1

E4 Acids, Bases, and SaltsE4 Acids, Bases, and Salts

Session two Parts 3, 4, and 5 Complete the teamreport and discussionpresentation.

Acid

Part 3. Neutralization and Indicators.Part 3. Neutralization and Indicators.

Study the use of indicators as monitors for anacid-base neutralization reaction.Neutralization:

acid base salt water

DEMO - proton transfer reaction: HCl(g) + NH3(g) → NH4Cl(s)

HCl(aq) + NH4OH(aq) → NH4Cl(aq) + HOH(l)+ +

Carboxylic Acids (Parts 3 - 5)Carboxylic Acids (Parts 3 - 5)

Monitor the neutralization of carboxylic acids with NaOH

Carboxylic acids

Carboxylic acids contain the group

Electronegativity of the elements* 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IVA VA VIA VIIA H 2.1

Li 1.0

Be 1.5

B 2.0

C 2.5

N 3.0

O 3.5

F 4.0

Na 0.9

Mg 1.2

Al 1.5

Si 1.8

P 2.1

S 2.5

Cl 3.0

K 0.8

Ca 1.0

Sc 1.3

Ti 1.5

V 1.6

Cr 1.6

Mn 1.5

Fe 1.8

Co 1.8

Ni 1.8

Cu 1.9

Zn 1.6

Ga 1.6

Ge 1.8

As 2.0

Se 2.4

Br 2.8

Rb 0.8

Sr 1.0

Y 1.2

Zr 1.4

Nb 1.6

Mo 1.8

Tc 1.9

Ru 2.2

Rh 2.2

Pd 2.2

Ag 1.9

Cd 1.7

In 1.7

Sn 1.8

Sb 1.9

Te 2.1

I 2.5

Cs 0.7

Ba 0.9

La -Lu

Hf 1.3

Ta 1.5

W 1.7

Re 1.9

Os 2.2

Ir 2.2

Pt 2.2

Au 2.4

Hg 1.9

Tl 1.8

Pb 1.8

Bi 1.9

Po 2.0

At 2.2

Fr 0.7

Ra 0.9

Ac 1.1

Th 1.3

Pa 1.5

U 1.7

Np-

N o 1.3

= Metalloids = Nonmetals = Metals

Carboxylic Acids and Ionizable Protons

The electronegativity of H and C are similar and therefore C-Hbonds are stable and the proton in the C-H bond is NOT ionizable

The electronegativity of H and O differ and therefore H in theCOOH group of carboxylic acids is ionizable.

2

Carboxylic acids (Parts 3 and 4)Carboxylic acids (Parts 3 and 4)

• H O Part 3: Acetic acid• | || HC2H3O2

• H− C − C − O − H CH3COOH• |• H

Carboxylic group containingone ionizable proton

• O O Part 4: Oxalic acid• || || H2C2O4

•H − O− C − C − O − H HOOCCOOH Two carboxylic groups each

with one ionizable proton

Acid-Base Neutralization EquilibriaAcid-Base Neutralization Equilibria

• Adding base to the weak (partially ionized) carboxylicacid drives the reaction (donation of proton/s) to completion

CH3COOH → + OH-

+ CH3COO-

→ HOHCH3COOH + NaOH CH3COONa + HOH

H +• At neutralization, only the products (salt and water)

are present.

Acid-Base Reaction StoichiometryAcid-Base Reaction Stoichiometry

Reaction stoichiometry is dependant on the number of ionizableprotons.

Acetic acid (Part 3):CH3COOH + NaOH → CH3COONa + HOH1 mol 1 mol

Oxalic acid (Part 4):HOOC-COOH + 2 NaOH → 2COONa + 2HOH1 mol 2 mol

Titration (Parts 3 - 5)Titration (Parts 3 - 5)

A procedure for determining the concentrationof a measured volume of acid (or base) bymeasuring the volume of base (or acid) of knownconcentration required for neutralization

A procedure for determining the identityidentityof a measured massof a measured mass of acid (or base)by measuring the volume of base (or acid) ofknown concentration required for neutralization

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TitrationTitration

Base delivered Indicator color changefrom burette signals the end-point or neutralizationDEMO

Q. Identify the acid (acetic or oxalic) if 10.0 mL of0.10 M NaOH neutralizes 10.0 mL of 0.10 M acid.Q. Identify the acid (acetic or oxalic) if 10.0 mL of0.10 M NaOH neutralizes 10.0 mL of 0.10 M acid.

Acetic acidCH3COOH + NaOH → CH3COONa + HOHOxalic acidHOOC-COOH + 2 NaOH → 2COONa + 2HOH

Answer. Acetic acid. mmol acid = mmol base 10.0 mL • 0.10 M = 10.0 mL • 0.10 M

2 (5.0 mL • 0.10 M) = 10.0 mL • 0.10 M

note: 5.0 mL of 0.10 M oxalic acid neutralizes 10.0 mL of0.10 M NaOH: 2 (mmol acid) = mmol base

• Bromothymol blue and bromophenol blue

• Phenolphthalein

Acids turn yellow Bases turn blue

Acids are colorless Bases turn pink

Part 3 IndicatorsPart 3 Indicators

Compare the use of three indicators for monitoringthe neutralization of acetic acid with sodium hydroxide

Indicator End Point pHIndicator End Point pH

The pH interval at which the indicator changescolor

↑pH 9.1

Acids are colorless Bases are pink

The indicator color change is observable at themid-point of the end point pH range.

Phenolphthalein: pH 8.2 - pH 10.1

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Indicator pH End PointsIndicator pH End Points

Indicators have different end-point pH color changes

For information on lab indicators, see the lab manual, p. 183

Equivalence Point pHEquivalence Point pH

The pH of the salt and water products of the acid-baseneutralization reaction = the equivalence point pH.

CH3COOH + NaOH → CH3COONa + HOH

“Ideally, the indicator end point pH equals theequivalence point pH = pH 8. “

DEMO

Equivalence point pH ≈ pH 8

Titration and Indicator pH End PointsTitration and Indicator pH End Points Titration of Oxalic Acid (Part 4) Titration of Oxalic Acid (Part 4)

Oxalic acid: MW = 90.04HOOC-COOH + 2 NaOH → 2COONa + 2HOH0.5 mol 1 mol45.02 g 1 mol (or 1000 mmol)

Q. You titrate 0.4502 g of oxalic acid with 1.0 M NaOH. Howmany mL of 1.0 M NaOH are required for neutralization?

Answer.Fact: 0.4502 g acid will neutralize 10 mmol of NaOH.

X mL • 1.0 M NaOH = 10 mmol NaOHX = 10 mL of 1.0 M NaOH

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Part 5. Identification of an Unknown AcidPart 5. Identification of an Unknown Acid

• Identify a carboxylic acid from melting point andtitration data

• The melting point range will be determined using a meltingpoint apparatus• The equivalent weight will be determined by titrating aknown mass of the unknown acid with NaOH

139-140

116cis-butenedioic(maleic) acid

1401572-chlorobenzoic acid

135-136

148trans-cinnamic acid

1351802-hydroxybenzoicacid acetate

MPEWMWFormulaSee Table, p.110

Name

COOH

OCOCH3

CH CH COOH

Cl

COOH

H HHOOC COOH

180

148

157

58

Equivalent Weight (EW)Equivalent Weight (EW)

Theoretical EW = Molecular Wt/# ionizable protons

Examples:H2SO4 = molecular weight of 98H2SO4 = equivalent weight of 49HCl = molecular weight of 36.5HCl = equivalent weight of 36.5

Unknowns and Carboxylic Acid StructureUnknowns and Carboxylic Acid Structure

• Benzoic acid• COOH• ••

No ionizable protons. Not an acid.

One ionizable proton.

Benzoic acid = C6 H5COOH

MW = = EW

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• O H H H H O Adipic acid• | | | | | | | | HOOC(CH2 )4COOHH - O − C − C − C − C − C − C − O − H • | | | |• H H H H

Two ionizable protons.

Molecular Weight = 146Equivalent weight = _____________?73

Q. An acid has an equivalent weight (EW) of 88.92. TheEW = molecular wt/#ionizable protons. List the numberof any compound from the table that may be the acid.

Compound Formula MW1. Ethanoic acid CH3COOH 602. Butanoic acid CH3CH2CH2COOH 883. Oxalic acid HOOC-COOH 904. 1-methyl, 1,2 benzene

dicarboxylic acidCH3C6H3(COOH)2 180

Answer. Compounds # 2 and # 4.

A teammate titrates a carboxylic acid and calculates theEW of the acid to be 72.50. The result is correct within2%. The MW of the acid is determined to be 1.5 x 102

A teammate titrates a carboxylic acid and calculates theEW of the acid to be 72.50. The result is correct within2%. The MW of the acid is determined to be 1.5 x 102

A. How many ionizable protons/mol are in the acid?that•

B. Circle each structure that is consistent with the data.

Adipic Acid, MW = 146: Trans-cinnamic acid, MW=148HO2CCH2CH2CH2CH2CO2H -CH=CH-COOH

Citric Acid, MW = 192: Oxalic acid, MW = 90HO2C-CH2-C(OH)-CH2-CO2H HO2C—CO2H CO2H

Let X = # ionizable protons.EW = 72.50 = MW/X = 150/X X= ~ 2

EW from Experiment Data (Part 5)

• EW = Mass (g) of acid neutralizing 1 mol OH-.

Determine the equivalent weight of an unidentifiedcarboxylic acid by determining the volume of base ofknown concentration required to neutralize a measuredmass of the unknown acid

EW = mass acid (g) moles of OH- neutralized

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EW of Oxalic Acid (Part 4) EW of Oxalic Acid (Part 4)

Oxalic acid: MW = 90.04HOOC-COOH + 2 NaOH → 2COONa + 2HOH0.5 mol 1 mol

45.02 g 1 mol (or 1000 mmol)

Answer. 0.4502 g neutralized 10.0 mmol of NaOH.The mass of acid needed to neutralize 1 mol = 45.02g = EW

Q. 10.0 mL of 1.0 M NaOH neutralizes 0.4502 grams ofoxalic acid, what is the equivalent weight of the acid?

Name Formula MW MP2-hydroxybenzoic acid acetate

COOH

OCOCH3

180 135

trans-cinnamic acid

CH CH COOH148 135-136

2-chlorobenzoic acid

Cl

COOH

157 140

cis-butenedioic (maleic) acid

H HHOOC COOH

116 139-140

Q. You titrate 0.175 g of an acid. 30.00 mL of 0.10 MNaOH neutralizes the sample.1) What is the acid’s EW based on the data?2) Identify the acid from the list below.

Q. You titrate 0.175 g of an acid. 30.00 mL of 0.10 MNaOH neutralizes the sample.1) What is the acid’s EW based on the data?2) Identify the acid from the list below.

Equivalent weight determination.

mol OH- = V(L) x M NaOH

= 0.0300L x 0.10 mol L = 3.0 x 10-3 mol

1) Determine mol of NaOH the acid has neutralized.Known: The acid mass neutralized 30.00 mL of 0.10 M NaOH

Equivalent Weight determination.

X g acid 1 mol OH-Known: 0.175 g = 0.0030 mol OH-

X = 58 g

2) Determine the mass of acid that wouldneutralize one mol of hydroxide ions.

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Name Formula MW MP2-hydroxybenzoic acid acetate

COOH

OCOCH3

180 135

trans-cinnamic acid

CH CH COOH148 135-136

2-chlorobenzoic acid

Cl

COOH

157 140

cis-butenedioic (maleic) acid

H HHOOC COOH

116 139-140

Q. What is the identity of the acid?Q. What is the identity of the acid?

√Questions?

Contact nkerner@umich.edu