Chem. 31 – 4/22 Lecture
Announcements
• Lab Reports
– Soda Ash report due 4/27
– Will be posting information about Formal Report
soon
• Today’s Lecture
– Chapter 8: Acid Base Chemistry
• Buffer Solutions
– Chapter 9: Polyprotic Acids
• Types of acids/functional groups
• Dissolution in water
Acid – Base Equilibria
• Buffer Solutions:
– A buffer solution is designed so that a small addition
of acid or base will only slightly change the pH
– Most buffer solutions have a weak acid and its
conjugate base both present
– Example: Determine pH of a mix of 0.010 M HCO2H
and 0.025 M Na+HCO2- solution (ignoring activity)
– Go to board to show if ICE approach is needed
Acid – Base Equilibria
• Buffer Solutions:
– Question: Was the ICE Problem set up
needed?
– Answer: No. The assumption of x << [HA],
[A-] is valid for all “traditional” buffers
– Traditional Buffer
• Weak acid (3 < pKa < 11)
• Ratio of weak acid to conjugate base in range 0.1
to 10
• mM+ concentration range
Acid – Base Equilibria
• Buffer Solutions:
– Since ICE not needed, can just use Ka
equation
– Ka = [H+][A-]/[HA] = [H+][A-]o/[HA]o
(always valid)
(valid for traditional buffer)
– But log version more common
– pH = pKa + log([A-]/[HA])
– Also known as Henderson-Hasselbalch
Equation
Acid – Base Equilibria
• Buffer Solutions:
– Ways to make buffer solution:
• Mix weak acid and conjugate base (done in making
reference solution for soda ash lab)
• Add strong base to weak acid (weak acid must be
in excess) – this converts some of the weak acid to
its conjugate base
• Add strong acid to weak base (weak base must be
in excess) – this converts some of weak base to its
conjugate acid
Acid – Base Equilibria
• Buffer Solutions:
– Why are they needed?/useful?
– The main reason is to keep the pH constant
so that the ratio of species of acids and bases
is constants
– Some examples:
• in water hardness titration, we want [Y4-]/[Y]total
constant so sample pH won’t affect results
• spectroscopy: Beer’s law only applies to single
species (e.g. separate laws for HIn and In-)
• chromatography: at pH = 2, benzoic acid is a
molecule and retained (reversed-phase HPLC)
Acid – Base Equilibria
• Buffer Solutions:
– Ways to make buffer solution:
• Mix weak acid and conjugate base (done in making
reference solution for soda ash lab)
• Add strong base to weak acid (weak acid must be
in excess) – this converts some of the weak acid to
its conjugate base
• Add strong acid to weak base (weak base must be
in excess) – this converts some of weak base to its
conjugate acid
Acid – Base Equilibria
• Example Problems:
– How many moles of hydroxyl ammonium
chloride (HONH3+Cl-) needs to be added to
500 mL of 0.020 M HONH2 to obtain a buffer
solution with a pH of 6.20? The pKa for
HONH3+ is 5.96.
– What is the pH of a solution made from
mixing 400 mL of 0.018 M CH3CO2H (pKa =
4.75) with 100 mL of 0.024 M NaOH?
(assume additive volumes)
Acid – Base Equilibria
• Example Qualitative Question:
– Which of the following mixtures will result in a
traditional buffer:
• 0.010 M CH3CO2H+ 0.014 M KOH
• 0.014 M NH4Cl + 0.010 M KOH
• 0.020 M NH2CH2CH2OH + 0.010 M NaOH
• 0.010 M Na2HPO4 + 1.0 x 10-4 M Na3PO4
• NH2CH2CH2OH is a weak base
Chapter 9 – Polyprotic Acids
• Polyprotic Acid – An acid that is capable of
releasing 2 or more moles of H+ per molecule.
• Diprotic Acid Types:
– Standard Acid: H2A
• examples - H2CO3, H2SO3
– Both Acid and Base Constituents: (H2L+ = acid form)
• Examples: NH2CH2CO2H, NH2C6H4OH
• One example is a zwitter ion (exist with both positive and
negative charges: NH3+CH2CO2-), but the other exists in an
uncharged form (e.g. NH2C6H6OH)
• Zwitter ion will occur when pKa (NH3+R group) > pKa (RCO2H
group)
– Difunctional Bases: (H2L2+ = acid form)
• Example: NH2CH2CH2NH2
Chapter 9 – Polyprotic Acids
• Diprotic Acid Forms:
– Acidic Form: H2A (for standard acid) or H2L+
or H2L2+ (acid/base and dibasic compounds)
– Intermediate form: HA- (for standard acid)
– Basic form: A2- (for standard acid)
Chapter 9 – Polyprotic Acids
• Polyprotic acids can be even more
complex
– Example EDTA – has 4 acid functional groups
and 2 base functional groups
– So 7 possible forms:
H6Y2+, H5Y+, …, HY3-, Y4Most acidic form
most basic form
Chapter 9 – Polyprotic Acids
• Example Questions:
1. Indicate what the “intermediate” form of
NH2CH2CH2NH2 (a compound with two
base groups) looks like
2. Is H3NC6H4OH+ (shown in the acidic
form) a zwitter ion? The pKa for the NH3
group is 4.8 and the pKa for the OH
group is 9.7.
Chapter 9 – Polyprotic Acids
• Dissolution in Water
– Acid Forms (e.g. 0.010 M HO2CCH2CO2H –
malonic acid)
– Intermediate Forms (0.010 M HO2CCH2CO2-)
– Basic Forms (0.010 M PO43-)
– Go Over Examples on Board