chm115 lecture15

8/8/2019 CHM115 Lecture15

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Lecture 15



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.



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.



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)



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



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



A ³photochemical reaction´ with visible light

Cl2 + h¯ 2 Cl·

2 Cl· + H2 2 HCl (net)

first step initiated by blue/violet light (visible)



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.



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



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



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



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



Spectr oscopy

Diff erent molecules can absorb light of diff erent color s (ener gies).

Demo



Absorption depends on concentration



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.



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



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



Detecting an IR Spectrum

IR Source

CH4

CH4

Spectrum



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/



20

Motions of CO2

Go to CO2



Interpreting IR stretching f requencies

IR absorption frequencies depend on Mass of atoms bonded together

Strength of the interaction between the atoms




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.




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.



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 &


C-H Stretches

C-C Stretches &


- increases

Ener gy increases



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



Question #2

Which molecule has the lowest H Xstretching frequency?

A. H2O

B. NH3

C. PH3



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



Three C4 Compounds

C2H5OC2H5

C2H5NHC2H5

C4H9OH

- increases

Ener gy increases



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!



Lecture 16 & 17

Reading 8.7-8.8, 12.1-12.3, 4.5, 14.1-14.3

chm115 lecture15

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