Experiment # Experiment #6 6 - Atomic Atomic Spectra Spectra

CHEM 7L Experiment 6: Atomic Spectra
Page 1
Experiment # Experiment #6 6 -- Atomic Atomic Spectra Spectra
Spectroscopy Spectroscopy
Hugely important branch of Hugely important branch of chemistry. chemistry.
Studies the interaction of light with matter Studies the interaction of light with matter
1
Analysis of the light emitted or absorbed by Analysis of the light emitted or absorbed by
substances substances
Atomic Spectroscopy Atomic Spectroscopy
Spectroscopy applied to atoms Spectroscopy applied to atoms
Atomic Atomic Spectra Spectra
Qualitative Data Qualitative Data
of absorption or emission can be used to of absorption or emission can be used to
identify the element identify the element
2
Quantitative Data Quantitative Data
IIntensity of absorption or emission can be ntensity of absorption or emission can be
used to determine the amount of element used to determine the amount of element
The Nature of Light The Nature of Light
3
Light travels through space as a wave similar to
an ocean wave
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The Nature of Light The Nature of Light
Wavelength ( Wavelength () ) -- peak peak to peak to peak distance between distance between
two adjacent minima or maxima of the wave two adjacent minima or maxima of the wave
Frequency ( Frequency (vv)) -- number number of oscillations per of oscillations per
4
Frequency ( Frequency (vv) ) number number of oscillations per of oscillations per
second second Units Units: : hertz (Hz) hertz (Hz) 1 1 Hz = 1 Hz = 1 ss
--11
Amplitude Amplitude -- height measured from the center of
the wave (related to the intensity of the radiation) of the radiation)
Speed of light (c) Speed of light (c) -- c c = = 2.998 x 10 2.998 x 10
88
m/s m/s
c c = = v v v v = = c / c / = = c / c / vv
Radiation Energy Radiation Energy
Electromagnetic radiation consists of particles
called photons. Each photon can be regarded
as a packet of energy
5
E
photon
= h v = h c (J/photon ~ 10
-19
)
h = Plancks constant = 6.626 10

-34
Js
To calculate E per mole: E
mole
= E
photon
N
A
Electromagnetic Radiation Spectrum Electromagnetic Radiation Spectrum
6
Page 3
Atomic Emission Spectrum Atomic Emission Spectrum
When When atoms are excited by heating or in an atoms are excited by heating or in an
electric discharge, they emit light at electric discharge, they emit light at discrete discrete
wavelengths wavelengths
Atomic Atomic emission spectrum: pattern of emission spectrum: pattern of
7
Atomic Atomic emission spectrum: pattern of emission spectrum: pattern of
electromagnetic radiation emitted by electromagnetic radiation emitted by an excited an excited
atom atom
Each Each element has its own, unique element has its own, unique spectrum spectrum
Useful for chemical analysis Useful for chemical analysis
When When an an electron electron makes makes aa transition transition from from a a higher higher energy energy level level
to to aa lower lower energy energy level, level, the the difference difference in in the the energy energy is is emitted emitted out out
as as aa photon photon.. Each Each transition transition produces produces aa photon photon of of aa certain certain
energy energy and and this this appears appears as as aa line line of of certain certain wavelength wavelength on on the the
spectrum spectrum.. These These lines lines constitute constitute the the emission emission spectrum spectrum.. pp pp
E = h E = h = E = E
ii
-- EE
ff
8
Emission Lines Emission Lines
9
Page 4
Spectroscope Spectroscope
10
11
Experiment #6: Atomic Spectra Experiment #6: Atomic Spectra
I. Spectroscope calibration I. Spectroscope calibration
Use Use mercury vapor lamp mercury vapor lamp
Obtain corrected Obtain corrected from scale reading from scale reading
12
II. Examine one e II. Examine one e- - system: system: Hydrogen Hydrogen
III. Examine two e III. Examine two e-- system: system: Helium Helium
IV. Examine multi e IV. Examine multi e- - system: system:
Alkali & Alkaline Earth Metals Alkali & Alkaline Earth Metals
V. Analyze an V. Analyze an unknown mixture unknown mixture ((22 metals) metals)
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Part I. Spectroscope Calibration Part I. Spectroscope Calibration
Use mercury vapor lamp Use mercury vapor lamp 4 lines in visible 4 lines in visible
emission spectrum: emission spectrum: =579.1nm,546.1nm, =579.1nm,546.1nm,
435.8nm,404.7nm 435.8nm,404.7nm
13
n
e
r
g
y
6
5
3
2
4
Part II. Hydrogen Emission Spectrum Part II. Hydrogen Emission Spectrum
E
Ultra Violet
Lyman
Series
Infrared
Paschen
Series
Visible
Balmer
Series
1
n
14 IN LAB- Use H discharge tube
Part II. Hydrogen Emission Spectrum Part II. Hydrogen Emission Spectrum
15
The results are a combination of measured visual
colored spectral lines and the calculated energy
levels for the electrons
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Hydrogen Emission Spectrum: Hydrogen Emission Spectrum: vv
Emission frequencies related to quantized
transitions by the following equation:
R 1 1
16
v = R
H
1 1
n
f
2
n
i
2
R
H
: Rydberg constant = 3.29 10
15
Hz
n
f
: relaxed electronic energy level
n
i
: excited electronic energy level
Hydrogen Emission Spectrum: E Hydrogen Emission Spectrum: E
E = h E = h v v
EE == (R (R hh)) 11 11
17
EE == (R (R
HH
hh) ) 11 11
nn
ff
22
nn
ii
22
(R (R
HH
hh) = 21.8 ) = 21.8 10 10
--19 19
J/photon J/photon
(R (R
HH
h h NN
AA
) = 1312.75 kJ/mole ) = 1312.75 kJ/mole
Ionization Ionization
Energy of Energy of
Hydrogen Hydrogen
Hydrogen Emission Spectrum: Hydrogen Emission Spectrum:
1/ 1/ = = v/ v/c c
1/ 1/ (R (R / ) / ) 11 11
18
1/ 1/ = (R = (R
HH
/c) /c) 11
11
nn
ff
22
nn
ii
22
IN IN LAB LAB::
Use Use this this equation equation with with n n
ff
== 22 to to determine determine which which
energy energy transition transition each each of of the the observed observed visible visible
lines lines represents represents
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Example: Hydrogen Spectra Example: Hydrogen Spectra -- vv
What What is is the the frequency, frequency, wavelength wavelength and and
energy energy of of a a nn = = 22 to to n n = = 11 transition transition in in
hydrogen? hydrogen? Will Will it it be be visible? visible?
19
y g y g
First First - - Determine Determine the the frequency frequency ((vv))::
vv == ((33..29 29 10 10
15 15
Hz) Hz) 11 11
11
22
22
22
== 22..47 47 10 10
15 15
Hz Hz (= (= s s
--11
))
Example: Hydrogen Spectra Example: Hydrogen Spectra --
energy energy of of a a nn = = 22 to to n n = = 11 transition transition in in
20
Second Second - - Determine Determine the the wavelength wavelength ( ())::
cc == v v == cc // vv
== ((22..998 998 10 10
88
m/s) m/s) // ( (22..47 47 10 10
15 15
ss
--11
))
== 121 121 10 10
--99
mm == 121 121 nm nm (= (= UV) UV)
Example: Hydrogen Spectra Example: Hydrogen Spectra -- EE
energy energy of of a a nn = = 2 2 to to n n = = 11 transition transition in in
21
Third Third - - Determine Determine the the Energy Energy (E) (E): :
EE == hh v v
EE== ((66..626 626 10 10
--34 34
Js) Js) ((22..47 47 10 10
15 15
ss
--11
))
== 16 16..4 4 10 10
--19 19
J/photon J/photon
EE= = 986 986 kJ/mol kJ/mol
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Part III. Helium Emission Spectrum Part III. Helium Emission Spectrum
Use Use same same method method as as Part Part II II to to observe observe the the
emission emission spectrum spectrum of of excited excited Helium Helium
22
Will use a helium discharge tube Will use a helium discharge tube
Will use quasi Rydberg equation to Will use quasi Rydberg equation to
attempt to calculate energy states attempt to calculate energy states
Parts IV Parts IV- -V. Alkali/Alkaline Earth Spectra V. Alkali/Alkaline Earth Spectra
Observe emission spectra of Alkali and Observe emission spectra of Alkali and
Alkaline earth metals Alkaline earth metals
23
Excitation source: Bunsen burner Excitation source: Bunsen burner
Salt samples (Li, Na, K, Ca, Sr, Ba) Salt samples (Li, Na, K, Ca, Sr, Ba)
These observations will allow you These observations will allow you
to identify an to identify an unknown unknown mixture mixture
Flame Test Flame Test
24
Sodium Sodium
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Practical Notes/Recommendations Practical Notes/Recommendations
Tilt Tilt Bunsen burner when burning metal to Bunsen burner when burning metal to
avoid plugging it up avoid plugging it up
25
"Unknown" has "Unknown" has two two metals! metals!
Lab report worksheet filled out in lab Lab report worksheet filled out in lab
Read the Lab Manual! Read the Lab Manual!
FOR MORE INFO... FOR MORE INFO...

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CHEM 7L Experiment 6: Atomic Spectra

h = Plancks constant = 6.626 10

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