Download 3/27 Lecture Slides

Chem. 31 – 3/27 Lecture
Announcements
• Quiz 3 Scores – not particularly high
• Additional Problem 3 Posted (it will be
a while before we cover those topics)
• Today’s Lecture
– Chapter 7 – Titrations
• Precipitation titrations – shape of curve
• What affects precipitation titration sharpness?
– Chapter 18 – Spectroscopy
• Light Basics
• Beer’s Law (if time)
Titrations
Shapes of Titration Curves – Precipitation Example
• Calculation of pHg22+:
• Example: 0.100 M Hg22+ (25 mL) titrated
with 0.0800 M CrO42• Before Equilibrium Point (excess Hg22+) –
e.g. 10 mL (do on board)
• At Equilibrium Point ([Hg22+] = [CrO42-] =
Ksp0.5)
• After Equilibrium Point (excess CrO42-) (do
e.g. 35 mL on board)
Titrations
Shapes of Titration Curves – Precipitation Example
• What affects sharpness at equivalence point?
– Ksp value (smaller means sharper)
– Concentration of ions (higher means sharper)
Ksp(CO32-) < Ksp(CrO42-) <Ksp(SO42-)
Hg Titration with Chromate
Hg Titration
8.0
16.0
7.0
14.0
6.0
12.0
pHg22+
mercury(I) sulfate
8.0
mercury (I) chromate
mercury (I) carbonate
6.0
pHg22+
5.0
10.0
0.10 M Hg
4.0
0.010 M Hg
0.001 M Hg
3.0
2.0
4.0
1.0
2.0
0.0
0.0
0
10
20
30
V(titrant)
40
50
60
0
10
20
30
V(CrO42-)
40
50
60
Titrations
Titration of a mixture
• Example: Titration of
a mixture of CrO42and CO32- by Hg22+
Hg Titration of chromate and carbonate
18.0
– Since Ksp value for
CO32- is smaller, it will
precipitate first
– After CO32precipitates to near
completion, pHg drops
to the point where
CrO42- starts to
precipitate
Which anion was initially
present at higher concentration?
16.0
14.0
pHg 2
2+
12.0
10.0
8.0
6.0
4.0
2.0
0.0
0
10
20
30
2+
V(Hg 2 ) (mL)
40
50
60
Chapter 18 - Spectroscopy
A. Introduction
1. One of the main branches of analytical chemistry
2. The interaction of light and matter (for purposes of
quantitative and qualitative analysis)
3. Topics covered:
- Properties of Light
- Absorption of Light
- Electromagnetic Spectrum
- Beer’s Law
- Spectrometers
Spectroscopy
B.
Fundamental Properties
of Light
1.
Wave-like properties:
λ
λ = wavelength = distance
between wave crests
n = frequency = # wave crests/s
n = wave number = # wave crests/length measure
c = speed of light (in vacuum) = 3.00 x 108 m/s
Relationships: c = λ·n and n = 1/λ
note: speed of light depends on medium (slower
in water than in vacuum) – not considered here
Spectroscopy
Fundamental Properties of Light
1. Other wave-like properties
- diffraction, interference
2. Particle-like properties
a) Idea of photons (individual entities of
light)
b) Energy of photons
E = hn = hc/l
Spectroscopy
Absorption vs. Emission
1. Absorption
- Associated with a
transition of matter
from lower energy to
higher energy
A + hn → A*
hn = photon
A* → A + hn
Energy
2. Emission
Excited State
Photon
out
- Associated with a
transition from high
energy to low energy
Ground State
Photon in
Spectroscopy
Regions of the Electromagnetic Spectrum
Many regions are defined as much by the types of
transitions occurring (e.g. outer shell electron) as by the
frequency or energy of the transitions
Short
wavelengths
Gamma
rays
High
Energies
Outer shell
electrons
X-rays
UV +
visible
Nuclear
Inner shell
transitions electrons
Bond
vibration
Infrared
Nuclear
spin
Microwaves
Molecular
rotations
Long
wavelengths
Radio waves
Electron spin
Low Energies
Spectroscopy
Some Example Questions
1.
2.
3.
A nuclear magnetic resonance (NMR) spectrometer absorbs light
at a frequency of 750 MHz. This is in the radio frequency and Hz
= s-1. What is the wavelength of this light?
An infrared absorption band occurs at a wavenumber of 812 cm-1.
What is the wavelength (in mm) and energy (J/photon) of that
light?
What type of light involves transitions of inner shell electrons?
Spectroscopy
Beer’s Law
Transmittance = T = P/Po
Absorbance = A = -logT
sample in cuvette
Light source
Absorbance used because it is
proportional to concentration
A = εbC
Where ε = molar absorptivity
and b = path length (usually in
cm) and C = concentration (M)
ε = constant for given
compound at specific λ value
Light
intensity
in = Po
b
Light
intensity
out = P
Spectroscopy
Beer’s Law Question
• Half of the 284 nm light is absorbed when
benzoic acid at a concentration of 0.0080
M is in a cuvette with a path length of 0.5
cm. What is the molar absorptivity of
benzoic acid at this wavelength?