofHSO4.
H3PO4 istheconjugate
acidofH2PO4.Afterconsidering,in
thismixture,HSO4willactasanacid.
Acids,BasesandSaltsHebden Unit4(page109182)
4. When HSO4- and H2PO4- are mixed, an equilibrium is set up.Which side of the equilibrium is favoured?
HSO ( ) H PO ( ) H PO ( ) SO 2 ( )
Compare the Ka of the two acids
HSO4- (aq) + H2PO4- (aq) H3PO4 (aq) + SO42- (aq) acid base conjugate acid conjugate base
52CHEM0012LectureNotes
HSO4- is a stronger acid than H3PO4 . The equilibrium will favour the products.
-
26/02/2012
27
Acids,BasesandSaltsHebden Unit4(page109182)
5. When CH3COOH and NH3 are mixed, an equilibrium is set up. Which side of the equilibrium is favoured?
CH3COOH (aq) + NH3 (aq) CH3COO- (aq) + NH4+ (aq)CH3COOH (aq) + NH3 (aq) CH3COO (aq) + NH4 (aq)
Compare the Ka of the two acids
CH COOHH+ + CH COO- 1 8x10-5
acid base conjugate base conjugate acid
53CHEM0012LectureNotes
CH3COOH is a stronger acid than NH4+. The equilibrium will favour the products.
CH3COOH H + CH3COO 1.8x10NH4+H+ + NH3 5.6x10-10
Acids,BasesandSaltsHebden Unit4(page109182)
6. Consider an amino acid, an equilibrium is set up between the amine group, R-NH2, and the carboxylic acid group, R-COOH .
Which side of the equilibrium is favoured?
base acid conjugate acid conjugate base
Compare the Ka of the two acidsCH COOH H+ + CH COO- 1 8x10-5
H2NCHRCOOH (aq) +H3NCHRCOO- (aq)
54CHEM0012LectureNotes
CH3COOH is a stronger acid than NH4+. The equilibrium will favour the products (i.e. a double charged species).
CH3COOH H + CH3COO 1.8x10NH4+ H+ + NH3 5.6x10-10
-
26/02/2012
28
Acids,BasesandSaltsHebden Unit4(page109182)
Direction of Reaction Example
Predict the direction favored in each of the following acid-base reactions That is does the reaction tend to go more in the forwardreactions. That is, does the reaction tend to go more in the forward or in the reverse direction?
a) NH4+ + OH- H2O + NH3b) HSO4- + NO3- HNO3 + SO42-c) HSO3- + CH3COO- SO32- + CH3COOH
Forward
Reverse
Reverse
55
d) CH3COOH + Cr(H2O)5(OH)2+ Cr(H2O)63+ + CH3COO-e) HNO2 + ClO4- HClO4 + NO2-f) H2CO3 + CO32- HCO3- + HCO3-
Reverse
Reverse
Forward
Acids,BasesandSaltsHebden Unit4(page109182)
Inaqueoussolutions,H3O+ (aq)&OH (aq)arethestrongest acids and bases that exist.strongestacidsandbasesthatexist.
StrongerbasesreactwithwatertoformOH (aq)O2 (aq)+H2O()OH (aq)+OH (aq)
Stronger acids react with water to form H3O+ (aq).
56
StrongeracidsreactwithwatertoformH3O (aq).HCl (aq)+H2O() H3O+ (aq)+Cl (aq)
ThisisknownastheLeveling Effect
-
26/02/2012
29
Acids,BasesandSaltsHebden Unit4(page109182)
The Self-Ionization of Water
57
Acids,BasesandSaltsHebden Unit4(page109182)
Ion Product of WaterH2O()+ H2O() H3O+(aq) + OH-(aq)
K= [H2O][H2O][H3O+][OH-]
KW= K[H2O][H2O] = [H3O+][OH-] = 1x10-14 at 25C
Consider HA(aq) + H2O() A-(aq) + H3O+(aq) Ka
58
A-(aq) + H2O() HA(aq) + OH-(aq) Kb
baw
w33
ba
KKK
KOHOHA
OHHAHA
OHAKK
-
26/02/2012
30
Acids,BasesandSaltsHebden Unit4(page109182)
pH and pOH
Thepotentialofthehydrogenionwasdefinedin1909asthenegativeofthelogarithmof[H+].
pH = -log[H3O+] pOH = -log[OH-]
-logKW = -log[H3O+]-log[OH-]= -log(1 0x10-14)
KW = [H3O+][OH-] = 1.0x10-14 at 25C
logKW log[H3O ] log[OH ] log(1.0x10 )
pKW = pH + pOH= -(-14)
pKW = pH + pOH = 14 at 25C
Acids,BasesandSaltsHebden Unit4(page109182)
Concentration and pH and pOH Examples at 25C
[H+] [OH] pH pOH Acidic/Basic[H ] [OH ] pH pOH Acidic/Basic
pH=5.52
0.15MHNO3
0.0100MCa(OH)2
1.0x1010 MCa(OH)2
-
26/02/2012
31
Acids,BasesandSaltsHebden Unit4(page109182)
Concentration and pH and pOH Examples at 25C
[H+] [OH] pH pOH Acidic/Basic[H ] [OH ] pH pOH Acidic/Basic
pH=5.52 3.0x106M 3.3x109M 5.52 8.48 Acidic
0.15MHNO3 0.15M 6.7x1014M 0.82 13.18 Acidic
0.0100MCa(OH)2 5.00x1013M 2.00x102M 12.301 1.699 Basic
1.0x1010 MCa(OH)2 1x107M 1x107M 7 7 Neutral
Acids,BasesandSaltsHebden Unit4(page109182)
Strong Acid and Bases Examples
1. CalculatethepHofasolutionof0.400gofHIin500.mlofsolution.
2. Calculatethethe massofSr(OH)2 whichmustbedissolvedin600.mlofsolutiontomakeapHof12.00.
-
26/02/2012
32
Acids,BasesandSaltsHebden Unit4(page109182)
Weak Acid and Bases Examples
1. Butyricacid,HC4H7O2(orCH3CH2CH2COOH)isusedtomakecompoundsemployedinartificialflavoringsandsyrups.A0.250MaqueoussolutionofHC4H7O2isfoundtohaveapHof2.72.DetermineKA forbutyricacid.
2. CalculatethepHofa0.20MsolutionofKF,KbforF is1.5x1011.
Acids,BasesandSaltsHebden Unit4(page109182)
Hydrolysis is the reaction between water and the ions contained in th t l ti Wh thi thi ld lt ithe water solution. When this occurs, this could result in producing an acidic or basic solution.
Spectator ions do NOT undergo hydrolysis. They are:1. Group 1 (alkali metals): Li+, Na+, K+, Rb+, Cs+, Fr+2. Group 2 (alkaline-earth metals): Be2+, Mg2+, Ca2+, Ba2+, Sr2+
64CHEM0012LectureNotes
3. Conjugate bases of strong acids: ClO4-, I-, Br-, Cl-, NO3-
-
26/02/2012
33
Acids,BasesandSaltsHebden Unit4(page109182)
Cation that undergo hydrolysis
NH4+, ammonia
Hydrolysis equation:
NH4+ (aq) + H2O () H3O+ (aq) + NH3 (aq)
65CHEM0012LectureNotes
Acids,BasesandSaltsHebden Unit4(page109182)
Hydrated cations that undergo hydrolysis
Transition metals have smaller ions and bigger chargesThi tt t H O l l This attracts H2O molecules
Example Fe3+ or iron (III) ions forms
Fe3+ (aq) + 6 H2O () Fe(H2O)63+ (aq)Hexaaquoiron,oriron(III)ionisaweakacid.
66CHEM0012LectureNotes
Other hydrated ions are chromium (III) andaluminum (III) ions
-
26/02/2012
34
Acids,BasesandSaltsHebden Unit4(page109182)
Anions that undergo hydrolysis
.
.
.Allionsinthissectioncanundergo
hydrolysis.
67CHEM0012LectureNotes
Anions that are not amphiprotic will act as weak bases in water.
Acids,BasesandSaltsHebden Unit4(page109182)
Anions that undergo hydrolysis
Examples of net ionic hydrolysis equations are:
IO3- (aq) + H2O () HIO3 (aq) + OH- (aq) NO2- (aq) + H2O () HNO2 (aq) + OH- (aq) CH COO ( ) + H O () CH COOH ( ) + OH ( )
68CHEM0012LectureNotes
CH3COO- (aq) + H2O () CH3COOH (aq) + OH- (aq)
-
26/02/2012
35
Acids,BasesandSaltsHebden Unit4(page109182)
Anions that undergo hydrolysis
These2ionsactasSTRONGBases.Theyundergo100%hydrolysistoformOH ions.
69CHEM0012LectureNotes
Anions that are not amphiprotic will act as weak bases in water.
Acids,BasesandSaltsHebden Unit4(page109182)
Anions that do not undergo hydrolysis
70CHEM0012LectureNotes
These5ionsdoNOTundergohydrolysis.Theyarespectators.
-
26/02/2012
36
Acids,BasesandSaltsHebden Unit4(page109182)
Amphiprotic anions that undergo hydrolysis
These anions start with H and have a - charge.
Examples: HSO4-, HSO3-, H2PO4-, HPO42-, HS- etc.
Amphiprotic anions hydrolyze as acids to produce H3O+, butThey also hydrolyze as bases to produce OH-. Compare the Ka and Kb values to determine the predominant hydrolysis.
71CHEM0012LectureNotes
Find the Ka of the ion by looking for the ion on the LEFT side of theacid table.Find the Kb of the ion by looking for the ion on the RIGHT side of the acid table. (Kb = Kw/Ka (conjugate acid))
Acids,BasesandSaltsHebden Unit4(page109182)
1. Is the salt NaCl acidic, basic or neutral in water?
NaCl (s) Na+ (aq) + Cl- (aq)( ) ( q) ( q)
Na+ andCl ionsarespectatorions.
Asaresult
72CHEM0012LectureNotes
Nohydrolysisreactionoccurs.Solutionisneutral.
-
26/02/2012
37
Acids,BasesandSaltsHebden Unit4(page109182)
2. Is the salt NaF acidic, basic or neutral in water?What is the net ionic equation?
NaF (s) Na+ (aq) + F- (aq)( ) ( q) ( q)Na+ ionsarespectatorions.
F ionsarefoundontherightsideoftheacidtable.Itisaweakbase.
As a result
73CHEM0012LectureNotes
F undergoeshydrolysisreaction.F (aq)+H2O() HF(aq)+OH (aq)
Solutionisbasic.
Asaresult
Acids,BasesandSaltsHebden Unit4(page109182)
3. Is the salt NH4NO3 acidic, basic or neutral in water?What is the net ionic equation?
NH4NO3 (s) NH4+ (aq) + NO3- (aq)4 3 ( ) 4 ( q) 3 ( q)NO3 ionsarespectatorions.
NH4+ ionsarefoundontheleftsideoftheacidtable.Itisaweakacid.
74CHEM0012LectureNotes
NH4+ undergoeshydrolysisreaction.NH4+ (aq)+H2O() NH3 (aq)+H3O+ (aq)
Solutionisacidic.
Asaresult
-
26/02/2012
38
Acids,BasesandSaltsHebden Unit4(page109182)
4. Is the salt NaHC2O4 acidic, basic or neutral in water?What is the net ionic equation?
NaHC2O4 (s) Na+ (aq) + HC2O4- (aq)2 4 ( ) ( q) 2 4 ( q)Na+ ionsarespectatorions.
HC2O4 ionsarefoundontherightandleftsideoftheacidtable.Itcaneitherbeaweakacidoraweakbase.
75CHEM0012LectureNotes
Ka >Kb.HC2O4 (aq)+H2O() H2C2O4 (aq)+H3O+ (aq)
Solutionisacidic.
Asaresult,compareKa andKb valuesofHC2O4.
13-2-
14
b5-
a x107.1x109.510K ;x104.6K
Acids,BasesandSaltsHebden Unit4(page109182)
5. Is the salt NH4NO2 acidic, basic or neutral in water? Write the net ionic equation.
NH4NO2 (s) NH4+ (aq) + NO2- (aq)4 2 ( ) 4 ( q) 2 ( q)
NH4+ ionsarefoundontheleftsideoftheacidtable.NO2 ionsarefoundontherightsideoftheacidtable.
76CHEM0012LectureNotesKa >Kb.Solutionisacidic.
Asaresult,compareKa ofNH4+ Kb ofNO2.
11-4-
14
b10-
a x102.2x106.410K ;x106.5K
-
26/02/2012
39
Acids,BasesandSaltsHebden Unit4(page109182)
6. Is the HCO3- ion acidic, basic or neutral in water?
HCO ions are found on the right and left side of the acid tableHCO ions are found on the right and left side of the acid tableHCO3 ionsarefoundontherightandleftsideoftheacidtable.
Asaresult,compareKa andKb ofHCO3.1410
HCO3 ionsarefoundontherightandleftsideoftheacidtable.
77CHEM0012LectureNotes
Kb >Ka.HCO3 (aq)+H2O() H2CO3 (aq)+OH (aq)
Solutionisbasic.
8-7-b
10- a x103.2x103.4
10K ;x106.5K
Acids,BasesandSaltsHebden Unit4(page109182)
Salt ExamplesPredict whether the following solutions are acidic or basic?
) N OCl N + h d l i OCl i j t b f HOCla) NaOCl
b) KCl
c) NH4Br
Na+ no hydrolysis, OCl- is conjugate base of HOCl.
Therefore solution is basic.
K+ no hydrolysis, Cl- no hydrolysis.
Therefore solution is neutral.
Br- no hydrolysis (HBr is strong acid) , ) 4
d) NH4CN
y y ( g ) ,
NH4+ is a weak acid. Therefore solution is acidic.
Both NH4+ and CN- hydrolyze in aqueous solutions. Must compare the values of Ka(NH4+) = 5.6x10-10and Kb(CN-) = 2.0x10-5. Since Kb>Ka the solution is basic.
-
26/02/2012
40
Acids,BasesandSaltsHebden Unit4(page109182)
Acids,BasesandSaltsHebden Unit4(page109182)
Is the salt Na2CO3 acidic, basic or neutral in water?Write the net ionic equation.
Na2CO3 (s) 2 Na+ (aq) + CO32- (aq)2 3 ( ) ( q) 3 ( q)Na+ ionsarespectatorions.
CO32 ionsarefoundontherightsideoftheacidtable.Itisaweakbase.
As a result
80CHEM0012LectureNotes
CO32 undergoeshydrolysisreaction.ThenetionicequationisCO32 (aq)+H2O() HCO3 (aq)+OH (aq)
Solutionisbasic.
Asaresult
-
26/02/2012
41
Acids,BasesandSaltsHebden Unit4(page109182)
Calculate the pH of 0.30 M Na2CO3.
Step 1: Identify the ions involved
Spectatorion Foundontherightsideoftheacid
table;undergobaseh d l i
Na2CO3 (s) 2 Na+ (aq) + CO32- (aq)
Step 1: Identify the ions involved.
81CHEM0012LectureNotes
hydrolysis
Acids,BasesandSaltsHebden Unit4(page109182)
Calculate the pH of 0.30 M Na2CO3.
Step 2: Write the net ionic equation for the hydrolysis. Set upICE table.
CO32-(aq) + H2O () HCO3- (aq) + OH- (aq)CO32(aq)+H2O() HCO3 (aq)+OH (aq)
82CHEM0012LectureNotes
[I] 0.30 0 0[C] x +x +x[E] 0.30 x x x
Stoichiometricratio
-
26/02/2012
42
Acids,BasesandSaltsHebden Unit4(page109182)
Calculate the pH of 0.30 M Na2CO3.
Step 3: Determine the Kb for the base hydrolysis.
CO32-(aq) + H2O () HCO3- (aq) + OH- (aq)Look up in the acid table the Ka for HCO3-
83CHEM0012LectureNotes
4-11-
-14
3a
wb 10 x 1.786 10 x 5.6
10 x 0.1)HCO(K
KK
Acids,BasesandSaltsHebden Unit4(page109182)
Calculate the pH of 0.30 M Na2CO3.
Step 4: Write the Kb expression for the base hydrolysis.
CO32-(aq) + H2O () HCO3- (aq) + OH- (aq)
]CO[]OH[][HCOK 2
-3
b
84CHEM0012LectureNotes
]CO[ 23
-
26/02/2012
43
Acids,BasesandSaltsHebden Unit4(page109182)
Calculate the pH of 0.30 M Na2CO3.
Step 5: Substitute in the equilibrium concentrations from the ICE table.
CO32(aq)+H2O() HCO3 (aq)+OH (aq)[I] 0.30 0 0
[C] x +x +x[E] 0 30 x x x
85CHEM0012LectureNotes
4-2
23
-3
b 10 x 1.786 x- 0.30x
]CO[]OH[][HCOK
[E] 0.30 x x x
Acids,BasesandSaltsHebden Unit4(page109182)
Calculate the pH of 0.30 M Na2CO3.
Step 6: Solve for x, where x = [OH-].
4-2
4-2
23
-3
b
10 x 786.10 30x
10 x 1.786 x- 0.30
x]CO[
]OH[][HCOK
Useassumption0.30 x 0.30
86CHEM0012LectureNotes
0.30 because[0.30]>1000x1.786x104
[OH-] = x = 7.319 x 10-3 M
-
26/02/2012
44
Acids,BasesandSaltsHebden Unit4(page109182)
Calculate the pH of 0.30 M Na2CO3.
Step 7: From the [OH-], calculate the pOH.
pOH = -log (7.319 x 10-3) = 2.1355
Step 8: Convert pOH to pH.
pH = 14 pOH = 14 2 1355 = 11 86
87CHEM0012LectureNotes
pH = 14 pOH = 14 2.1355 = 11.86
Step 9: Check that your answer makes sense.We expect CO32- ions will undergo basehydrolysis. pH > 7.
Acids,BasesandSaltsHebden Unit4(page109182)
Dissolve 45.0 g of NH4Cl in 1.50 L of water. What is the pH of this solution? Molar mass of NH4Cl is 53.5 g/mole.
88CHEM0012LectureNotes
-
26/02/2012
45
Acids,BasesandSaltsHebden Unit4(page109182)
Dissolve 45.0 g of NH4Cl in 1.50 L of water. What is the pH of this solution? Molar mass of NH4Cl is 53.5 g/mole.
Step 1: Determine the concentration of the solution.
ClNHM5610mole 0.841
mole 841.0g/mole 53.5
g 45.0
89CHEM0012LectureNotes
ClNHM561.0L 1.50 4
Acids,BasesandSaltsHebden Unit4(page109182)
Dissolve 45.0 g of NH4Cl in 1.50 L of water. What is the pH of this solution? Molar mass of NH4Cl is 53.5 g/mole.
Step 2: Identify the ions involved.
NH4Cl (s) NH4+ (aq) + Cl- (aq)
SpectatorionFoundontheleftside of the acid
90CHEM0012LectureNotes
sideoftheacidtable;undergoacid
hydrolysis
-
26/02/2012
46
Acids,BasesandSaltsHebden Unit4(page109182)
Dissolve 45.0 g of NH4Cl in 1.50 L of water. What is the pH of this solution? Molar mass of NH4Cl is 53.5 g/mole.
Step 3: Write the net ionic equation for the hydrolysis.Set up ICE table.
NH4+ (aq) + H2O () NH3 (aq) + H3O+ (aq)NH4+ (aq)+H2O() NH3 (aq)+H3O+ (aq)
91CHEM0012LectureNotes
[I] 0.561 0 0[C] x +x +x[E] 0.561 x x x
Stoichiometricratio
Acids,BasesandSaltsHebden Unit4(page109182)
Dissolve 45.0 g of NH4Cl in 1.50 L of water. What is the pH of this solution? Molar mass of NH4Cl is 53.5 g/mole.
Step 4: Determine the Kb for the base hydrolysis.
NH4+ (aq) + H2O () NH3 (aq) + H3O+ (aq)Look up in the acid table the Ka for NH4+
92CHEM0012LectureNotes
Ka
-
26/02/2012
47
Acids,BasesandSaltsHebden Unit4(page109182)
Dissolve 45.0 g of NH4Cl in 1.50 L of water. What is the pH of this solution? Molar mass of NH4Cl is 53.5 g/mole.
Step 5: Write the Ka expression for the acid hydrolysis.
NH4+ (aq) + H2O () NH3 (aq) + H3O+ (aq)
]OH[][NHK 33
93CHEM0012LectureNotes
]NH[][][K
4
33a
Acids,BasesandSaltsHebden Unit4(page109182)
Dissolve 45.0 g of NH4Cl in 1.50 L of water. What is the pH of this solution? Molar mass of NH4Cl is 53.5 g/mole.
Step 6: Substitute in the equilibrium concentrations from the ICE table.
NH4+ (aq)+H2O() NH3 (aq)+H3O+ (aq)[I] 0.561 0 0[C] x + x + x
94CHEM0012LectureNotes
10-2
4
33a x106.5x - 0.561
x]NH[
]OH[][NHK
[ ] x +x +x[E] 0.561 x x x
-
26/02/2012
48
Acids,BasesandSaltsHebden Unit4(page109182)
Dissolve 45.0 g of NH4Cl in 1.50 L of water. What is the pH of this solution? Molar mass of NH4Cl is 53.5 g/mole.
Step 7: Solve for x, where x = [H3O+].
10-2
4
33a x106.5x - 0.561
x]NH[
]OH[][NHK
Make assumption: 0.561 x 0.5612
95CHEM0012LectureNotes
10-2
a x106.5 0.561xK
[H3O+] = x = 1.8 x 10-5 M
Checkassumption:[Initial]>1000xKa
Assumption is valid because0.561 > 1000 x 5.6 x 10-10
Acids,BasesandSaltsHebden Unit4(page109182)
Dissolve 45.0 g of NH4Cl in 1.50 L of water. What is the pH of this solution? Molar mass of NH4Cl is 53.5 g/mole.
Step 8: Convert to pH.
[H3O+] = x = 1.8 x 10-5 M
pH = - log (1.8 x 10-5 ) = 4.74
96CHEM0012LectureNotes
-
26/02/2012
49
Acids,BasesandSaltsHebden Unit4(page109182)
Titrations
Equivalence point:Equivalencepoint: Thepointinthereactionatwhichbothacidandbasehavebeen
consumed. Neitheracidnorbaseispresentinexcess.
Endpoint: Thepointatwhichtheindicatorchangescolor.
Titrant:
97CHEM0012LectureNotes
Thesolutionaddedtothesolutionintheflask. TitrationCurve:
AplotofpHofthesolutionbeinganalyzedasafunctionoftheamountoftitrantadded.
Acids,BasesandSaltsHebden Unit4(page109182)
Acid-Base Indicators ColorofsomesubstancesdependsonthepH.p p
HIn(aq)+H2O() In(aq)+H3O+(aq) >90%acidformthecolorappearstobetheacidcolor >90%baseformthecolorappearstobethebasecolor Intermediatecolorisseeninbetweenthesetwostates.
Completecolorchangeoccursover2pHunitsM k h d i f i i b h i
98CHEM0012LectureNotes
Markstheendpointofatitrationbychangingcolor.
Theequivalencepointisnot necessarilythesameastheendpoint.
-
26/02/2012
50
Acids,BasesandSaltsHebden Unit4(page109182)
Theacidandbaseformsoftheindicatorphenolphthalein.Intheacidform(Hln),themoleculeiscolorless.Whenaproton(plusH2O)isremovedtogivethebaseform(ln),thecolorchangestoi k
99CHEM0012LectureNotes
pink.
Acids,BasesandSaltsHebden Unit4(page109182)
100CHEM0012LectureNotes
-
26/02/2012
51
Acids,BasesandSaltsHebden Unit4(page109182)
Titration of a Strong Acid with a Strong Base
The pH has a low value at the beginning ThepHhasalowvalueatthebeginning. ThepHchangesslowly
untiljustbeforetheequivalencepoint. ThepHrisessharply
perhaps6unitsper0.1mL additionoftitrant. The pH rises slowly again.
101CHEM0012LectureNotes
ThepHrisesslowlyagain. AnyAcidBaseIndicatorwilldo.
AslongascolorchangeoccursbetweenpH4and10.
Acids,BasesandSaltsHebden Unit4(page109182)
The pH curve for the titration of 100 0 mL oftitration of 100.0 mL of 0.10 M HCl with 0.10 MNaOH.
Note: At the equivalence
point pH = 7
102CHEM0012LectureNotes
point pH 7 Either indicator will
give a fairly accurate result
-
26/02/2012
52
Acids,BasesandSaltsHebden Unit4(page109182)
For the titration of a strong base with a strong acid the titration curve is just flipped over.
The pH curve
103CHEM0012LectureNotes
The pH curve for the titration of 100.0 mL of 0.50 M NaOHwith 1.0 M HCl.
Acids,BasesandSaltsHebden Unit4(page109182)
Weak Acid - Strong Base Titration
Step 1 A stoichiometry problem reaction is Step1 Astoichiometry problem reactionisassumedtoruntocompletion thendetermineremainingspecies.
Step2 An equilibriumproblem determineposition of eak acid eq ilibri m and calc late
104CHEM0012LectureNotes
positionofweakacidequilibriumandcalculatepH.
-
26/02/2012
53
Acids,BasesandSaltsHebden Unit4(page109182)
The pH curve for the titration of 50.0 mL of 0.100 M HC2H3O2 with
Weak Acid - Strong Base Titration
2 3 20.100 M NaOH.
Note: At the equivalence point pH > 7 Choice of indicator is important pKa = pH at the point halfway to
the equivalence pointTh H i f i l t t d
105CHEM0012LectureNotes
The pH is fairly constant around halfway to the equivalence point i,.e it is a buffer solution
Acids,BasesandSaltsHebden Unit4(page109182)
Comparison of strong and weak acid titration curves.
106CHEM0012LectureNotes
-
26/02/2012
54
Acids,BasesandSaltsHebden Unit4(page109182)
The pH curves for the titrations of 50.0-mL samples of 0.10 Macids with various Kavalues with 0.10 MNaOH.
107CHEM0012LectureNotes
Acids,BasesandSaltsHebden Unit4(page109182)
For the titration of a weak base with a strong acid thestrong acid the titration curve is just flipped over.
The pH curve for the titration of 100.0 mL of 0.050 M NH3 with 0.10 M
108CHEM0012LectureNotes
HCl.
Note at equivalence point pH < 7
-
26/02/2012
55
Acids,BasesandSaltsHebden Unit4(page109182)
Titration of a Weak Polyprotic AcidNaOHNaOH NaOH
H3PO4 H2PO4- HPO42- PO43-3 4 2 4 4 4
109CHEM0012LectureNotes
Acids,BasesandSaltsHebden Unit4(page109182)
Buffer Solutions TwocomponentsystemsthatchangepHonly
li htl dditi f id bslightlyonadditionofacidorbase. Thetwocomponentsmustnotneutralizeeachotherbutmustneutralizestrongacidsandbases.
Aweakacidanditsconjugatebase.A k b d it j t id
110CHEM0012LectureNotes
Aweakbaseanditsconjugateacid
Afteradditionofstrongacidorbase,dealwithstoichiometry first,thenequilibrium.
-
26/02/2012
56
Acids,BasesandSaltsHebden Unit4(page109182)
How A Buffer Works
111CHEM0012LectureNotes
Acids,BasesandSaltsHebden Unit4(page109182)
The Henderson-Hasselbalch EquationA variation of the ionization constant expression.
Consider a hypothetical weak acid, HA, and its salt NaA:HA(aq) H+(aq) + A-(aq)
HAAlogHlogKlog
HAAHK aa
112CHEM0012LectureNotes
acidconjugate baseconjugatelogpKpHHAAlogpKpHor
HAAlogpHpK
a
aa
-
26/02/2012
57
Acids,BasesandSaltsHebden Unit4(page109182)
The Henderson-Hasselbalch Equation
Onlyusefulwhenyoucanuseinitial concentrationsofacidy yandsalt. Thislimitsthevalidityoftheequation.
Limitscanbemetby: 10A10
113CHEM0012LectureNotes
aa K10HAandK10Abothand
10HA
1.0
Acids,BasesandSaltsHebden Unit4(page109182)
Characteristics of the Henderson-Hasselbalch Equation
pHdependsonlyontheratio[A]/[HA] ThismeansthatthepHofabuffersolutiondoesnotchangeondilution.
When[A]=[HA]thenpH=pKa.Thisisalsothepointwheretheratiochangesleastupontheadditionofeitheracidorbase.ThereforeasystemisbestatbufferingatapH=pKa.
pH changes as the Log of the ratio [A]/[HA] Therefore
114CHEM0012LectureNotes
pHchangesastheLogoftheratio[A ]/[HA].ThereforepHisnotverysensitivetothevalueof[A]/[HA]
Thelargerthevalueof[A]and[HA]themoreacidorbasethatcanbeaddedwithoutsignificantlychangingthepH(i.e.thegreaterthebufferingcapacityofthesolution).
-
26/02/2012
58
Acids,BasesandSaltsHebden Unit4(page109182)
Buffer Solutions
ConsiderthecaseofaceticacidCH3COOH(aq)+H2O() H3O+ (aq)+CH3COO(aq)
pKa =4.75
[CH3COO]/[CH3COOH] pH0.10 5
3.754 45
115CHEM0012LectureNotes
0.51210
4.454.755.055.75
Acids,BasesandSaltsHebden Unit4(page109182)
Preparing a Buffer Solution of a Desired pH
What mass of NaC2H3O2 must be dissolved in 0.300 L of 0.25 MWhatmassofNaC2H3O2 mustbedissolvedin0.300Lof0.25MHC2H3O2 (Ka =1.8x105)toproduceasolutionwithpH=5.09?(Assumethatthesolutionvolumeisconstantat0.300L)
116CHEM0012LectureNotes
top related