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11
The Chemistry The Chemistry of Acids and of Acids and BasesBases
The Chemistry The Chemistry of Acids and of Acids and BasesBases
Chapter 17Chapter 17
22
Acid and BasesAcid and Bases
33
Acid and BasesAcid and Bases
44
Acid and BasesAcid and Bases
55
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
• Generally divide acids and bases into Generally divide acids and bases into STRONG or WEAK ones.STRONG or WEAK ones.
STRONG ACID:STRONG ACID: HNO HNO33(aq) + H(aq) + H22O(liq) --->O(liq) --->
HH33OO++(aq) + NO(aq) + NO33--(aq)(aq)
HNOHNO33 is about 100% dissociated in water. is about 100% dissociated in water.
66
HNO3, HCl, H2SO4 and HClO4 are among the only known strong acids.
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
77
• Weak acids are much less than 100% ionized in Weak acids are much less than 100% ionized in
water.water.
One of the best known is acetic acid = CHOne of the best known is acetic acid = CH33COCO22HH
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
88
• Strong Base:Strong Base: 100% dissociated in 100% dissociated in water.water.
NaOH(aq) ---> NaNaOH(aq) ---> Na++(aq) + OH(aq) + OH--(aq)(aq)
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
Other common strong Other common strong bases include KOH and bases include KOH and Ca(OH)Ca(OH)22..
CaO (lime) + HCaO (lime) + H22O -->O -->
Ca(OH)Ca(OH)22 (slaked lime) (slaked lime)CaOCaO
99
• Weak base:Weak base: less than 100% ionized less than 100% ionized in waterin water
One of the best known weak bases is One of the best known weak bases is ammoniaammonia
NHNH33(aq) + H(aq) + H22O(liq) O(liq) ee NH NH44++(aq) + OH(aq) + OH--(aq)(aq)
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
1010
ACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIES• The most general theory for common The most general theory for common
aqueous acids and bases is the aqueous acids and bases is the BRØNSTED - LOWRY BRØNSTED - LOWRY theorytheory
• ACIDS DONATE HACIDS DONATE H++ IONS IONS
• BASES ACCEPT HBASES ACCEPT H++ IONS IONS
1111
The Brønsted definition means NHThe Brønsted definition means NH33 is a is a BASEBASE in water — and water is itself an in water — and water is itself an ACIDACID
BaseAcidAcidBaseNH4
+ + OH-NH3 + H2OBaseAcidAcidBase
NH4+ + OH-NH3 + H2O
ACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIES
1212
ACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIES
NHNH33 is a is a BASEBASE in water — and water is itself in water — and water is itself an an ACIDACID
NHNH3 3 / NH/ NH44++ is a is a conjugate pairconjugate pair — —
related by the gain or loss of Hrelated by the gain or loss of H++
Every acid has a conjugate Every acid has a conjugate base - and vice-versa.base - and vice-versa.
1313
Conjugate PairsConjugate Pairs
1414
More About WaterMore About Water
HH22O can function as both an ACID and a BASE.O can function as both an ACID and a BASE.
In pure water there can be In pure water there can be AUTOIONIZATIONAUTOIONIZATION
Equilibrium constant for autoion = KEquilibrium constant for autoion = Kww
KKww = [H = [H33OO++] [OH] [OH--] = ] = 1.00 x 101.00 x 10-14-14 at 25 at 25 ooCC
1515
More About WaterMore About Water
KKww = [H = [H33OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 at 25 at 25 ooCC
In a In a neutralneutral solution [Hsolution [H33OO++] = [OH] = [OH--]]
so Kso Kww = [H = [H33OO++]]22 = [OH = [OH--]]22
and so [Hand so [H33OO++] = [OH] = [OH--] = 1.00 x 10] = 1.00 x 10-7-7 M M
OH-
H3O+
OH-
H3O+
AutoionizationAutoionization
1616Calculating [HCalculating [H33OO++] & ] & [OH[OH--]]
You add 0.0010 mol of NaOH to 1.0 L of You add 0.0010 mol of NaOH to 1.0 L of pure water. Calculate [Hpure water. Calculate [H3OO++] and [OH] and [OH--].].
SolutionSolution
2 H2 H22O(liq) O(liq) e e H H3OO++(aq) + OH(aq) + OH--(aq)(aq)
Le Chatelier predicts equilibrium shifts to Le Chatelier predicts equilibrium shifts to the ____________. the ____________.
[H[H3OO++] < 10] < 10-7-7 at equilibrium. at equilibrium.
Set up a ICE table.Set up a ICE table.
1717Calculating [HCalculating [H33OO++] & ] & [OH[OH--]]
You add 0.0010 mol of NaOH to 1.0 L of You add 0.0010 mol of NaOH to 1.0 L of pure water. Calculate [Hpure water. Calculate [H3OO++] and [OH] and [OH--].].
SolutionSolution
2 H2 H22O(liq) O(liq) ee H H33OO++(aq) + OH(aq) + OH--(aq)(aq)
initialinitial 00 0.00100.0010 changechange +x+x +x+x equilibequilib xx 0.0010 + x0.0010 + xKKww = (x) (0.0010 + x) = (x) (0.0010 + x)
Because x << 0.0010 M, assume Because x << 0.0010 M, assume [OH[OH--] = 0.0010 M] = 0.0010 M
[H[H33OO++] = K] = Kww / 0.0010 = 1.0 x 10 / 0.0010 = 1.0 x 10-11-11 M M
1818Calculating [HCalculating [H33OO++] & ] & [OH[OH--]]
You add 0.0010 mol of NaOH to 1.0 L of You add 0.0010 mol of NaOH to 1.0 L of pure water. Calculate [Hpure water. Calculate [H3OO++] and [OH] and [OH--].].
SolutionSolution
2 H2 H22O(liq) O(liq) e e H H3OO++(aq) + OH(aq) + OH--(aq)(aq)
[H[H3OO++] = K] = Kww / 0.0010 = 1.0 x 10 / 0.0010 = 1.0 x 10-11-11 M M
This solution is _________
because
[H3O+] < [OH-]
1919
[H[H33OO++], [OH], [OH--] and pH] and pH
A common way to express acidity and basicity A common way to express acidity and basicity is with pHis with pH
pH = log (1/ [HpH = log (1/ [H33OO++]) ])
= - log [H= - log [H33OO++]]In a neutral solution, In a neutral solution,
[H[H3OO++] = [OH] = [OH--] = ] =
1.00 x 101.00 x 10-7-7 at 25 at 25 ooCC
pH = -log (1.00 x 10pH = -log (1.00 x 10-7-7) ) = = - (-7) = 7- (-7) = 7
2020
[H[H33OO++], [OH], [OH--] and pH] and pH
What is the pH of the What is the pH of the 0.0010 M NaOH solution? 0.0010 M NaOH solution?
[H[H3OO++] = 1.0 x 10] = 1.0 x 10-11-11 M M
pH = - log (1.0 x 10pH = - log (1.0 x 10-11-11) = 11.00) = 11.00
General conclusion —General conclusion —
Basic solution Basic solution pH > 7pH > 7
Neutral Neutral pH = 7pH = 7
Acidic solutionAcidic solution pH < 7pH < 7
2121
Figure 17.1Figure 17.1
2222
[H[H33OO++], [OH], [OH--] and pH] and pH[H[H33OO++], [OH], [OH--] and pH] and pH
If the pH of Coke is 3.12, it is ____________.If the pH of Coke is 3.12, it is ____________.
Because pH = - log [HBecause pH = - log [H3OO++] then] then
log [Hlog [H3OO++] = - pH] = - pH
Take antilog and getTake antilog and get
[H[H3OO++] = 10] = 10-pH-pH
[H[H3OO++] = 10] = 10-3.12-3.12 = =
7.6 x 107.6 x 10-4-4 M M
2323
pH of Common pH of Common SubstancesSubstances
2424
Other pX ScalesOther pX ScalesOther pX ScalesOther pX Scales
In generalIn general pX = -log XpX = -log X
and so and so pOH = - log [OHpOH = - log [OH--]]
KKww = [H = [H3OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 at 25 at 25 ooCC
Take the log of both sidesTake the log of both sides
-log (10-log (10-14-14) = - log [H) = - log [H3OO++] + (-log [OH] + (-log [OH--])])
pKpKww = 14 = pH + pOH = 14 = pH + pOH
2525
Equilibria Involving Equilibria Involving Weak Acids and Weak Acids and
BasesBases
Equilibria Involving Equilibria Involving Weak Acids and Weak Acids and
BasesBases
Aspirin is a good example Aspirin is a good example
of a weak acid, of a weak acid,
KKaa = 3.2 x 10 = 3.2 x 10-4-4
2626
Equilibria Involving Equilibria Involving Weak Acids and BasesWeak Acids and BasesEquilibria Involving Equilibria Involving
Weak Acids and BasesWeak Acids and Bases
AcidAcid Conjugate BaseConjugate Base
acetic, CHacetic, CH33COCO22HH CHCH33COCO22--, acetate, acetate
ammonium, NHammonium, NH44++ NHNH33, ammonia, ammonia
bicarbonate, HCObicarbonate, HCO33-- COCO33
2-2-, carbonate, carbonate
A weak acid (or base) is one that ionizes A weak acid (or base) is one that ionizes to a VERY small extent (< 5%).to a VERY small extent (< 5%).
2727
Equilibria Involving Equilibria Involving Weak Acids and BasesWeak Acids and Bases
Consider acetic acid, CHConsider acetic acid, CH33COCO22H (HOAc)H (HOAc)
HOAc + HHOAc + H22O O ee H H33OO++ + OAc + OAc--
AcidAcid Conj. base Conj. base
Ka [H3O+][OAc- ]
[HOAc] 1.8 x 10-5Ka
[H3O+][OAc- ][HOAc]
1.8 x 10-5
(K is designated K(K is designated Kaa for for ACIDACID))
Because [HBecause [H33OO++] and [OAc] and [OAc--] are SMALL, K] are SMALL, Kaa << 1. << 1.
2828
Equilibrium Constants Equilibrium Constants for Weak Acidsfor Weak Acids
Equilibrium Constants Equilibrium Constants for Weak Acidsfor Weak Acids
Weak acid has KWeak acid has Kaa < 1 < 1
Leads to small [HLeads to small [H33OO++] and a pH of 2 - 7 ] and a pH of 2 - 7
2929
Equilibrium Constants Equilibrium Constants for Weak Basesfor Weak Bases
Equilibrium Constants Equilibrium Constants for Weak Basesfor Weak Bases
Weak base has KWeak base has Kbb < 1 < 1
Leads to small [OHLeads to small [OH--] and a pH of 12 - 7 ] and a pH of 12 - 7
3030Ionization Constants for Ionization Constants for Acids/Bases Acids/Bases
AcidsAcids ConjugateConjugateBasesBases
Increase strength
Increase strength
3131
Relation Relation
of Kof Kaa, K, Kbb, ,
[H[H33OO++] ]
and pHand pH
3232
K and Acid-Base Reactions K and Acid-Base Reactions
Reactions always go from the Reactions always go from the stronger A-B pair (larger K) to the stronger A-B pair (larger K) to the
weaker A-B pair (smaller K).weaker A-B pair (smaller K).
ACIDSACIDS CONJUGATE BASESCONJUGATE BASESACIDSACIDS CONJUGATE BASESCONJUGATE BASES
STRONGSTRONGSTRONGSTRONG
weakweakweakweak
weakweakweakweak
STRONGSTRONGSTRONGSTRONG
3333
K and Acid-Base ReactionsK and Acid-Base ReactionsK and Acid-Base ReactionsK and Acid-Base Reactions
A strong acid is 100% dissociated.A strong acid is 100% dissociated.
Therefore, a Therefore, a STRONG ACIDSTRONG ACID—a good H—a good H++ donor— donor—must have a must have a WEAK CONJUGATE BASEWEAK CONJUGATE BASE—a —a poor Hpoor H++ acceptor. acceptor.
HNOHNO33(aq) + H(aq) + H22O(liq) O(liq) ee H H33OO++(aq) + NO(aq) + NO33--(aq)(aq)
STRONG ASTRONG A basebase acid acid weak Bweak B
•Every A-B reaction has two acids and two Every A-B reaction has two acids and two bases.bases.•Equilibrium always lies toward the weaker pair.Equilibrium always lies toward the weaker pair.•Here K is very large.Here K is very large.
•Every A-B reaction has two acids and two Every A-B reaction has two acids and two bases.bases.•Equilibrium always lies toward the weaker pair.Equilibrium always lies toward the weaker pair.•Here K is very large.Here K is very large.
3434
K and Acid-Base ReactionsK and Acid-Base ReactionsK and Acid-Base ReactionsK and Acid-Base Reactions
We know from experiment that HNOWe know from experiment that HNO33 is a is a strong acid.strong acid.
1.1. It is a stronger acid than HIt is a stronger acid than H33OO++
2.2. HH22O is a stronger base than NOO is a stronger base than NO33--
3.3. K for this reaction is largeK for this reaction is large
WEAK BASE
ACID
STRONG ACID
BASEH3O+ + NO3
-HNO3 + H2OWEAK BASE
ACID
STRONG ACID
BASEH3O+ + NO3
-HNO3 + H2O
3535
K and Acid-Base ReactionsK and Acid-Base ReactionsK and Acid-Base ReactionsK and Acid-Base Reactions
Acetic acid is only 0.42% ionized when [HOAc] = 1.0 M. Acetic acid is only 0.42% ionized when [HOAc] = 1.0 M. It is a It is a WEAK ACIDWEAK ACID
HOAc + HHOAc + H22O O ee H H33OO++ + OAc + OAc--
WEAK AWEAK A basebase acid acid STRONG BSTRONG B
Because [HBecause [H33OO++] is small, this must mean] is small, this must mean
1.1. HH33OO++ is a stronger acid than HOAc is a stronger acid than HOAc
2.2. OAcOAc-- is a stronger base than H is a stronger base than H22OO
3.3. K for this reaction is smallK for this reaction is small
3636
Types of Acid/Base ReactionsTypes of Acid/Base Reactions
Strong acid + Strong baseStrong acid + Strong base
HH++ + Cl + Cl-- + Na + Na++ + OH + OH-- ee H H22O + NaO + Na++ + Cl + Cl--
Net ionic equationNet ionic equation
HH++(aq) + OH(aq) + OH--(aq) (aq) ee H H22O(liq)O(liq)
K = 1/KK = 1/Kww = 1 x 10 = 1 x 101414
Mixing equal molar quantities of a strong acid and strong
base produces a neutral solution.
3737
Types of Acid/Base ReactionsTypes of Acid/Base Reactions
Weak acid + Strong baseWeak acid + Strong base
CHCH33COCO22H + OHH + OH- - ee H H22O + CHO + CH33COCO22--
This is the reverse of the reaction of CHThis is the reverse of the reaction of CH33COCO22--
(conjugate base) with H(conjugate base) with H22O.O.
OHOH-- stronger base than CH stronger base than CH33COCO22--
K = 1/KK = 1/Kbb = 5.6 x 10 = 5.6 x 1044
Mixing equal molar quantities of a weak acid and strong base produces the acid’s conjugate
base. The solution is basic.
Mixing equal molar quantities of a weak acid and strong base produces the acid’s conjugate
base. The solution is basic.
3838
Types of Acid/Base ReactionsTypes of Acid/Base Reactions
Strong acid + Weak baseStrong acid + Weak base
HH33OO++ + NH + NH33 ee H H22O + NHO + NH44++
This is the reverse of the reaction of NHThis is the reverse of the reaction of NH44++
(conjugate acid of NH(conjugate acid of NH33) with H) with H22O.O.
HH33OO++ stronger acid than NH stronger acid than NH44++
K = 1/KK = 1/Kaa = 5.6 x 10 = 5.6 x 1044
Mixing equal molar quantities of a strong acid and weak base produces the bases’s conjugate
acid. The solution is acid.
Mixing equal molar quantities of a strong acid and weak base produces the bases’s conjugate
acid. The solution is acid.
3939
Types of Acid/Base ReactionsTypes of Acid/Base Reactions
Weak acid + Weak baseWeak acid + Weak base
•Product cation = conjugate acid of weak base.Product cation = conjugate acid of weak base.•Product anion = conjugate base of weak acid.Product anion = conjugate base of weak acid.•pH of solution depends on relative strengths pH of solution depends on relative strengths
of cation and anion.of cation and anion.
4040
Types of Acid/Base Reactions: Types of Acid/Base Reactions: SummarySummary
4141
Calculations Calculations with with
Equilibrium Equilibrium ConstantsConstants
pH of an acetic pH of an acetic
acid solution.acid solution.
What are your What are your
observations?observations?
pH of an acetic pH of an acetic
acid solution.acid solution.
What are your What are your
observations?observations?
0.0001 M0.0001 M
0.003 M0.003 M
0.06 M0.06 M
2.0 M2.0 M
a pH meter, Screen 17.9a pH meter, Screen 17.9
4242Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
You have 1.00 M HOAc. Calc. the You have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, Hequilibrium concs. of HOAc, H33OO++, OAc, OAc--, ,
and the pH.and the pH.
Step 1.Step 1. Define equilibrium concs. in ICE Define equilibrium concs. in ICE
table.table.
[HOAc][HOAc] [H[H33OO++]] [OAc[OAc--]]
initialinitial
changechange
equilibequilib
4343Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
You have 1.00 M HOAc. Calc. the You have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, Hequilibrium concs. of HOAc, H33OO++, OAc, OAc--, ,
and the pH.and the pH.
Step 1.Step 1. Define equilibrium concs. in ICE Define equilibrium concs. in ICE
table.table.
[HOAc][HOAc] [H[H33OO++]] [OAc[OAc--]]
initialinitial 1.001.00 00 00
changechange -x-x +x+x +x+x
equilibequilib 1.00-x1.00-x xx xx
Note that we neglect [HNote that we neglect [H33OO++] from H] from H22O.O.
4444Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Step 2.Step 2. Write K Write Kaa expression expression
You have 1.00 M HOAc. Calc. the equilibrium concs. You have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, Hof HOAc, H33OO++, OAc, OAc--, and the pH., and the pH.
Ka 1.8 x 10-5 = [H3O+][OAc- ]
[HOAc]
x2
1.00 - xKa 1.8 x 10-5 =
[H3O+][OAc- ][HOAc]
x2
1.00 - x
This is a quadratic. Solve using quadratic This is a quadratic. Solve using quadratic formula or method of approximations (see formula or method of approximations (see
Appendix A).Appendix A).
4545Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Step 3.Step 3. Solve K Solve Kaa expression expression
You have 1.00 M HOAc. Calc. the equilibrium concs. You have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, Hof HOAc, H33OO++, OAc, OAc--, and the pH., and the pH.
Ka 1.8 x 10-5 = [H3O+][OAc- ]
[HOAc]
x2
1.00 - xKa 1.8 x 10-5 =
[H3O+][OAc- ][HOAc]
x2
1.00 - x
First assume x is very small because First assume x is very small because KKaa is so small. is so small.
Ka 1.8 x 10-5 = x2
1.00Ka 1.8 x 10-5 =
x2
1.00
Now we can more easily solve this Now we can more easily solve this approximate expression.approximate expression.
4646
Equilibria Involving A Weak AcidEquilibria Involving A Weak AcidEquilibria Involving A Weak AcidEquilibria Involving A Weak Acid
Step 3.Step 3. Solve K Solve Kaa approximateapproximate expression expression
You have 1.00 M HOAc. Calc. the equilibrium concs. You have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, Hof HOAc, H33OO++, OAc, OAc--, and the pH., and the pH.
Ka 1.8 x 10-5 = x2
1.00Ka 1.8 x 10-5 =
x2
1.00
x = [x = [HH33OO++] = [] = [OAcOAc--] = [K] = [Kaa • 1.00] • 1.00]1/21/2
x = x = [[HH33OO++] = [] = [OAcOAc--] = 4.2 x 10] = 4.2 x 10-3-3 M M
pH = - log [pH = - log [HH33OO++] = -log (4.2 x 10] = -log (4.2 x 10-3-3) = ) = 2.372.37
4747Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Consider the approximate expression Consider the approximate expression
Ka 1.8 x 10-5 = x2
1.00Ka 1.8 x 10-5 =
x2
1.00 x [H3O+ ] = [Ka • 1.00]1/2x [H3O+ ] = [Ka • 1.00]1/2
For many weak acidsFor many weak acids
[H[H33OO++] = [conj. base] = [K] = [conj. base] = [Kaa • C • Coo]]1/21/2
where Cwhere C00 = initial conc. of acid = initial conc. of acid
Useful Rule of Thumb:Useful Rule of Thumb:
If 100•KIf 100•Ka a < C< Coo, then [H, then [H33OO++] = [K] = [Kaa•C•Coo]]1/21/2
4848Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Equilibria Involving A Weak Equilibria Involving A Weak AcidAcid
Calculate the pH of a 0.0010 M solution of Calculate the pH of a 0.0010 M solution of formic acid, HCOformic acid, HCO22H.H.
HCOHCO22H + HH + H22O O ee HCO HCO22-- + H + H33OO++
KKaa = 1.8 x 10 = 1.8 x 10-4-4
Approximate solutionApproximate solution
[H[H33OO++] ] = = [K[Kaa • C • Coo]]1/21/2 = 4.2 x 10 = 4.2 x 10-4-4 M, M, pH = 3.37pH = 3.37
Exact SolutionExact Solution
[H[H33OO++] = [] = [HCOHCO22--] = 3.4 x 10] = 3.4 x 10-4-4 M M
[[HCOHCO22HH] = 0.0010 - 3.4 x 10] = 0.0010 - 3.4 x 10-4-4 = 0.0007 M = 0.0007 M
pH = 3.47 pH = 3.47
4949
Weak BasesWeak Bases
5050Equilibria Involving A Weak Equilibria Involving A Weak BaseBase
You have 0.010 M NHYou have 0.010 M NH33. Calc. the pH.. Calc. the pH.
NHNH33 + H + H22O O ee NH NH44++ + OH + OH--
KKbb = 1.8 x 10 = 1.8 x 10-5-5
Step 1.Step 1. Define equilibrium concs. in ICE table Define equilibrium concs. in ICE table
[NH[NH33]] [NH[NH44++]] [OH[OH--]]
initialinitial
changechange
equilibequilib
5151Equilibria Involving A Weak Equilibria Involving A Weak BaseBase
You have 0.010 M NHYou have 0.010 M NH33. Calc. the pH.. Calc. the pH.
NHNH33 + H + H22O O ee NH NH44++ + OH + OH--
KKbb = 1.8 x 10 = 1.8 x 10-5-5
Step 1.Step 1. Define equilibrium concs. in ICE table Define equilibrium concs. in ICE table
[NH[NH33]] [NH[NH44++]] [OH[OH--]]
initialinitial 0.0100.010 00 00
changechange -x-x +x+x +x+x
equilibequilib 0.010 - x0.010 - x x x xx
5252Equilibria Involving A Weak Equilibria Involving A Weak BaseBase
You have 0.010 M NHYou have 0.010 M NH33. Calc. the pH.. Calc. the pH.
NHNH33 + H + H22O O e e NH NH44++ + OH + OH--
KKbb = 1.8 x 10 = 1.8 x 10-5-5
Step 2.Step 2. Solve the equilibrium expression Solve the equilibrium expression
Kb 1.8 x 10-5 = [NH4
+][OH- ][NH3 ]
= x2
0.010 - xKb 1.8 x 10-5 =
[NH4+][OH- ]
[NH3 ] =
x2
0.010 - x
Assume x is small (100•KAssume x is small (100•Kbb < C < Coo), so), so
x = [OHx = [OH--] = [NH] = [NH44++] = 4.2 x 10] = 4.2 x 10-4-4 M M
and [NHand [NH33] = 0.010 - 4.2 x 10] = 0.010 - 4.2 x 10-4-4 ≈ 0.010 M ≈ 0.010 M
The approximation is validThe approximation is valid !!
5353Equilibria Involving A Weak Equilibria Involving A Weak BaseBase
You have 0.010 M NHYou have 0.010 M NH33. Calc. the pH.. Calc. the pH.
NHNH33 + H + H22O O ee NH NH44++ + OH + OH--
KKbb = 1.8 x 10 = 1.8 x 10-5-5
Step 3.Step 3. Calculate pH Calculate pH
[OH[OH--] = 4.2 x 10] = 4.2 x 10-4-4 M M
so pOH = - log [OHso pOH = - log [OH--] = 3.37] = 3.37
Because pH + pOH = 14,Because pH + pOH = 14,
pH = 10.63pH = 10.63
5454
MX + HMX + H22O ----> acidic or basic solution?O ----> acidic or basic solution?
Consider NHConsider NH44ClCl
NHNH44Cl(aq) ----> NHCl(aq) ----> NH44++(aq) + Cl(aq) + Cl--(aq)(aq)
(a)(a) Reaction of ClReaction of Cl-- with H with H22OO
ClCl-- + + HH22O ---->O ----> HCl + HCl + OHOH--
basebase acidacid acidacid basebase
ClCl-- ion is a VERY weak base because its ion is a VERY weak base because its conjugate acid is strong. conjugate acid is strong.
Therefore, ClTherefore, Cl-- ----> neutral solution ----> neutral solution
Acid-Base Properties of SaltsAcid-Base Properties of SaltsAcid-Base Properties of SaltsAcid-Base Properties of Salts
5555
NHNH44Cl(aq) ----> NHCl(aq) ----> NH44++(aq) + Cl(aq) + Cl--(aq)(aq)
(b)(b) Reaction of NHReaction of NH44++ with H with H22OO
NHNH44++ + H + H22O ---->O ----> NHNH33 + + HH33OO++
acidacid basebase basebase acidacid
NHNH44++ ion is a moderate acid because its ion is a moderate acid because its
conjugate base is weak. conjugate base is weak.
Therefore, NHTherefore, NH44++ ----> acidic solution ----> acidic solution
See TABLE 17.4 for a summary of See TABLE 17.4 for a summary of acid-base properties of ions.acid-base properties of ions.
Acid-Base Properties of SaltsAcid-Base Properties of Salts
5656
Acid-Base Properties of Acid-Base Properties of SaltsSalts
5757
Calculate the pH of a 0.10 M solution of NaCalculate the pH of a 0.10 M solution of Na22COCO33. .
NaNa++ + H + H22O ---> neutralO ---> neutral
COCO332-2- + + HH22O O ee HCOHCO33
-- + OH + OH--
basebase acidacid acidacid basebase
KKbb = 2.1 x 10 = 2.1 x 10-4-4
Step 1.Step 1. Set up concentration tableSet up concentration table
[CO[CO332-2-]] [HCO[HCO33
--]] [OH[OH--] ] initialinitial
changechange equilibequilib
Acid-Base Properties of SaltsAcid-Base Properties of Salts
5858
Calculate the pH of a 0.10 M solution of NaCalculate the pH of a 0.10 M solution of Na22COCO33. .
NaNa++ + H + H22O ---> neutralO ---> neutral
COCO332-2- + + HH22O O ee HCOHCO33
-- + OH + OH--
basebase acidacid acidacid basebase
KKbb = 2.1 x 10 = 2.1 x 10-4-4
Step 1.Step 1. Set up ICE tableSet up ICE table
[CO[CO332-2-]] [HCO[HCO33
--]] [OH[OH--] ] initialinitial 0.100.10
00 00
changechange -x-x +x+x +x+x
equilibequilib 0.10 - x0.10 - x xx xx
Acid-Base Properties of SaltsAcid-Base Properties of Salts
5959
Calculate the pH of a 0.10 M solution of NaCalculate the pH of a 0.10 M solution of Na22COCO33. .
NaNa++ + H + H22O ---> neutralO ---> neutral
COCO332-2- ++ HH22O O ee HCOHCO33
-- ++ OHOH--
basebase acidacid acidacid basebase
KKbb = 2.1 x 10 = 2.1 x 10-4-4
Acid-Base Properties of SaltsAcid-Base Properties of Salts
Kb = 2.1 x 10-4 = [HCO3
- ][OH- ]
[CO32 ]
x2
0.10 - xKb = 2.1 x 10-4 =
[HCO3- ][OH- ]
[CO32 ]
x2
0.10 - x
Assume 0.10 - x ≈ 0.10, because 100•Kb < Co
x = [HCO3-] = [OH-] = 0.0046 M
Step 2.Step 2. Solve the equilibrium expressionSolve the equilibrium expression
6060
Calculate the pH of a 0.10 M solution of NaCalculate the pH of a 0.10 M solution of Na22COCO33. .
NaNa++ + H + H22O ---> neutralO ---> neutral
COCO332-2- + H+ H22O O ee HCOHCO33
-- + OH + OH--
basebase acidacid acidacid basebase
KKbb = 2.1 x 10 = 2.1 x 10-4-4
Acid-Base Properties of SaltsAcid-Base Properties of Salts
Step 3.Step 3. Calculate the pHCalculate the pH
[OH[OH--] = 0.0046 M] = 0.0046 M
pOH = - log [OHpOH = - log [OH--] = 2.34] = 2.34
pH + pOH = 14, pH + pOH = 14,
so so pH = 11.66pH = 11.66, and the solution is ________., and the solution is ________.