NAME:
DATE:
FLAME LAB
SPECTROSCOPY
PRELAB
Read the entire lab and answer the following questions on a separate sheet of paper (you may need to consult your textbook or the internet):
1. Is a flame test an emission or an absorption spectroscopy technique?
2. Predict the color of calcium and sodium in a flame.
3. Prepare a data table in the space provided. Include the compound formula, metal identity, flame color, wavelength of light emitted and energy for each sample. Remember units.
PURPOSE
To examine how elements produce interesting effects when heated in a flame. To use flame color as a form of emission spectroscopy to identify metals.
THEORY
The flame test is a type of spectroscopy, which is an analytical technique that analyzes light energy released or absorbed by compounds and atoms. Spectroscopy can be classified as emission or absorption depending on whether you purposely excite the atoms for analysis of if you allow light to be absorbed by the elements. Emission spectroscopy causes substances to emit light of a certain frequency and wavelength when excited electrons return to ground state. The flame test is an emission spectroscopy analysis. Absorption spectroscopy measures how much light of a certain wavelength and frequency can be absorbed by elements and compounds. UV-VIS spectrophotometery (used in AP chemistry) is an example of absorption spectroscopy.
The unique pattern of frequencies of light emitted by an atom corr4esponds to the set of energies given off as electrons drop from a higher energy level to a lower energy level closer to the nucleus. Electrons must be previously excited by current, heat, or light energy. Excited electrons cannot remain excited very long. When they return to lower energy levels, the energy may be calculated using p=Planck’s equation: E = h n, with each frequency corresponding to a discrete amount of energy. The energy is released as a photon of light. By observing the wavelength of light released, the energy of the electron transition may be determined by the relationship. E= hc/l.
In this lab, each compound contains a cation (positively charged metal ion) and an anion (negatively charge non-metal ion) that form a salt that has no net charge. The non-metal atoms do not emit light in the visible region, so all colors that appear can be attributed to the metals in the compound. We will categorize each cation by the color of light it emits and use that knowledge to determine the identity of an unknown metal in solution.
MATERIALS
Salts – see containers in lab. Write formulas for each salt and find names to write in your materials section. Show the metal that will be tested from each salt.
Bunsen burner
matches
goggles
mystery cation solution
spectroscopes
geisler tubes
power supply
PROCEDURE
Flame excitation
1. Light the Bunsen burner. Adjust the flame to about 3” and make sure it is dark blue in color.
2. Obtain a wooden splint that has been soaked in deionized water.
3. Dip the splint into the salt to obtain a few crystals at the tip.
4. Hold the split/salt in the flame and observe the color of light emitted. Record the color, salt, and metal identity in your data table. When you observe the potassium salt, use a piece of cobalt glass to help distinguish it from the sodium ion.
5. Test the unknown solution. Obtain a splint that has been soaked in an unknown metal solution. Determine the identity of the metal based on colors observed for the known metals.
Current excitation/ spectroscope analysis
6. Observe the Hg geisler tube with a spectroscope. Draw the discrete line spectrum for Hg. Color the bands, record wavelengths, calculate frequency and energy for each line observed.
DATA
CALCUALTIONS
Show a sample calculation for 1 metal:
1. calculate wavelength in meters
2. calculate frequency in hertz
3. calculate energy in Joules
Show a sample calculation for one of the lines in Hg spectrum.
1. calculate wavelength in meters
2. calculate frequency in hertz
3. calculate energy in Joules
POST LAB QUESTIONS
1. Pretend for the moment that you are an analytical chemist (a chemist who uses instruments to determine the composition and identity of unknown samples). Your boss asks you to use a flame test to identify a particular unknown sample. Keeping in mind that your continued employment is dependent on obtaining correct, unambiguous results, which ions do you hope are not contained in the sample? Explain your choices.
2. If you were an analytical chemist, you probably wouldn’t depend on your eyes to determine the ions in solution. Find 2 methods of analysis that include emission spectroscopy. Name the methods and describe the processes.
3. You have a lab partner who consistently left the wood splint in the flame too long, and the wood caught fire for each sample ion tested. For which ion or ions, would this not have interfered with a correct color identification?
4. List the following ions in increasing order of the energy of the light emitted:
Cu+2, Na+, K+ (underlying color, viewed through the cobalt glass), Li+, Hg+2.