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Transcript of CHM115 Lecture15
8/8/2019 CHM115 Lecture15
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Lecture 15
8/8/2019 CHM115 Lecture15
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Concentration and Spectr oscopy
This laboratory will give you an opportunityto work with an example of a very common
analytical technique used in all types of labs,
not just chemistry labs.
8/8/2019 CHM115 Lecture15
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Week 9: Concentration and Spectr oscopy
Goals:
Generate a calibration curve from a set of
standard solutions.
Find the concentration of a solution from its
absorbance as measured via UV-Vis
spectrophotometer and your calibration curve.
8/8/2019 CHM115 Lecture15
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Exam 2
Elliot Hall of Music, 6:30 PM, T Oct. 26
Be there by 6:10 PM
Bring PUID, pencils, non-programmable calculator,
and seat location. The exam covers
Lectures 7-14, OWL homework, learning objectives for
those lectures, labs (Amount of acid in solution, chemical
synthesis, does human breathing, molecular geometry)
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Not everything that is clear and colorless is
the same substance: How can we tell them
apart?
DEMO: colored water, pure water, water with salt, tonic water;
Conductivity setup, black (UV) light source
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Ener gy (³light´) + Matter (molecules)
region of spectru
m
wavelength
range
molecular eff ects «and this means«
gamma pm ± nm ionizing damage DNA
UV-C 200-280 nm break bondsHighly damaging; totally absorbed by O2
and O3 in upper atmosphere
UV-B 280-320 nm break bonds Sunbur n and skin cancer.
Absorbed by O3 in stratosphere.
UV- A 320-400 break bonds/
electr on
transitions
A lot reaches sur f ace of earth
thr ough atmosphere; can result
in eye damage; tanning
Visible 400-700 nm electr on
transitions
Results in color
IR (near) 700-5000 nm bond vibrations spectr oscopy to look at
structures of molecules
Micr owave 1 mm ± 1 m bond r otations Micr owave ovens
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A ³photochemical reaction´ with visible light
Cl2 + h¯ 2 Cl·
2 Cl· + H2 2 HCl (net)
first step initiated by blue/violet light (visible)
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o c em s ry n e ra osp ere: an
ozone
O3 + h¯nm) O2 + O·
O2 + O· O3
Ozone in upper atmosphere absorbs in the UV-B range. It is
then spontaneously regenerated from products.
Ozone protects us from damaging radiation.
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How do we see stuff that is colored?
White Light
Wavelengths we
don¶t see
because they ar
absorbed
We see the wavelengths that
get ref lected (or transmitted)
demo
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The eye as a visible light detector
Although there is a lar ge range in the wavelength and f requency of electr omagnetic
radiation, the eye can only detect in the visible region. How does the eye detect
visible light?
Retinal
Absorption of light by
retinal in rhodopsin
trigger s a cis to trans
geometry change
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Absorption spectra for retinal in diff erent areas in
rhodopsin
The wavelength where retinal absorbs depends on the envir onment in which it
is in, which generates diff erent ³cones.´ Together, the r od and cones absorb
over the entire visible region of the electr omagnetic spectrum
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The Amazing Eye!
Just because organic molecules dont absorbVISIBLE doesnt mean they dont absorb
anything.
We may not be able to see UV or IR light with
our eyes, but we can detect it with our bodies
IR radiation is detected as heat
UV radiation is detected as sunburn
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Spectr oscopy
Diff erent molecules can absorb light of diff erent color s (ener gies).
Demo
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Absorption depends on concentration
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Absorption depends on concentration
Absorbance = § l C
§ = molar absorptivity
l = path length
C = concentation
The next two laboratory experiments deal with absorption of
visible light and determination of concentration.
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Not everything that is clear and colorless
is the same: how can we tell them apart?
Although we cant always see differences in
organic materials with our eyes, we cancharacterize them if we look for absorption in
the IR
IR absorption: happens due to vibrations of bonds in
molecules; best suited for distinguishing/identifyingfunctional groups
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17
unct ona gr oups are st ngu s e y
their spectra
1 micr on = 1 micr ometer = 1 ®m
Wavenumber =
Recipr ocal - in cm-1
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Detecting an IR Spectrum
IR Source
CH4
CH4
Spectrum
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Atoms bonded together are not stationary, they vibrate. Bondsstretch and angles bend.
Simple model: two atoms connected by a spring
Adding energy (via IR absorption) makes the atoms vibrate withmore energy; the vibrational energies are quantized (just like n
levels in atoms)
Molecular vibrations: IR absorption
http://icn2.umeche.maine.edu/genchemlabs/IR/
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20
Motions of CO2
Go to CO2
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Interpreting IR stretching f requencies
IR absorption frequencies depend on Mass of atoms bonded together
Strength of the interaction between the atoms
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Interpreting IR stretching f requencies
For the same elements, larger mass atoms willvibrate at a lower frequency (more slowly)
(spring demo)
The C-D stretching frequency is lower than the C-H stretching frequency.
So, what would that indicate about the relative
wavelengths?
C
-D would absorb at a longer wavelength thanC
-H.
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Interpreting IR stretching f requencies
Pairs with higher bond energies will generally vibrateat a higher frequency and
have shorter wavelength IR absorption, c = ¯*-
Trend in vibrational frequency, largest to smallest.
HF > HCl > HBr > HI
Bond 565 427 363 295
Energies
*Note that this means that HI absorbs at the largest wavelength.
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CH3CH2CH2CH2CECH
CqC, C=C, & CEC
CH3CH2CH2CH2CH2CH3
C-H Stretches
C-C Stretches &
H-C-H bends
CH3CH2CH2CH2CH=CH2C=C stretch
=CH2 stretchCEC stretch
EC-H stretch
C-H Stretches
C-C Stretches &
H-C-H bendsCEC stretch
C-H Stretches
C-C Stretches &
H-C-H bendsCEC stretch
C-H Stretches
C-C Stretches &
H-C-H bendsCEC stretch
- increases
Ener gy increases
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Question #1
Which pair of atoms in the table below has the
weakest bond?
Pair Absorption Wavelength
C-C 9.1 m
C-H 3.4 m
C-O 9.7 m
C-Cl 14.3 m
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Question #2
Which molecule has the lowest H Xstretching frequency?
A. H2O
B. NH3
C. PH3
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Using IR spectr oscopy to Identif y Functional
Gr oups
Because IR frequencies provide informationabout bonds between atoms, it is very useful
for identifying functional groups:
Functional Gr oup Characteristic Vibration wavelength
-OH O-H stretch 2.7 m
-Cl C-Cl stretch 14.3 m
C=O C=O stretch 5.8 mCN CN stretch 4.4 m
The IR stretching f requency is of ten highly
characteristic for a given f unctional gr oup
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Three C4 Compounds
C2H5OC2H5
C2H5NHC2H5
C4H9OH
- increases
Ener gy increases
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Detection of light by biological molecules
(not including the eye)
Your skin contains a wide variety of organic
molecules, including those that contain double
bonds. Thus, you can detect
Sunlight (UV), and
Heat (IR)
with your eyes closed!
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Lecture 16 & 17
Reading 8.7-8.8, 12.1-12.3, 4.5, 14.1-14.3