CHEM 110 Lecture Notes - Lecture 4: Flame Test, Probability Distribution, Planetary System

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31 Aug 2016

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Visible light contains a continuous distribution of frequencies of electromagnetic radiation. Spectrum- distribution of frequencies of light absorbed or emitted by an object. Due to its wave nature, light can undergo diffraction around an object or through a prism or series of small slits. Each element has a unique line spectrum. Flame test- an analytical procedure used by chemists to detect the presence of particular metal ions, based on the color or the ame produced. Observation of line spectra implies that atoms have discrete(quantized) energy levels. Line spectra are results of light interacting with atoms. The absorption or emission lines are the results of electron transitions: electrons moving from one allowed energy state to another. Atom is similar to planetary system with electron orbiting central nucleus. Only certain speci c radii (orbits) are allowed! Rn = (n2/z) a0 = n2 a0. Where z = nuclear charge of h-atom, a0 = bohr radius (0. 529 ),

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Procedure for Part 1 1.Using the TV Specs, observe and record atomic emission spectra from H lamp. 2.Sketch the atomic emission spectrum of H. Include color and wavelength.3.Repeat step 1 with the Hg lamp.4.Sketch the atomic emission spectrum of Hg. Include color and wavelength.5.Repeat step 1 with the Helamp.6.Sketch the atomic emission spectrum of He. Include color and wavelength.

Procedure for Part 2

4.Conduct a full spectrum analysis of a colored solution.a.Fill a cuvette Â¾ full with the colored solution and place it in the spectrometer. Align the cuvette so that the clear sides are facing the light source of the spectrometer.b.Click . A full spectrum graph of the colored solution will be displayed.c.Once signal has stabilized, click d.Examine the graph, noting the peak or peaks of very high absorbance or other distinguishing features. Compare the color of the solution with the wavelength associated with that color. Use the ârainbowâ chart drawn above.e.To Save your graph, select Store Latest Run from the Experiment menu.

Post lab question

1. Briefly explain why the gas discharge tube glows when the high voltage is

applied. Why does the gas not glow when the voltage is turned off.

2. When you look directly at the gas discharge tube through the grating, do you see the color

of a single wavelength of light or a blend of wavelengths?

3.Other than our blending of colors of different wavelengths emitted from a single source,

what is another limitation of using our eyes to detect electromagnetic radiation?

4. Explain how the emission phenomenon can be used

to find out what elements are present in an unknown sample.

5. What is a continuous spectra? What type of source creates a continuous spectra?

6. Explain why atomic emission spectra are not continuous.

7. How many lines are there in the Hg line spectra? How many

did you see? What might be the reason that these two answers are different?

8. Why do atoms absorb and emit light of the same frequency? In other words, why donât

atoms absorb light at one set of frequencies and emit light at another set of frequencies?

9. Why does the atomic emission spectrum vary from element to element? Explain how

atomic structure influences the spectrum.

10. In part 2, does the wavelength of maximum absorbance (ie: the color that corresponds to

the wavelength) for each solution correspond to the color of the solution that you âseeâ?

Describe why or why not? (For instance, for the red solution, was the highest absorbance

measured at the red wavelength and describe why or why not?)

plz help me answer as much as possible thanks

Use a ruler to calibrate the spectral limits in the figure above. In other words, what are the right and left limits in nm, and what is the equation that converts distance on this page in cm to spectral units of nm? This âcalibrationâ will help you with the next item.

Use the NIST Atomic Spectral Database to assign the three bright spectral lines in the hydrogen spectrum.

Abstract This physical chemistry laboratory exercise will have students work with provided emission spectra for several elements and data collected on the NIST Atomic Spectral Database. First, students will develop a method for calculating what wavelengths of visible light are associated with the most intense spectral lines in the provided visible emission spectra. Then, the NIST Atomic Spectral Database will be accessed in order to identify select properties associated with the four most intense spectral lines. Introduction The emission process can be described in two general parts. Initially, energy is absorbed by an atom, and its electrons can be moved into higher energy (excited) states from their ground state The excitedrelectrons then "relax" and drop back to their ground state. Energy lost during an electron's retaxation process is emitted as a photon. Recall that the energy of a photon is photon upper E lower Where Ephoton is the energy of the photon that is emitted when the emitter drops from an upper energy state to a lower energy state, h is Planck's constant, c is the speed of light, and v is the wavenumber. Remember that the length units should match for c and v This is a good time to revisit a few concepts. Atoms and molecules possess discrete energy levels their energy levels are quantized and not continuous. Using eq 7.1, it is possible to determine the energy associated with a specific excited state to ground state transition if we can determine the wavelength of emitted light. Also keep in mind that there are selection rules that limit the type of transitions we can expect to observe. All of these points are reflected in an elemental or ionic emission spectrum. The visible light spectrum falls within a fairly narrow range of wavelengths, from approximately 400 to 700 nm. Even so, discrete spectral lines are seen in each element's visible light emission spectrum. Each (take some time to reflect on why that would be), each element also has a unique emission spectrum. This provides a diagnostic tool for determining the composition of a sample. As an example, the elements present in celestial objects can be determined by the light that they produce line represents one possible transition. Since each element has unique energy levels Some of the transitions observed in this lab exercise involve d-orbitals. As such, a discussion regarding d-orbitals and related selection rules is necessary at this point. The first goal is to determine which character table should be used. Start by observing the shape and relative orientations of the d-orbitals. Refer to Figure 1 below

Light visible to the human eye is a tiny portion of the entireelectromagnetic spectrum. The most
obvious feature of visible light is color. Each color of light isidentified by its wavelength, which is usually
measured in nanometers (nm). The visible spectrum is the entirerange of colors or wavelengths your
eye can see. When white light from a lamp or the sun travelsthrough a drop of water, it is dispersed
into its component colors. This is the familiar rainbow of colorsand is called a continuous spectrum.
You can produce the same effect by passing white light through aprism or a diffraction grating.
When atoms are excited, either with heat or in an electricdischarge, they absorb energy and are said to
be in an excited state. The excited atoms reemit the energy in theform of light energy when they relax
back to a ground state. The light is emitted only at certainwavelengths that are specific to the atoms in
the sample. Consequently, if this light is passed through a prismor diffraction grating it will appear quite
different from the familiar rainbow. There will be a pattern ofcolored lines separated by dark areas of
no color. These lines of color and their corresponding wavelengthvalues constitute a line emission
spectrum for the element and reveal detailed information regardingthe electronic structure of the
atom.
Every element has a unique line spectrum and the characteristiclines can be used to identify an
element. If a gaseous element is heated and its emitted lightenergy viewed through a diffraction grating
or a prism, you can see the emission lines of that element that arein the visible region of the
electromagnetic spectrum.
As an example, when you look through a diffraction grating atfluorescent light you see three very
noticeable lines or narrow bands of bright color superimposed on acontinuous rainbow. These three
lines are caused by the light emission of glowing gaseous mercuryatoms in the fluorescent tube and the
continuous rainbow is produced by the white light in thefluorescent tube. In the mercury line spectrum,
one line is violet, the second is green and the third is yellow.Each of these lines has a characteristic
position in the visible spectrum, which is the same as saying eachof these lines has a specific
wavelength. For mercury, the visible emission lines are identifiedas follows:
In this experiment, you will use a spectroscope to view lightsources that produce continuous and line
spectra. You will also observe the emission colors produced byheating various salt

Experimental Procedure
Part A: Fluorescent Light
Obtain a spectroscope box. Make sure it has a slide mount whichcontains a diffraction grating taped at
one end. On the side opposite to the grating, there should be anarrow slit made by placing black tape
over an opening. Look through the diffraction grating while holdingthe box up to a fluorescent light.
Light will enter the box through the taped slit. You should see anarrow rainbow band superimposed
over the plastic ruler. Superimposed on this rainbow band should bethe emission lines of mercury. On
your report form, describe what you see with words and apicture.
Part B: Candle and Light Bulb
Take the spectroscope box back to where the candle and light bulbare located. Observe the spectrum
for each. CAUTION: BE CAREFUL NOT TO PUT THE SPECTROSCOPE BOX TOOCLOSE TO THE CANDLE
FLAME. On your report form, describe what you see with words and apicture for the light bulb and the
candle. Pay particular attention to the width of each coloredband.
Part C: Emission Tubes
At various places in the laboratory there are elemental emissiontubes. View each emission tube
through the spectroscope box. On your report form, record the colorof the glowing gaseous element
and the color of each emission line for each element. Also on yourreport form, record where the line is
superimposed over the ruler (in cm).
Part D: Salt Emissions
For this section you need to wear your safety glasses. Your TA willburn various salts. Observe the color
of the flame for each different salt burned. Record the colors onyour report form with the chemical
name and formula of the salt compound.

The questions are:

1. There are safety hazards in this lab. Specifically, what arethese safety hazards? Explain why they are

hazardous.

2. Give the order of colors in a rainbow from shortestwavelength to longest wavelength and provide

approximate numeric values for the highest and lowest visiblewavelengths of light.

3. In the electromagnetic spectrum, as the wavelength increases,what happens to frequency? Give the

mathematical expression that supports your answer.

4. In your own words, describe the difference between acontinuous spectrum and a line spectrum.

5. Which radio station broadcasts at a longer wavelength: 93.9FM or 104.3 FM? To support your

answer, list the actual wavelength values and units for each ofthese radio stations.

CHEM 110 Lecture Notes - Lecture 4: Flame Test, Probability Distribution, Planetary System

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