Acid - Base Neutralization (Parts 3 - 5)
E4 Acids, Bases, and Salts
Neutralization:
HCl(aq) + NH4OH(aq) → NH4Cl(aq) + HOH(l)
acid
+ base
→
salt +
water
Session two
 Parts 3, 4, and 5
 Complete the team
report and discussion
presentation.
DEMO - proton transfer reaction:
HCl(g) + NH3(g) →
Carboxylic Acids (Parts 3 - 5)
 Neutralization of carboxylic acids with NaOH
Carboxylic acids
Carboxylic acids contain the
group
NH4Cl(s)
Carboxylic Acids and Ionizable Protons
Electronegativity of the elements*
1
2
3
4
5
6
7
8
IA IIA IIIB IVB VB VIB VIIB
9
VIIIB
10
11
IB
12 13 14 15 16 17
IIB IIIA IVA VA VIA VIIA
H
2.1
Li Be
B C N O F
1.0 1.5
2.0 2.5 3.0 3.5 4.0
Na Mg
Al Si P S Cl
0.9 1.2
1.5 1.8 2.1 2.5 3.0
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br
0.8 1.0 1.3 1.5 1.6 1.6 1.5 1.8 1.8 1.8 1.9 1.6 1.6 1.8 2.0 2.4 2.8
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I
0.8 1.0 1.2 1.4 1.6 1.8 1.9 2.2 2.2 2.2 1.9 1.7 1.7 1.8 1.9 2.1 2.5
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At
0.7
The0.9electronegativity
H and
O differ
and1.9
therefore
in the
-Lu 1.3 1.5 1.7 of1.9
2.2 2.2
2.2 2.4
1.8 1.8 H1.9
2.0 2.2
COOH
carboxylic
Fr Ra group
Ac Thof Pa
U Np-acids
N o is ionizable.
1.1 1.3 1.5 1.7 of H and
1.3 C are similar and therefore C-H
0.7
The0.9electronegativity
= Metalloids
Metals
bonds
are stable and= Nonmetals
the proton in =the
C-H bond is NOT ionizable
1
Carboxylic acids
•
•
H
|
O
||
• H− C − C − O − H
•
|
•
H
•
•
O O
||
||
•H − O− C − C − O − H
Acid-Base Reaction Stoichiometry
Part 3: Acetic acid
HC2H3O2
CH3COOH
Carboxylic group containing
one ionizable proton
Part 4: Oxalic acid
H2C2O4
HOOCCOOH
 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
Two carboxylic groups each
with one ionizable proton
Acid-Base Neutralization Equilibria
Titration (Parts 3 - 5)
• Adding base to the weak (partially ionized) carboxylic
 A procedure for determining the concentration
acid drives the reaction (donation of proton/s) to completion
of a measured volume of acid (or base) by
measuring the volume of base (or acid) of known
concentration required for neutralization
+ + CH
→ the H
• AtCH
neutralization,
only
products
(salt3COO
and water)
3 COOH
are present.
+
- →
CH3COOH + NaOH
OH
CH
HOH + HOH
3 COONa
 A procedure for determining the identity
of a measured mass of acid (or base)
by measuring the volume of base (or acid) of
known concentration required for neutralization
2
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.
Titration
Acetic acid
CH3COOH + NaOH → CH3COONa + HOH
Oxalic acid
HOOC-COOH + 2 NaOH → 2COONa + 2HOH
Answer. Acetic acid.
mmol acid = mmol base
10.0 mL • 0.10 M = 10.0 mL • 0.10 M
note: 5.0 mL of 0.10 M oxalic acid neutralizes 10.0 mL of
0.10 M NaOH:
DEMO
Base delivered
from burette
2 (mmol acid) = mmol base
2 (5.0 mL • 0.10 M) = 10.0 mL • 0.10 M
Indicator color change
signals the end-point
or neutralization
Part 3 Acid-Base Neutralizations and 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
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.
3
Indicator pH End Points
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
Equivalence point pH
≈ pH 8
“Ideally, the indicator end point pH equals the
equivalence point pH = pH 8. “
DEMO
 Indicators have different end-point pH color changes
 For information on lab indicators, see the lab manual, p. 183
Titration and Indicator pH End Points
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.
Fact: 0.4502 g acid will neutralize 10 mmol of NaOH.
X mL • 1.0 M NaOH = 10 mmol NaOH
X = 10 mL of 1.0 M NaOH
4
Part 5. Identification of an Unknown Acid
•
Identify a carboxylic acid from melting point and
titration 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 acid with NaOH
See Table, p.110
Name
2-hydroxybenzoic
acid acetate
Formula
MW EW MP
180 180 135
COOH
OCOCH3
trans-cinnamic acid
CH
Cl
2-chlorobenzoic acid
148 148
135136
157 157
140
CH COOH
COOH
cis-butenedioic
(maleic) acid
H
COOH
116
58 139140
H
Unknowns and Carboxylic Acid Structure
Equivalent Weight (EW)
Theoretical
 EW = Molecular Wt/# ionizable protons
HOOC
•
•
•
•
•
Examples:
H2SO4 = molecular weight of 98
H2SO4 = equivalent weight of 49
HCl = molecular weight of 36.5
HCl = equivalent weight of 36.5
Benzoic acid
COOH
 One ionizable proton.
Benzoic acid = C6 H5COOH
MW = EW
 No ionizable protons.
 Not an acid.
5
Q. An acid has an equivalent weight (EW) of 88.92. The
•
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
Two ionizable protons.
Molecular Weight = 146
73
Equivalent weight = _____________?
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. Compounds # 2 and # 4.
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
•
Let X = # ionizable protons.
EW = 72.50 = MW/X = 150/X
X= ~ 2
B. Circle each structure that is consistent with the data.
Adipic Acid, MW = 146:
HO2CCH2CH2CH2CH2CO2H
Citric Acid, MW = 192:
HO2C-CH2-C(OH)-CH2-CO2H

CO2H
Trans-cinnamic acid, MW=148
-CH=CH-COOH
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
Oxalic acid, MW = 90
HO2C—CO2H
6
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)
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
Answer. 0.4502 g neutralized 10.0 mmol of NaOH.
The mass of acid needed to neutralize 1 mol = 45.02g = EW
Equivalent weight determination.
1) Determine mol of NaOH the acid has neutralized.
Known: The acid mass neutralized 30.00 mL of 0.10 M NaOH
mol OH- = V(L)
x M NaOH
= 0.0300L x 0.10 mol
L
-3
= 3.0 x 10 mol
Cl
cis-butenedioic (maleic) acid
COOH
H
H
HOOC
MW
180
MP
135
148
135-136
157
140
116
139-140
COOH
Equivalent Weight determination.
2) Determine the mass of acid that would
neutralize one mol of hydroxide ions.
Known:
0.175 g
X g acid
=
0.0030 mol OH1 mol OH-
X = 58 g
7
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
Questions?
Contact [email protected]
COOH
8