Atomic Theory: Spectroscopy and Flame Tests

Transcription

1 Atomic Theory: Spectroscopy and Flame Tests Introduction: When sunlight strikes your skin, you feel its heat. This is a sign that you are absorbing some of the sun's energy. Light is only one form of energy known as electromagnetic radiation. The electromagnetic spectrum below shows all of the other forms of radiation: visible light, X-rays, radio waves, infrared (IR), ultraviolet (UV) rays and microwaves. Light that can be seen with the naked eye falls within a small range of wavelengths (~ nm) called the visible region. Note that these different forms of radiation possess different energies and present different potential health hazards. For example, patients often wear lead shields when getting dental X-rays, and sunscreens are used to block UVA or UVB rays, forms of ultraviolet radiation that can not only tan skin but can also cause severe sunburns. However, people usually do not worry about exposure to the radio waves being broadcast throughout a city, so they can all listen to their favorite station. These different forms of radiation have electric and magnetic components that travel in the form of a wave. Imagine throwing a pebble into a still pond and watching the circular ripples moving outward. Like those ripples, each energy wave has a series of high points known as crests and a series of low points known as troughs. The figure below shows two different waves : λ A crest λ B trough The wavelength, symbolized by the Greek letter lambda (λ), is the distance between two wave crests, which is equal to the distance between two troughs. Notice that the wavelength for the top wave, indicated by λ A, is greater than the wavelength for the bottom wave, λ B. Since energy waves move, we can count the number of crests or peaks that pass a given point in one second. GCC CHM 151LL: Atomic Theory: Spectroscopy and Flame Tests GCC, 2016 page 1 of 9

2 This is called the wave s frequency, which is symbolized by the Greek letter nu (v) and measured in units of cycles per second called Hertz (Hz = 1/s = s -1 ). For two waves moving at the same speed, the longer the wavelength, the fewer crests or troughs that pass per second. Thus, the frequency of waves is inversely proportional to their wavelengths. These two properties can be related to one another using a proportionality constant equal to the speed of the wave. Light and other forms of energy move at the fastest speed possible in a vacuum, defined as the speed of light. The speed of light is given the symbol c and is exactly equal to m/s. For convenience sake, the speed of light is often rounded to m/s. Since this value is rounded, we don t use it to limit significant figures in calculations. Frequency and wavelength are related according to the following equation: c = λ v Equation 1 Because the heat emitted by the sun and other energy sources is constant, most scientists believed that energy existed as continuous waves. This belief continued for centuries until a German physicist named Max Planck proposed a controversial new theory: Energy was not only a wave but also a particle. Based on his experiments on Blackbody radiation, Planck theorized that energy is emitted in small bundles called quanta or quantum for a single bundle, which led to the theory s name: quantum mechanics. Each quantum of energy has a specific frequency associated with it, and the frequency is directly proportional to its energy: E = hv h=6.626 x J s Equation 2 where h is Planck's constant, which is equal to x J s. Albert Einstein later applied Planck s theory to light, so a particle of light is now called a photon. Given its wavelength, the energy of a photon or other kind of energy can be determined by combining Equations 1 and 2. In this lab, you will calculate the wavelength, frequency, and/or energy of various forms of electromagnetic radiation. I. USING LIGHT AND COLOR TO ANALYZE SAMPLES A spectrophotometer (often abbreviated as Spec-20 ) is an instrument that measures the intensity of a light beam passing through a solution. Most Spec-20 s operate in the visible and IR regions; for example, the Genesys Spec-20 s used in our labs use wavelengths ranging from 325 nm to 1100 nm. Spec-20 s generally have wavelength control buttons, a display, and a zero button. Inside, there is a white light source, a prism to separate the white light into the spectrum of colors (each with a different wavelength), a sample compartment, and a detector. Spec-20 s are generally used to analyze colored solutions. Light of a given wavelength (selected by the control buttons) passes through the sample, hits the detector, and the detector measures the solution s absorbance (A), the amount of light absorbed by the solution s molecules and/or GCC CHM 151LL: Atomic Theory: Spectroscopy and Flame Tests GCC, 2016 page 2 of 9

3 ions. In general, darker solutions are more concentrated (i.e., containing more molecules or ions), and thus have a higher absorbance. In this experiment, you will use a Spec-20 to determine the color of light at various wavelengths. Note that as you change the wavelength you are actually turning the prism so different colors shine through the slit onto the sample. II. ATOMIC EMISSION SPECTRA AND FLAME TESTS The sun is 93 million miles away, and other stars are many light years away. (Note that one light year = six trillion miles or 6 x miles). In spite of these great distances, the elements in stars can be determined by analyzing the light they give off since the atoms of every element selectively absorb and emit light of specific wavelengths, and thus, specific energies. These characteristic wavelengths account for the different colors substances emit when heated. The unique colors emitted by each element provided experimental evidence for Danish physicist Neils Bohr to propose that electrons in an atom occupy orbitals, and each orbital has a specific energy. Because negatively charged electrons are attracted to the positively charged nucleus, they prefer to be in the lowest energy orbitals close to the nucleus; thus, an atom is in its ground (or lowest energy) state when its electrons occupy the lowest energy orbitals. However, when atoms are heated, they absorb the specific energies needed for their electrons to jump up to higher energy orbitals. Now, the atom is in an excited state, which is unstable. Since the electrons prefer to be closer to the nucleus, they quickly return to lower energy orbitals, and the excess energy is emitted. When the energy emitted falls within the visible range, specific colors are observed. Since every element has a different number of protons and electrons, then the energy gap between its orbitals varies. As a result, different elements release light at different wavelengths, and each element emits a characteristic emission spectrum, often called an atomic fingerprint. When an element is heated, it may emit a characteristic color, usually corresponding to one of the colors in its emission spectrum. This accounts for the different colors observed with fireworks. In this experiment, you will observe the characteristic colors given off by various elements using flame tests and then use your observations to identify an unknown. III. ABSORPTION AND EMISSION SPECTRA The third part of this lab involves an interactive online tutorial to help explain the process electrons go through when emission and absorption spectra are obtained from pure substances. Procedure: PART I. SPECTROPHOTOMETRIC ANALYSIS OF LIGHT Turn on the Spec-20 Genesys spectrophotometer, using the switch on the back side of the instrument at the start of lab; it will take about 15 minutes to warm up before use. On the top right of the instrument is a sample holder that holds the sample cells, called cuvettes. Never insert chemicals directly into the Spec-20, or you will contaminate and damage it. To the left of the sample holder is a panel of buttons that control the wavelength (in nanometers) and output (percent transmittance or absorbance) settings. The up/down arrows by GCC CHM 151LL: Atomic Theory: Spectroscopy and Flame Tests GCC, 2016 page 3 of 9

4 wavelength turn the prism-like device in the Spec-20, so light of only one wavelength strikes the sample in the cuvette. Use the up/down arrows to adjust the wavelength to 400 nm. A piece of filter paper will be placed in the sample holder to reflect the light beam within the spectrophotometer. Stand where each student can see the side of the filter paper that faces the light source. You should see a tiny dot of color on the paper. The color observed is the color of light with a wavelength of 400 nm. Use the up/down arrows to scroll through wavelength values from 400 nm to 750 nm in 25 nm increments to find the colors corresponding to the different wavelengths of the visible spectrum. Use colored pencils to color in the spectrum in the box on your report sheet to show the correlation between color and wavelength. Note: There will be many more shades of color than the seven used in the often-used pneumonic device ROYGBIV (which means: Red, Orange, Yellow, Green, Blue, Indigo, and Violet). Therefore, you may have to use some creative color blending of colors. PART II: Gas Discharge Demonstration Your instructor will show samples of gas collected in thin glass tubes known as gas discharge tubes. The ends of the tubes have electrodes that allow a current to pass through the gas and light up. When inserted into a voltage source, the samples will glow a characteristic color. When the light is diffracted (e.g., with a prism or the diffraction glasses we will be using in class), you can see the separate spectral lines that make up each sample. In the 2 boxes at the top of your Lab Report, draw color representations of the diffracted light from the two gas samples your instructor shows. PART III: FLAME TESTS **Lab Notebook** In this lab, you will record the flame test data directly into your lab notebook. Read through the directions for the flame tests at least once and then think about how you would create a table to organize your experimental data. Here are a couple of hints: You will be testing 6 known solutions: LiNO 3,Cu(NO 3 ) 2,Sr(NO 3 ) 2, Ba(NO 3 ) 2,KNO 3, and NaNO 3. You will label test tubes for your known solutions (from 1-6). Each known solution should correspond to a unique number. For each solution, you will need to describe the emission color of the flame during the test. You will also be testing an unknown solution. You will need to describe the emission color of the flame and determine the identity of the solution. You will conduct flame tests to observe the flame emission colors for your known solutions. Since nitrates (NO 3 - ) do not emit color, you will be observing the color of the emission of the metallic cations. Safety precaution: Do not touch the chemicals on the splints with your fingers! Wash your hands immediately if you accidentally touch the chemicals on any of the splints. GCC CHM 151LL: Atomic Theory: Spectroscopy and Flame Tests GCC, 2016 page 4 of 9

5 1. Label your 6 test tubes with a pencil one for each solution listed above. From the reagent station, put about drops of each solution into the appropriate test tube. Place one splint into each test tube. These should soak for about 5 10 minutes in order to absorb enough solution. 2. At this time, each group should obtain one unknown from your instructor. Get a splint, and place it in the test tube of the unknown to allow sufficient time to soak. Be sure to record your unknown number in your lab notebook. 3. Prepare your 150 ml beaker with about ml of water to dispose of the used splints. 4. Light your Bunsen burner with a striker. Your instructor will check to make sure that your flame is adjusted properly for the activity. You should see two blue cones of flame. 5. Grasp the LiNO 3 wood splint by the tip and place the damp end of the splint in the middle of the flame (in the tip of the inner cone) for a short time (about 2 3 seconds). You should see the color of the metal ion burning in the first few seconds. 6. Avoid burning the wood splint itself. A wet splint cannot burn. If you start to notice the splint burning, it was in the flame past the point of dryness. If it does start to burn, you should immediately dip the splint back into the correct test tube to put out the flame, re-wet the splint and test the flame color again. You can repeat this several times if you have difficulty seeing the color. 7. When you are done testing a splint and its solution, dispose of the burned splint in the 150 ml beaker. 8. Observe and carefully describe the color of the flame on the data table. For example, describe the color as pinkish red or violet red instead of just red. 9. Continue with the other solutions, recording the flame color for each solution. Be as descriptive and accurate as possible. 10. Observe and record the color given off by your unknown in the same manner. Identify the metal in your unknown solution. 11. When you have finished with all splints and solutions, dispose of all materials in the proper solid or liquid waste container in the hood. PART IV. ABSORPTION AND EMISSION SPECTRA: A WEB TUTORIAL The fourth activity of this lab involves an interactive tutorial to help explain the process electrons go through when absorption and emission spectra are obtained from pure substances. Go to the Website link provided in the discussion section for this week s experiment. Your Lab Report Should include the following: ü Header completed in your lab notebook. ü A purpose statement (in notebook) ü Each section of the experiment clearly labeled Observations (in notebook) Data table with a title and proper headings (in notebook) Calculations clearly written showing all work (in notebook) Results/Summary of the unknown solution with the unknown number included (in notebook) ü Conclusion (in notebook) ü Discussion Questions (answers written on pages 7-9) GCC CHM 151LL: Atomic Theory: Spectroscopy and Flame Tests GCC, 2016 page 5 of 9

6 Name: Section: Pre-Lab Questions Write your answers on this page and turn it in to your instructor before starting this experiment. 1. How are wavelength and frequency related? Directly or Inversely 2. How are wavelength and energy related? Directly or Inversely 3. Calculate the frequency of red light with a wavelength of 705 nm? 4. Why is the range of wavelengths used on the spectrophotometer in today s lab only 400 nm 750 nm? 5. A student burns a wood splint soaked in a solution of calcium nitrate, Ca(NO 3 ) 2, and it burns a brick red color. Is it the electrons in the calcium ion or the nitrate ion of Ca(NO 3 ) 2 that is causing the observed color? GCC CHM 151LL: Atomic Theory: Spectroscopy and Flame Tests GCC, 2016 page 6 of 9

7 Name: Section: Discussion Questions Write your answers on these pages and attach them to your report. I: Spectrophotometric Analysis of Visible Light 8 points Shade in all colors between the wavelengths (in nm) listed below: II: Gas Charge Demonstration 12 points Gas Discharge Demo Before putting the diffraction glasses on, identify the element being shown in the discharge tube and its color (be descriptive with the color). Now put the glasses on, and draw exactly what you see as best you can, emphasizing specific lines of color. You should include one full spectrum of diffracted light. This data should be recorded directly in your notebook, then copied here as neatly as possible for your report. Sample 1 Gas: Color of lamp: Spectral Lines Sample 2 Gas: Color of lamp: Spectral Lines GCC CHM 151LL: Atomic Theory: Spectroscopy and Flame Tests GCC, 2016 page 7 of 9

8 Part III: Flame Tests (The data and results for the flame tests should be recorded in the lab notebook only) 1. (3 pts) Describe the difference between the lithium emission and the strontium emission in the flame tests. Part IV: Online interactive tutorial: ü Click on Light Emission and Absorption only for the correct video tutorial. Answer these questions in your own words. Do not restate what the tutorial says word for word, as this would be plagiarism. 1. (4 pts) Describe the difference between the ground state and the excited state of an atom. 2. (4 pts) Describe what happens to an electron when it absorbs energy. Describe the location of the electrons in terms of energy levels and distance from the nucleus. 3. (4 pts) Describe what happens when an electron in an excited atom returns to its ground state. 4. (4 pts) The emission spectrum of hydrogen gives four distinct wavelengths representing four colors of light. The tutorial lists the colors as: violet (410nm), blue (434nm), green (486nm), and red (656nm). Which color of light in the visible emissions spectrum for hydrogen has the highest energy photons? Explain. GCC CHM 151LL: Atomic Theory: Spectroscopy and Flame Tests GCC, 2016 page 8 of 9

9 Questions and Calculations: (You must show all of your work on calculations to receive full credit.) 1. (4 pts) Determine if energy is absorbed or emitted for each electron transition in a hydrogen atom: a. from n = 4 to n = 2 emitted absorbed b. from n = 2 to n = 6 emitted absorbed 2. (4 pts) You are the student representative for Glendale Christmas fireworks display. Explain which chemicals should be used in the fireworks to have the fireworks match the traditional Christmas decorations. (Hint: Consider your flame test results.) 3. (4 pts) KSLX FM radio rocks out at MHz (FM radio station frequencies are in megahertz). Calculate the wavelength in meters for this Phoenix station. 4. (4 pts) Which travels faster X rays or microwaves? Explain your answer. 5. (10 pts) Several Predators (aliens that see IR radiation) have landed near your house one dark night, and Arnold is nowhere to be found. Your filtered lamp is emitting radiation of 5.45 x J per photon of energy. If Predators can only sense IR radiation, will they see the radiation from your lamp? Calculate the frequency and wavelength of the lamp emissions and then look at Figure 3.1 in the book to determine if this radiation is in the IR range. ν = Hz λ = nm Type of radiation GCC CHM 151LL: Atomic Theory: Spectroscopy and Flame Tests GCC, 2016 page 9 of 9