E4 Acids, Bases, and Salts
Acid
Session two
Parts 3, 4, and 5.
Part 3. Neutralization and Indicators.

Study the use of indicators as monitors for an
acid-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
1
Strength of Carboxylic Acids
Electronegativity of the elements*
1
2
3
4
5
6
7
8
IA IIA IIIB IVB VB VIB VIIB
H
2.1
Li
1.0
Na
0.9
Be
1.5
Mg
1.2
K
0.8
Rb
0.8
Cs
0.7
Fr
0.7
Ca
1.0
Sr
1.0
Ba
0.9
Ra
0.9
Sc
1.3
Y
1.2
La
-Lu
Ac
1.1
Ti
1.5
Zr
1.4
Hf
1.3
Th
1.3
V
1.6
Nb
1.6
Ta
1.5
Pa
1.5
Cr
1.6
Mo
1.8
W
1.7
U
1.7
Mn
1.5
Tc
1.9
Re
1.9
Np-
Fe
1.8
Ru
2.2
Os
2.2
No
1.3
9
10
11
IB
VIIIB
Co
1.8
Rh
2.2
Ir
2.2
Ni
1.8
Pd
2.2
Pt
2.2
Cu
1.9
Ag
1.9
Au
2.4
12
IIB
13 14 15
IIIA IVA VA
16 17
VIA VIIA
Zn
1.6
Cd
1.7
Hg
1.9
B
2.0
Al
1.5
Ga
1.6
In
1.7
Tl
1.8
C
2.5
Si
1.8
N
3.0
P
2.1
O
3.5
S
2.5
F
4.0
Cl
3.0
Ge
1.8
Sn
1.8
Pb
1.8
As
2.0
Sb
1.9
Bi
1.9
Se
2.4
Te
2.1
Po
2.0
Br
2.8
I
2.5
At
2.2
Carbon
based oxy =(carboxylic)
acids
are weak acids!
= Metalloids
Nonmetals
= Metals
Note that the electronegativity of C and H are close in value; the
electronegativity of C and O differ.
Carboxylic acids (Parts 3 and 4)
•
•
H
|
O
||
Part 3: Acetic acid
HC2H3O2
• H− C − C − O − H
•
|
•
H
•
•
CH3COOH
Carboxylic group containing
one ionizable proton
O O
||
||
•H − O− C − C − O − H
Part 4: Oxalic acid
H2C2O4
HOOCCOOH
Two carboxylic groups each
with one ionizable proton
Acid-Base Neutralization Equilibria
• Adding base to the weak (partially ionized) carboxylic
acid drives the reaction (donation of proton/s) to completion
CH3COOH
→
H + + CH3 COO+
OH- → HOH
• At neutralization, only the products (salt and water)
are present.
CH3COOH + NaOH
CH3 COONa + HOH
2
Acid-Base Reaction Stoichiometry
 Reaction stoichiometry is dependant on the number of ionizable
protons.
Acetic acid (Part 3):
CH3COOH + NaOH → CH3COONa + HOH
1 mol
1 mol
Oxalic acid (Part 4):
HOOC-COOH + 2 NaOH → 2COONa + 2HOH
1 mol
2 mol
Titration (Parts 3 - 5)
 A procedure for determining the unknown
concentration of a measured volume of acid (or base)
by measuring the volume of base (or acid) of known
concentration required for neutralization
 A procedure for determining the unknown
identity of a measured mass of acid (or base)
by measuring the volume of base (or acid) of known
concentration required for neutralization
Titration
DEMO
Base delivered
from burette
Indicator color change
signals the end-point
or neutralization
3
Q. Identify the acid (acetic or oxalic) if 10.0 mL of
0.10 M NaOH neutralizes 10.0 mL of 0.10 M acid.
Acetic acid
CH3COOH + NaOH → CH3 COONa + HOH
Oxalic acid
HOOC-COOH + 2 NaOH → 2COONa + 2HOH
Answer: _______________________
Titration of Oxalic Acid (Part 4)
Q. You titrate 0.4502 g of oxalic acid with 1.0 M NaOH. How
many mL of 1.0 M NaOH are required for neutralization?
Oxalic acid: MW = 90.04
HOOC-COOH + 2 NaOH → 2COONa + 2HOH
0.5 mol
1 mol
45.02 g
1 mol (or 1000 mmol)
Answer: ___________________________________
Part 3 Indicators
 Compare the use of three indicators for monitoring
the neutralization of acetic acid with sodium hydroxide
• Bromothymol blue and bromophenol blue
Acids turn yellow
Bases turn blue
• Phenolphthalein
Acids are colorless
Bases turn pink
4
Indicator End Point pH
 The pH interval at which the indicator changes
color
Acids are colorless Bases are pink
Phenolphthalein: pH 8.2
- pH 10.1
↑
pH 9.1
 The indicator color change is observable at the
mid-point of the end point pH range.
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-base
neutralization reaction = the equivalence point pH.
CH3COOH + NaOH → CH3COONa + HOH
↓
~ pH 8 (equiv.pt.)
“Ideally, the indicator end
point = pH 8. “
DEMO
5
Titration and Indicator pH End Points
Part 5. Identification of an Unknown Acid
•
Identify a carboxylic acid from melting point and
equivalent weight data
• The melting point range will be determined using a melting
point apparatus
• The equivalent weight will be determined by titrating a
known mass of the unknown acid with NaOH
See Table, p.110
Name
2-hydroxybenzoic
acid acetate
Formula
COOH
MW EW MP
180 180 135
OCOCH3
trans-cinnamic acid
CH
CH COOH
Cl
2-chlorobenzoic acid
148 148
135136
157 157
140
COOH
cis-butenedioic
(maleic) acid
HOOC
H
COOH
H
116
58 139140
6
Equivalent Weight (EW)
Theoretical
 EW = Molecular Wt/# ionizable protons
Examples:
H2SO4 = molecular weight of 98
H2SO4 = equivalent weight of 49
HCl = molecular weight of 36.5
HCl = equivalent weight of 36.5
Unknowns and Carboxylic Acid Structure
Benzoic acid
•
•
•
•
•
COOH
Benzoic acid = C6 H5COOH
MW = 122
EW = _________?
 No ionizable protons.
 Not an acid.
•
O H H H H O
Adipic acid
•
||
|
|
|
|
||
HOOC(CH2 )4COOH
H-O−C−C−C−C−C−C−O−H
•
|
|
|
|
•
H H H H
Molecular Weight = 146
Equivalent weight = _____________?
7
Q. An acid has an equivalent weight (EW) of 88.92. The
EW = molecular wt/#ionizable protons. List the number
of any compound from the table that may be the acid.
1.
2.
3.
4.
Compound
Ethanoic acid
Butanoic acid
Oxalic acid
1-methyl, 1,2 benzene
dicarboxylic acid
Formula
CH3COOH
CH3CH2CH2COOH
HOOC-COOH
CH3C6H3(COOH)2
MW
60
88
90
180
Answer. _________________
A teammate titrates a carboxylic acid and calculates the
EW of the acid to be 72.50. The result is correct within
2%. The MW of the acid is determined to be 1.5 x 102
A. How many ionizable protons/mol are in the acid?that
•
Answer: __________________
B. Circle each structure that is consistent with the data.
Adipic Acid, MW = 146:
HO2CCH2CH2CH2CH2 CO2H
Citric Acid, MW = 192:
HO2C-CH2-C(OH)-CH2-CO2H

CO2H
Trans-cinnamic acid, MW=148
-CH=CH-COOH
Oxalic acid, MW = 90
HO2C—CO2H
EW from Experiment Data (Part 5)
 Determine the equivalent weight of an unidentified
carboxylic acid by determining the volume of base of
known concentration required to neutralize a measured
mass of the unknown acid
• EW = Mass (g) of acid neutralizing 1 mol OH-.
EW =
mass acid (g)
moles of OH- neutralized
8
EW of Oxalic Acid (Part 4)
Q. 10.0 mL of 1.0 M NaOH neutralizes 0.4502 grams of
oxalic acid, what is the equivalent weight of the acid?
Oxalic acid: MW = 90.04
HOOC-COOH + 2 NaOH → 2COONa + 2HOH
0.5 mol
1 mol
45.02 g
1 mol (or 1000 mmol)
Answer. _________________________________________
Q.
You titrate 0.175 g of an acid. 30.00 mL of 0.10 M
NaOH neutralizes the sample.
1) What is the acid’s EW based on the data?
2) Identify the acid from the list below.
Name
2-hydroxybenzoic acid acetate
Formula
COOH
OCOCH3
trans-cinnamic acid
CH CH COOH
2-chlorobenzoic acid
cis-butenedioic (maleic) acid
Cl
COOH
H
H
HOOC
MW
180
MP
135
148
135-136
157
140
116
139-140
COOH
Equivalent weight determination.
1) Determine mol of NaOH the acid has neutralized:
mol OH- = V(L)
x M NaOH
9
Equivalent Weight determination.
2) Determine the mass of acid that would
neutralize one mol of hydroxide ions.
Mass of acid = ___________g
Q. What is the identity of the acid?
Name
2-hydroxybenzoic acid acetate
Formula
MP
135
148
135-136
Cl
157
140
COOH
H
116
139-140
OCOCH3
trans-cinnamic acid
CH CH COOH
2-chlorobenzoic acid
cis-butenedioic (maleic) acid
MW
180
COOH
H
HOOC
COOH
Questions?
Contact [email protected]
10