November 6, 2003
Flame Test and Atomic Spectra 26
Objective:
Students will experiment and observe the flame colors of various salts and compare the color given off by an unknown with the known metal salts. In part II, electricity will be passed through an unknown gas and a comparison of known gases will be used to identify the gases. Line spectrums of hydrogen, helium, and neon/argon gas will be used to compare.
Safety:
1. Wear goggles and an apron.
2. Secure loose items.
3. Caution using the Bunsen burner.
Part 1. Flame Test
Procedures:
1. Obtain a cobalt blue glass and nine wooden splints that have been soaking in the salt solutions located on the front table.
2. Label each wooden splint with the names of the salts so that they do not get mixed up.
3. Light the Bunsen burner and open the air vent to obtain a luminous flame with two double cones. For instructions, see part one of “Lab Investigation 1: Lab Techniques.” Be sure that it is not a yellow colored flame. If it is, re-light the Bunsen burner.
4. Place the end of the wooden splint that was soaked in the salt solution. Hold the other end of the wooden splint using a crucible tong. Burn the spilt at the top of the inner blue cone.
5. Repeat step four with each of the 8 wooden splints and record the colors of the flame as precisely as possible. The color given off by the salt is its initial color, not the yellow-orange color produced by the burning wood.
6. For the sodium potassium mixture, observe the colors as before and once again by looking through the cobalt blue glass. The cobalt blue glass cuts out any orange color.
7. If more observations are needed, dip the wooden splint in the solutions
for a few minutes and repeat. Otherwise, discard the wooden splints in the
trashcan at the end of the experiment.
Observations:
|
Metal found in the Salt |
Flame Color and Intensity |
|
Lithium |
Bright “cinnamon” red |
|
Barium |
Fuchsia red |
|
Strontium |
Reddish orange |
|
Calcium |
Pale orange |
|
Copper |
Bright green |
|
Sodium |
Medium orange, yet brighter then calcium |
|
Potassium |
Lavender |
|
Sodium and Potassium |
Without Cobalt blue glass: light orange With Cobalt blue glass: bright purple |
|
Unknown |
Lavender |
Part 2. Atomic Spectra
1. Go to a lab station where a hydrogen gas discharge is set up.
2. Turn on the electricity and observe the color given off by the reaction of electricity passing through gas. Record your observations in the data table.
3. Look through a diffraction grating and record the colored lines (spectrum) produced by the gas. Record only one set of colors. Be sure to draw the color lines seen.
4. Go to the rest of the lab tables and repeat with each unknown gas.
5. Turn off the gas discharge tube when not in use and return the diffraction
grading.
Observations:
|
|
Color of the discharge tube |
Sketch of the line spectrum |
|
Hydrogen |
|
|
|
Helium |
|
|
|
Argon |
|
|
|
Unknown |
|
|
Analysis:
Part 1. Flame Test
1. List the colors observed in this lab from the lowest energy to the highest energy.
Lithium-bright “cinnamon” red, Barium-Fuchsia red, Strontium-Reddish orange, Calcium-pale orange, Sodium-Medium orange, yet brighter then calcium , Sodium and Potassium- light orange, Copper-bright green, Unknown-lavender, Potassium-lavender.
2. List the colors observed in this lab from the lowest frequency to the highest frequency.
See answer to question #1.
3. List the colors observed in this lab from the longest wavelength to the shortest wavelength.
See answer to question #1
4. What is the relationship between energy, frequency, and wavelength?
High energy = high frequency = short wavelength
low energy = low frequency = long wavelength
5. Based on the results of your experiment, what metal is found in the unknown? Explain.
Potassium is the metal that was unknown. I concluded this result because the color recorded of both salt solutions produced the same color.
6. What is the purpose of the cobalt blue glass? Why is only the purple color of the potassium seen through the cobalt glass?
The cobalt blue glass blocks out any orange light. Since orange light is blocked out, the colors with the higher energy, frequency, and shorter wavelength can be seen through the cobalt blue glass.
7. Do you think we can use the flame test to determine the identity of the unknowns in a mixture? Why or why not?
No, you cannot determine the identity of an unknown mixture because since
it is mixed, it can create new substances and form a different color, so
you can not tell apart from individual elements.
8. How are electrons “excited” in this part of the experiment? What does it mean the electrons are “excited”?
Electrons are excited because it is heated by the fire. When the electrons are “excited,”, it means that the electrons go to a higher energy level, rising above the ground state.
9. Explain why we see distinct lines when metal salts are burned.
Because the number n can only come in certain definite values. This could be explain by the line emission spectrum when an electron of an atom is able to exist in the energy states. There is no diffraction grating slide.
10. How do fireworks give off color? (see p.73 and 102).
When a fuse is lit, it supplies the energy to cause the fireworks to explode. When they explode, their chemical contents react to produce hot gases. The atoms in these gases absorb the energy released by the explosion, causing a chain reaction. When it is absorbed, some if it is released in the form of light. The different colors of fireworks form when different elements are combined and heated at certain temperatures.
11. Give the short hand electron configuration of the following elements.
Lithium: 1s22s1
Barium: 1s22s22p63s23p64s23d104p65s24d105p66s2
Copper: 1s22s22p63s23p64s23d9
Strontium: 1s22s22p63s23p64s23d104p65s2
Calcium: 1s22s22p63s23p64s2
Sodium: 1s22s22p63s1
Potassium: 1s22s22p63s23p64s1
Part 2. Atomic Spectra
12. Each line in the emission spectrum of the hydrogen corresponds to an electromagnetic
radiation with a specific wavelength. Match the 4 observed colors with the
following wavelengths:
a. 410 nm- purple c. 486 nm- green
b. 434 nm- blue d. 656 nm- red
13. How are electrons “excited” in this part of the lab? What happens when the electrons relax?
When electrons are “excited,” they absorb energy. When the electrons relax, they release energy in the form of light. A current passes through a glass tube containing a gas at low pressure.
14. Draw an orbital energy diagram for hydrogen, helium, and neon.
Hydrogen Helium Neon
15. It has been calculated that the observed colors in a hydrogen atom correspond to the relaxation of the electron from a higher energy level to the second energy level. Which color responds to: Explain your reasoning.
a. 6 > 2 transition: violet (not intense) c. 4 >2 transition: blue/green line
b. 5 >2 transition: violet line d. 3 >2 transition: red line
When the electron drops to a lower energy level, it looses a certain amount of energy that corresponds to one quantum light.
16. What gas do you think is found in the unknown?
I think hydrogen is found in the unknown because as the element moves down the periodic table, the colors of the gas tube and the line spectrum varies as energy levels increases or decreases.
17. Each element has its own unique line emission spectrum, just like fingerprints. Explain how this technique can be used to determine the elemental composition of stars.
Since each element produces a different set of spectra lines, the energy difference between energy levels are unique for atoms of each element. Since it has its own “fingerprint,” the light given off by stars can be looked through a diffraction grating disk to see its spectra lines and compared with other known elements.
Conclusion
Summarize your understanding of how fireworks, neon signs, lasers, and street lamps emit light. Include your understanding of the internal structure of atoms and the relationship between electrons and light.
In fireworks, , neon signs, lasers, and street lamps, the energy is supplied when a fuse (electricity) ignites the chemicals. When it reaches a certain temperature, the energy moves the electrons to an excited state and release energy when they return to their ground state in the form of light.
