Pisgah High School Chemistry

1
The Hydrogen Spectrum
Mike Jones
Pisgah High School Chemistry
Canton, NC
Rev 020804
2
In this experiment you will measure the
wavelengths of the lines in the visible portion
of the hydrogen spectrum and compare them to the
calculated values predicted by the Bohr Model.
3
This is a spectroscope
We can use it to measure the wavelengths of the
lines in atomic spectra.
4
A high voltage is applied to a narrow tube filled
with hydrogen gas and light is produced.
5
Behind the eyepiece is a transmission diffraction
grating which separates the light into its
components.
6
The spectral lines produced by the the
diffraction grating appear superimposed over the
scale.
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The light from the hydrogen discharge tube passes
through a narrow slit.
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The spectral lines appear superimposed over the
scale so that their wavelengths can be determined.
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The wavelength is the number times 10-7 meters.
Or the wavelength is the number times 100
nanometers.
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Light from the spectrum tube coming through the
slit
Measure the wavelength of each line in meters and
nanometers.
2 3 4 5 6
Actual photograph of hydrogen spectrum. - M.
Jones 2/5/04
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Now you will predict the wavelength of light
emitted as the electron moves from a higher
energy level to a lower energy level.
You will be given the energy level the electron
comes from, and the energy level the electron
goes to.
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Use Bohrs equation to predict the energy of each
allowed state.
n is the principal quantum number and has values
of 1, 2, 3 7
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Rh is called the Rydberg Constant,
Rh has a value of 2.18 x 10-18 J.
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Next, find the difference between the two allowed
energy states.
DE E final E initial
Then, compute the wavelength.
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Complete a table like the one below.
Initial energy level ninitial
Final energy level nfinal
Initial energy (J) Einital
Final energy (J) Efinal
Energy difference in (J) DE
Wavelength (m) l
Wavelength (nm) l
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You will now combine your results with the
results of other students to compile a list of
all of the possible transitions from a higher
energy level to a lower energy level, which give
off light.
A table like the following one will be on the
board.
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Electrons come from these higher energy levels and go to Electrons come from these higher energy levels and go to Electrons come from these higher energy levels and go to Electrons come from these higher energy levels and go to Electrons come from these higher energy levels and go to Electrons come from these higher energy levels and go to Electrons come from these higher energy levels and go to
7 6 5 4 3 2
6
5
4
3
2
1
These transitions do not result in an emission.
these lower levels
Enter the wavelengths in nanometers here.
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Finally, compare the wavelengths you measured to
the wavelengths predicted by the Bohr model.
How do they compare? Were all of these lines
visible?
In what parts of the electromagnetic spectrum do
they appear?
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Do the results of your experiment support the
Bohr Model? Why or why not?
What additional experiments could you do in order
to further test the model?
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Send questions, comments, and suggestions to
Mike Jones Pisgah High School 1 Black Bear
Drive Canton, NC 28716 828-646-3463 mjones_at_haywoo
d.k12.nc.us