Emission Spectra Lab: Gas Discharge Tubes and Flame Tests

Transcription

1 Introduction: Spectroscopes can be used to determine the unique electronic spectrum for each element, a result of the individual and discrete arrangement of electrons around each atom. When atoms absorb energy, electrons move into higher energy levels. These electrons then lose energy by emitting light when they return to lower levels. Each discrete line in an emission spectrum corresponds to one exact frequency of light emitted by the atom. A spectroscope works by breaking light into the wavelengths (or spectra) that make it up. Some spectroscopes are made of diffraction grating- a material that has lots of little parallel lines that are approximately one wavelength apart. When light hits the lines, it bends. Different wavelengths (colors) of light bend by different amounts, so it splits the light into its colors. Scientists can tell the elements present in a star by looking at its light through a spectroscope. Each element will have its own unique spectral lines of color, just as people each have a unique fingerprint. Much knowledge about the composition of the universe comes from studying the atomic spectra of stars, which are hot glowing bodies of gases. No two elements have the same emission spectrum. In this lab we will be looking at the emission spectrum of various gas discharge tubes using the spectroscopes. These different gas discharge tubes will give off different colors depending on the element in each tube. For example, mercury vapor lamps produce a blue glow and nitrogen gas gives off a yellowish-orange light.

2 Just as a fingerprint is unique to each person, the color of light emitted by metals heated in a flame is unique to each metal. In this laboratory activity, the characteristic colors of light emitted for calcium, lithium, sodium, potassium, copper, and strontium will be observed. When a substance is heated in a flame, the substance's electrons absorb energy from the flame. This absorbed energy allows the electrons to be promoted to excited energy levels. From these excited energy levels, the electrons naturally want to make a transition, or relax, back down to the ground state. When an electron makes a transition from a higher energy level to a lower energy level, a particle of light called a photon is emitted. An electron may relax all the way back down to the ground state in a single step, emitting a photon in the process. Or, an electron may relax back down to the ground state in a series of smaller steps, emitting a photon with each step. In either case, the energy of each emitted photon is equal to the difference in energy between the excited state and the state to which the electron relaxes. The energy of the emitted photon determines the color of light observed in the flame. Seen through a spectroscope, the light given off by these elements is observed as being composed of discrete lines with characteristic wavelengths. Wavelengths are commonly listed in units of nanometers (1 nm = 1 x 10-9 m or 1 x 10 9 nm = 1 m), so a conversion between meters and nanometers is generally made. The color of light observed when a substance is heated in a flame varies from substance to substance. Because each element has a different electronic arrangement, the electronic transitions for a given metal are unique. Therefore, the exact energy of the emitted photon, and its corresponding wavelength and color are unique to each substance. As a result, the color observed when a substance is heated in a flame can be used as a means of identification.

3 Procedure: This lab will be performed in pairs. YOU MUST WEAR YOUR GOGGLES AT ALL TIMES AND ALL HAIR MUST BE TIED BACK. Gas Discharge Tubes: 1. Using the spectroscopes, record the emission spectra for the various gases from the gas discharge tubes provided by the instructor. Record on the report sheet the gas tube tested, the observed color of the light, and the color of the emission lines. Flame Tests: 1. One partner will obtain a wet wooden Q-tip. With the Q-tip, take a small sample of the salt to be tested and hold the sample over the Bunsen burner. Allow some of the moisture to burn away, a characteristic flame will result for each compound tested. While the salt is burning, the other partner will observe the spectrum using the spectroscope. Record on the report sheet the compound tested, the observed color of the flame, and the color of the emission lines. The salts to be tested are calcium chloride (CaCl2), lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), copper (II) chloride (CuCl2) and strontium chloride (SrCl2). The metals in these compounds are responsible for the colors one will see, i.e. it is the sodium that causes the color in NaCl, not the chloride.

4 Pre-lab Requirement: Read the lab handout carefully. The following items are needed in your composition notebook. Title of Lab Summary of introduction Purpose Summary of procedure 2 Emission Spectrum Data Sheets Answer the following Pre-lab questions. Answer in complete sentences where appropriate. 1. How does a spectroscope work? 2. How are spectra lines similar to fingerprints? 3. How did scientists gather information about stars? 4. What is the difference between an excited electron and a ground state electron? How does an electron move from a ground state to an excited state? How does an electron in the excited state move to the ground state? Analysis Questions: 1. Compare the results you obtained to the actual results posted by your teacher. Did you see the same spectra lines in your experiments? 2. How could a forensic scientist (a person who tests evidence found at a crime scene) use knowledge of spectra lines to test chemicals collected as evidence? 3. Paper logs soaked in solutions of metal salts and dried are sold for producing colored flames in fireplaces. If you were producing one of these logs explain which compounds you would use, and why you would use them, to produce a flame to celebrate Halloween? Christmas? 4. A bright line in the bright line spectrum of sodium has a wavelength of 590 nm. What is the frequency of this line? What is the energy emitted by this line? Wavelengths are commonly listed in units of nanometers (1 m = 1 x 10 9 nm), so a conversion between meters and nanometers is generally made. 5. When an electron in a hydrogen atom falls from the fourth to the first energy level, 1.04 x J of energy are released. What is the frequency of the corresponding electromagnetic radiation? What is the wavelength?

5 Emission Spectra Data

6 Emission Spectra Data