Part 2: Qualitative SpectroscopyPart 3: Quantitative SpectroscopyGas Tube: _________________Ch100: Fundamentals for Chemistry 1Instructor: Tony ZableLAB: SpectroscopyNeon lights are orange. Sodium lamps are yellow. Mercury lights are bluish. Electricity is doing something to the electrons of these elements to produce light of a distinctive color.PURPOSE:1. To build a simple spectroscope2. To observe visible continuous and discrete spectra of various light sources3. To measure the wavelength of spectral lines from a hydrogen light source.4. To calculate the energy of a photon of light using the relation: E =h.fTHEORY:Elements that exist in the ground state (i.e. unexcited) emit no light. Energy applied to the atoms, in the form of an electric current, may be absorbed by their electrons. As the energy isabsorbed, electrons become excited and are bumped up to higher orbits. According to the Bohr model of the atom, only quantum levels of excitation are allowed. Electrons do not remain in the excited state forever. They eventually drop back to the ground state. The energy that made them excited is released as electromagnetic radiation (in other words, light).This is why neon lights glow when plugged in.Electromagnetic radiation seen by the human eye is called visible light. Differences in visible light energy result in color. At the macro level, color gives us a subjective measure of light energy. In the micro view, color is the result of electrons bouncing up and down between orbits. Different orbits give different colors. This objective measure of energy is expressed by frequency, f, and wavelength, (lambda).Light travels in waves much like those seen on the surface of the ocean before they crash ontothe shore. The distance from wave peak to wave peak is called the wavelength ()1. You’ve probably seen ocean waves with wavelengths of 3 meters or more. Visible light is commonly expressed in wavelengths of 300-700 nanometers. Wavelength determines color, and color indicates energy. If the wavelength of light is known, then the energy of that wavelength may be calculated via the following equations from laws of physics:Energy is h times frequency………………….. E = h . f h is Planck’s constant…………….. h = 6.626 x 10-34 J*s c is the speed of light…………….. c = 2.998 x 108 m/sVelocity is frequency times wavelength….. c = f. Frequency is velocity divided by wavelength. f = c / Putting it all together:34 8(6.626 10 )(2.998 10 / )l l-״��= =h c J s m sEIn summary, we see different colors because light consists of different wavelengths (corresponding to each color). The amount energy (in Joules) that is associated with a given color, actually the energy per photon of that color, can be calculated by a “simple” equation (see above).Spectroscopy:1 (Note: is different from peak height which is of interest to surfers and small craft)Ch100: Fundamentals for Chemistry 2Instructor: Tony ZableA single source of light may contain radiation of many different wavelengths. Evidence of this is the way a rainbow reveals that plain sunlight is actually composed of many colors. Dropletsof water in the atmosphere act as thousands of prisms to produce this effect. The simple spectroscope uses a special prism, called a diffraction grating, to do the same thing. In this experiment, light from an energized source will be viewed through a spectroscope. A spectroscope is an instrument that uses a diffraction grating to split up the light into its component colors. The component colors may then be viewed against the calibrated scale inside the spectroscope. EXPERIMENTPart 1: Building a Spectroscope (time permitting)A simple spectroscope can be constructed from the following items: a thin box (such as a shoe box or cereal box) a diffraction grating black electrical tape a scalpel (or razor blade).Procedure:1. Cut a 2 cm square at each end of the box2. Cover one hole with two pieces of tape so that you have a slit about 1 mm wide (see diagram below)3. Cover the other hole with the diffraction grating (be sure grating is aligned with the slit)4. Hold the box so that the grating is close to your eye and point the other end toward a light source. 5. Congratulations. You have just constructed a simple spectroscope.Ch100: Fundamentals for Chemistry 3Instructor: Tony ZablePart 2: Qualitative SpectroscopyIn the following steps you will observe several light sources with your spectroscope. The spectroscope will allow you identify the individual colors that make up the spectrum of that light source. Using colored ink pens or crayons (sorry but yah gotta do it…) sketch the spectrum or line spectra for the following light sources: {feel free to include any additional observations you make…}1. an incandescent or fluorescent light2. hydrogen gas tube3. helium gas tube 4. other light source or gas tube: _____________Part 3: Quantitative SpectroscopyObtain a spectroscope (with wavelength scale) from the instructor. Set-up and view the line spectra for the hydrogen gas tube. Observe and record the color and wavelength of each line.From the wavelength, calculate the energy of each line. Use the results of your calculations toprepare a table showing the “ranking” of colors (from highest to lowest) in terms of the energyand wavelength.Ch100: Fundamentals for Chemistry 4Instructor: Tony ZableData Sheet:Gas Tube: _________________Color Wavelength (nm)Wavelength (m)Energy (J)QUESTIONS:1. Arrange in order of increasing energy: blue, orange, green, violet, red2. Use your data to predict the color of these lines: 444 nm, 500 nm, 650 nm3. What is the explicit relationship between energy and frequency to wavelength as the latter increases or decreases?4. How do the values of the wavelength measured for light compare to the size of an atom (roughly 1x10-10 m)? For discussion, consider an atom to be a sphere, i.e. a Bohr atom.5. Is the gas in a blue “neon” light actually neon? Explain.Ch100: Fundamentals for Chemistry 5Instructor: Tony ZableAppendix 1. Historical Development of SpectroscopyMax Planck (in the 1890’s): explained thermal spectra by assuming that radiant energy (like light) was emitted in discrete packets. Planck proposed that the energy of each energy packet (E) is: E = h.f {where f is the frequency & h = 6.626x10-34 J.s} did not believe this explanation nor did his contemporaries
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