Chem. 31 – 4/1 Lecture
Announcements I
• Additional Problem 2.2 due Today
• Quiz 4 also Today
• Lab Report – Resubmissions
– Cl lab report resubmission due today (2 weeks after deadline
excluding Spring Break)
– Water Hardness lab report resubmissions due next
Wednesday
• Homework Set 2
– Set 2.3 problem solutions to be posted soon
Announcements II
• Today’s Lecture
– Chapter 7
• General Comments on the Systematic Method
– Chapter 17 Spectroscopy (skipping ahead to keep up with
lab)
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Introduction
Nature of Light
Absorption of Light/Regions of the Electromagnetic Spectrum
Beer’s Law/Basics on Instrumentation
The Systematic Method
Stong Acid/Strong Base Problems
• When do we need to use the systematic
approach?
– when more than 1 coupled reaction occur
(unless coupling is insignificant)
– examples: 4.0 x 10-3 M HCl. 7.2 x 10-3 M
NaOH
– Key point is the charge balance equation:
- for strong acid HX, [H+] = [X-] + [OH-]
- If [X-] >> [OH-], then [H+] = [X-]
– for strong base NaOH, [H+] + [Na+] = [OH-]
The Systematic Method
General Comments
• Effects of secondary reactions
– e.g. MgCO3 dissolution
– Additional reactions increase solubility
– Secondary reactions also can affect pH (CO32+ H2O will produce OH- while Mg2+ + H2O will
produce H+)
• Software is also available to solve these
types of problems (but still need to know
steps 1 → 5 to get problems solved)
Chapter 17 - 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?
Spectroscopy
More on Beer’s Law
– Law not valid for high
concentrations
– Deviations to law appear to
occur when multiple
wavelengths of light used
or when multiple species
exist but absorb light
differently
– Uncertainties are lowest
when 0.1 < A < 1
Example of deviations to Beer’s Law:
Unbuffered Indicator with ε(In-) = 300 M1 cm-1, ε(HIn) = 20 M-1 cm-1; pKa = 4.0
HIn ↔ H+ + In-
Absorbance
• Useful for determination
of analyte concentrations
• Some limitations
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
0.005
0.01
Total HIn Conc.
0.015
Spectroscopy
Spectrometers
sample in
cuvette
light detector –
measures light
intensity by converting
it to an electrical signal
Data
processor
light source
light discriminator:
monochromator
(passes only a
small range of
wavelengths)
Components can look very different in different types of
spectrometers, but spectrometers will have all of the major
components (except other methods of wavelength discrimination
may replace monochromators)
Spectroscopy
Example Measurement: Ozone
• Can also make measurements
remotely (e.g. absorbance between
two skyscrapers)
light source
(l = 254 nm)
reference cell
– compares absorbance through sample
cell vs.
– absorbance through reference cell
sample cell
• Ozone (O3) is a pollutant (lower
chopper
atmosphere) and in stratosphere
air in
provides UV protection
• Instrument is used for measurement
at station or in airplane
O3 scrubber
light detector