Additional Aspects of Aqueous Equilibria. Reaction of Weak Bases with Water The base reacts with...

Additional Aspects Additional Aspects of Aqueous of Aqueous EquilibriaEquilibria

Reaction of Weak Bases with Reaction of Weak Bases with WaterWater

The base reacts with water, producing The base reacts with water, producing its conjugate acid and hydroxide ion:its conjugate acid and hydroxide ion:

CH3NH2 + H2O CH3NH3+ + OH- Kb = 4.38 x 10-

4

4 3 3

3 2

[ ][ ]4.38 10

[ ]b

CH NH OHK x

CH NH

KKbb for Some Common Weak for Some Common Weak BasesBases

Base FormulaConjugat

e AcidKb

Ammonia   NH3  NH4+  1.8 x 10-5 

 Methylamine  CH3NH2  CH3NH3+  4.38 x 10-4 

 Ethylamine  C2H5NH2  C2H5NH3+  5.6 x 10-4 

 Diethylamine  (C2H5)2NH  (C2H5)2NH2+  1.3 x 10-3 

 Triethylamine   (C2H5)3N   (C2H5)3NH+  4.0 x 10-4 

 Hydroxylamine  HONH2   HONH3+

   1.1 x 10-8 

 Hydrazine H2NNH2  H2NNH3+

   3.0 x 10-6 

 Aniline  C6H5NH2   C6H5NH3+

   3.8 x 10-10 

 Pyridine  C5H5N   C5H5NH+    1.7 x 10-9 

Many students struggle with identifying weak bases and their conjugate acids.What patterns do you see that may help you?

Reaction of Weak Bases with Reaction of Weak Bases with WaterWater

The generic reaction for a base The generic reaction for a base reacting with water, producing its reacting with water, producing its conjugate acid and hydroxide ion:conjugate acid and hydroxide ion:

B + H2O BH+ + OH-

[ ][ ]

[ ]b

BH OHK

B

(Yes, all weak bases do this – DO (Yes, all weak bases do this – DO NOTNOTendeavor to make this endeavor to make this complicated!)complicated!)

Buffered SolutionsBuffered Solutions

A solution that resists a change A solution that resists a change in pH when either hydroxide ions in pH when either hydroxide ions oror protons are added. protons are added.

Buffered solutions contain either:Buffered solutions contain either: A weak acid and its saltA weak acid and its salt A weak base and its saltA weak base and its salt

Acid/Salt Buffering PairsAcid/Salt Buffering Pairs

Weak AcidFormula

of the acidExample of a salt of the

weak acid Hydrofluoric  HF   KF – Potassium fluoride 

 Formic   HCOOH   KHCOO – Potassium formate 

 Benzoic   C6H5COOH   NaC6H5COO – Sodium benzoate

 Acetic   CH3COOH   NaH3COO – Sodium acetate 

 Carbonic   H2CO3   NaHCO3 - Sodium bicarbonate

 Propanoic   HC3H5O2    NaC3H5O2  - Sodium propanoate

 Hydrocyanic   HCN   KCN - potassium cyanide 

The salt will contain the anion of the The salt will contain the anion of the acid, and the cation of a strong base acid, and the cation of a strong base ((NaNaOH, OH, KKOH)OH)

Base/Salt Buffering PairsBase/Salt Buffering PairsThe salt will contain the cation of the The salt will contain the cation of the base, and the anion of a strong acid base, and the anion of a strong acid (H(HClCl, H, HNONO33))

BaseFormula of the base

Example of a salt of the weak acid

Ammonia   NH3  NH4Cl - ammonium chloride

 Methylamine

 CH3NH2

 CH3NH2Cl – methylammonium chloride

 Ethylamine  C2H5NH2

 C2H5NH3NO3 -  ethylammonium nitrate

 Aniline  C6H5NH2  C6H5NH3Cl – aniline hydrochloride

 Pyridine  C5H5N    C5H5NHCl – pyridine hydrochloride

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00m illiliters NaOH (0.10 M)

pH

Titration of an Unbuffered Titration of an Unbuffered SolutionSolution

A solution that is 0.10 M CH3COOH is titrated with 0.10 M NaOH

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m illiliters NaOH (0.10 M)

pH

Titration of a Buffered Titration of a Buffered SolutionSolution

A solution that is 0.10 M CH3COOH and 0.10 M NaCH3COO is titrated with 0.10 M NaOH

Comparing ResultsComparing Results

Graph

0

2

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10

12

14

0 5 10 15 20 25 30 35 40 45

mL 0.10 M NaOH

pH

BufferedBuffered

UnbufferedUnbuffered

Comparing ResultsComparing Results

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m illiliters NaOH (0.10 M)

pH

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00m illiliters NaOH (0.10 M)

pH

Unbuffered Buffered

In what ways are the graphs different?In what ways are the graphs similar?

Henderson-Hasselbalch Henderson-Hasselbalch EquationEquation

][

][log

][

][log

acid

basepK

HA

ApKpH aa

[ ] [ ]log log

[ ] [ ]b b

BH acidpOH pK pK

B base

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00m illiliters NaOH (0.10 M)

pH

Weak Acid/Strong Base Weak Acid/Strong Base TitrationTitration

A solution that is 0.10 M CH3COOH is titrated with 0.10 M NaOH

Endpoint is above pH 7

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m illiliters NaOH (0.10 M)

pH

Strong Acid/Strong Base Strong Acid/Strong Base TitrationTitration

A solution that is 0.10 M HCl is titrated with 0.10 M NaOH

Endpoint is at pH 7

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m illiliters HCl (0.10 M)

pH

Strong Acid/Strong Base Strong Acid/Strong Base TitrationTitration

A solution that is 0.10 M NaOH is titrated with 0.10 M HCl

Endpoint is at pH 7 It is important to

recognize that titration curves are not always increasing from left to right.

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m illiliters NH3 (0.10 M)

pH

Strong Acid/Weak Base Strong Acid/Weak Base TitrationTitration

A solution that is 0.10 M HCl is titrated with 0.10 M NH3

Endpoint is below pH 7

Selection of Selection of IndicatorsIndicators

Some Acid-Base IndicatorsSome Acid-Base IndicatorsIndicator

pH Range in whichColor Change Occurs

Color Changeas pH Increases

Crystal violetThymol blueOrange IVMethyl orangeBromcresol greenMethyl redChlorophenol redBromthymol bluePhenol redNeutral redThymol bluePhenolphthaleinThymolphthaleinAlizarin yellowIndigo carmine

0.0 - 1.61.2 - 2.81.4 - 2.83.2 - 4.43.8 - 5.44.8 - 6.25.2 - 6.86.0 - 7.66.6 - 8.06.8 - 8.08.0 - 9.6

8.2 - 10.09.4 - 10.6

10.1 - 12.011.4 - 13.0

yellow to bluered to yellowred to yellowred to yellowyellow to bluered to yellowyellow to redyellow to blueyellow to redred to amberyellow to bluecolourless to pinkcolourless to blueyellow to blueblue to yellow

pH pH IndicatorIndicator

s s and theirand theirrangesranges

KKspsp Values for Some Salts at Values for Some Salts at 2525CC

Name Formula Ksp

 Barium carbonate   BaCO3   2.6 x 10-9 

 Barium chromate   BaCrO4   1.2 x 10-10 

 Barium sulfate   BaSO4   1.1 x 10-10 

 Calcium carbonate   CaCO3   5.0 x 10-9 

 Calcium oxalate   CaC2O4   2.3 x 10-9 

 Calcium sulfate   CaSO4   7.1 x 10-5 

 Copper(I) iodide   CuI   1.3 x 10-12 

 Copper(II) iodate   Cu(IO3)2   6.9 x 10-8 

 Copper(II) sulfide   CuS   6.0 x 10-37 

 Iron(II) hydroxide   Fe(OH)2   4.9 x 10-17 

 Iron(II) sulfide   FeS   6.0 x 10-19 

 Iron(III) hydroxide   Fe(OH)3   2.6 x 10-39 

 Lead(II) bromide   PbBr2   6.6 x 10-6 

 Lead(II) chloride   PbCl2   1.2 x 10-5 

 Lead(II) iodate   Pb(IO3)2   3.7 x 10-13 

 Lead(II) iodide   PbI2   8.5 x 10-9 

 Lead(II) sulfate   PbSO4   1.8 x 10-8 

Name Formula Ksp

 Lead(II) bromide   PbBr2   6.6 x 10-6 

 Lead(II) chloride   PbCl2   1.2 x 10-5 

 Lead(II) iodate   Pb(IO3)2   3.7 x 10-13 

 Lead(II) iodide   PbI2   8.5 x 10-9 

 Lead(II) sulfate   PbSO4   1.8 x 10-8 

 Magnesium carbonate   MgCO3   6.8 x 10-6 

 Magnesium hydroxide   Mg(OH)2   5.6 x 10-12 

 Silver bromate   AgBrO3   5.3 x 10-5 

 Silver bromide   AgBr   5.4 x 10-13 

 Silver carbonate   Ag2CO3   8.5 x 10-12 

 Silver chloride   AgCl   1.8 x 10-10 

 Silver chromate   Ag2CrO4   1.1 x 10-12 

 Silver iodate   AgIO3   3.2 x 10-8 

 Silver iodide   AgI   8.5 x 10-17 

 Strontium carbonate   SrCO3   5.6 x 10-10 

 Strontium fluoride   SrF2   4.3 x 10-9 

 Strontium sulfate   SrSO4   3.4 x 10-7 

 Zinc sulfide   ZnS   2.0 x 10-25 

Solving Solubility ProblemsSolving Solubility Problems

For the salt AgI at 25C, Ksp = 1.5 x 10-16

AgI(s) Ag+(aq) + I-(aq)

I

C

E

OO

+x +x

x x

1.5 x 10-16 = x2

x = solubility of AgI in mol/L = 1.2 x 10-8 M

Solving Solubility ProblemsSolving Solubility Problems

For the salt PbCl2 at 25C, Ksp = 1.6 x 10-

5PbCl2(s) Pb2+(aq) + 2Cl-(aq)

I

C

E

OO

+x +2x

x 2x

1.6 x 10-5 = (x)(2x)2 = 4x3

x = solubility of PbCl2 in mol/L = 1.6 x 10-2 M

Solving Solubility with a Common Solving Solubility with a Common IonIon

For the salt AgI at 25C, Ksp = 1.5 x 10-16

What is its solubility in 0.05 M NaI?

AgI(s) Ag+(aq) + I-(aq)

I

C

E

0.05O

+x 0.05+x

x 0.05+x

1.5 x 10-16 = (x)(0.05+x) (x)(0.05)

x = solubility of AgI in mol/L = 3.0 x 10-15 M

Precipitation and Qualitative Precipitation and Qualitative AnalysisAnalysis

Complex IonsComplex Ions

A Complex ion is a charged species composed of:

1. A metallic cation

2. Ligands – Lewis bases that have a lone electron pair that can form a covalent bond with an empty orbital belonging to the metallic cation

NHNH33, CN, CN--, and H, and H22O O are Common Ligandsare Common Ligands

O

H

HNH

HH

C N-

Coordination NumberCoordination Number Coordination number refers to the number of ligands attached to the cation 2, 4, and 6 are the most common coordination numbersCoordinatio

n number

Example(s)

2 Ag(NH3)2+

4 CoCl42- Cu(NH3)42+

6 Co(H2O)62+ Ni(NH3)6

2+

Complex Ions and SolubilityComplex Ions and Solubility

AgCl(s) Ag+ + Cl- Ksp = 1.6 x 10-10

Ag+ + NH3 Ag(NH3)+ K1 = 2.1 x 103

Ag(NH3)+ NH3 Ag(NH3)2+ K2 = 8.2 x 103

AgCl + 2NH3 Ag(NH3)2+ +

Cl-K = KspK1K2

23

233

][

]][)([108.2

NH

ClNHAgxK