e4 acids, bases, and salts - university of michiganchem125/w08/e4w08lab2.pdf · e4 acids, bases,...
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E4 Acids, Bases, and Salts
Session twoParts 3, 4, and 5.
Acid
Part 3. Neutralization and Indicators.
Study the use of indicators as monitors for anacid-base neutralization reaction.
Neutralization:HCl(aq) + NH4OH(aq) → NH4Cl(aq) + HOH(l) acid base salt water
DEMO - proton transfer reaction:
HCl(g) + NH3(g) → NH4Cl(s)
Carboxylic Acids (Parts 3 - 5)
Monitor the neutralization of carboxylic acids with NaOH
Carboxylic acids
Carboxylic acids contain the group
2
Electronegativity of the elements*1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IVA VA VIA VIIA
H2.1
Li1.0
Be1.5
B2.0
C2.5
N3.0
O3.5
F4.0
Na0.9
Mg1.2
Al
1.5
Si1.8
P2.1
S2.5
Cl3.0
K0.8
Ca1.0
Sc1.3
Ti1.5
V1.6
Cr1.6
Mn1.5
Fe1.8
Co1.8
Ni1.8
Cu1.9
Zn1.6
Ga1.6
Ge1.8
As2.0
Se2.4
Br2.8
Rb0.8
Sr1.0
Y1.2
Zr1.4
Nb1.6
Mo1.8
Tc1.9
Ru2.2
Rh2.2
Pd2.2
Ag1.9
Cd1.7
In1.7
Sn1.8
Sb1.9
Te2.1
I2.5
Cs0.7
Ba0.9
La-Lu
Hf1.3
Ta1.5
W1.7
Re1.9
Os2.2
Ir2.2
Pt2.2
Au2.4
Hg1.9
Tl1.8
Pb1.8
Bi1.9
Po2.0
At2.2
Fr0.7
Ra0.9
Ac1.1
Th1.3
Pa1.5
U1.7
Np- No1.3
= Metalloids = Nonmetals = Metals
Strength of Carboxylic Acids
Carbon based oxy (carboxylic) acids are weak acids!Note that the electronegativity of C and H are close in value; theelectronegativity of C and O differ.
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 Equilibria
• Adding base to the weak (partially ionized) carboxylicacid drives the reaction (donation of proton/s) to completion
CH3COOH → + OH-
+ CH3COO-
→ HOH
CH3COOH + NaOH CH3COONa + HOH
H +
• At neutralization, only the products (salt and water) are present.
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Acid-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)
A procedure for determining the unknownconcentration of a measured volume of acid (or base)by measuring the volume of base (or acid) of knownconcentration required for neutralization
A procedure for determining the unknownunknownidentity of a measured massidentity of a measured mass of acid (or base)by measuring the volume of base (or acid) of knownconcentration required for neutralization
Titration
Base delivered Indicator color changefrom burette signals the end-point or neutralizationDEMO
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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: _______________________
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: ___________________________________
• Bromothymol blue and bromophenol blue
• Phenolphthalein
Acids turn yellow Bases turn blue
Acids are colorless Bases turn pink
Part 3 Indicators
Compare the use of three indicators for monitoringthe neutralization of acetic acid with sodium hydroxide
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Indicator 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
Indicator pH End Points
Indicators have different end-point pH color changes
For information on lab indicators, see the lab manual, p. 183
Equivalence Point pH
The pH of the salt and water products of the acid-baseneutralization reaction = the equivalence point pH.
CH3COOH + NaOH → CH3COONa + HOH ↓
~ pH 8 (equiv.pt.)
“Ideally, the indicator endpoint = pH 8. “
DEMO
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Titration and Indicator pH End Points
Part 5. Identification of an Unknown Acid
• Identify a carboxylic acid from melting point andequivalent weight 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 H
HOOC COOH
180
148
157
58
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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 Structure
• Benzoic acid• COOH•••
No ionizable protons. Not an acid.
Benzoic acid = C6 H5COOH
MW = 122
EW = _________?
• O H H H H O Adipic acid• | | | | | | | | HOOC(CH2 )4COOHH - O − C − C − C − C − C − C − O − H• | | | |• H H H H
Molecular Weight = 146Equivalent weight = _____________?
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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 MW
1. Ethanoic acid CH3COOH 60
2. Butanoic acid CH3CH2CH2COOH 88
3. Oxalic acid HOOC-COOH 90
4. 1-methyl, 1,2 benzene
dicarboxylic acid
CH3C6H3(COOH)2 180
Answer. _________________
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
Answer: __________________
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)
Oxalic acid: MW = 90.04HOOC-COOH + 2 NaOH → 2COONa + 2HOH0.5 mol 1 mol
45.02 g 1 mol (or 1000 mmol)
Answer. _________________________________________
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 H
HOOC 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.
Equivalent weight determination.
mol OH- = V(L) x M NaOH
1) Determine mol of NaOH the acid has neutralized:
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Equivalent Weight determination.
Mass of acid = ___________g
2) Determine the mass of acid that wouldneutralize one mol of hydroxide ions.
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 H
HOOC COOH
116 139-140
Q. What is the identity of the acid?
Questions?Contact [email protected]