Test solution Color of flame Which positive ion is present? Purified water

Transcription

1 Experiment 19: Spectra Name: Part 1 Flame Tests Positive ion tested K + Color of flame Ba 2+ Ca 2+ Cu 2+ Sr 2+ Na + Test solution Color of flame Which positive ion is present? Purified water Tap water A raisin Pink liquid soap Unknown number: Report sheet for Exp. 19 Page 1 of 11 Alscher

2 Part 2 Spectroscopic Examination Use crayons to record the spectrum that you see in each case. Daylight or a regular light bulb, or other Hydrogen Mercury Helium Report sheet for Exp. 19 Page 2 of 11 Alscher

3 Questions (Note: to answer these questions, refer to your textbook and your lecture notes.) 1. Fill in the following table regarding the hydrogen spectrum. Refer to your textbook for the initial and final values of n. Color Known λ (nm) n initial n final red turquoise dark blue purple Show the calculation all of the above wavelengths using: E = J n f n and E = hc i λ (Plug in values of n i and n f to calculate E, then solve for λ.) Report sheet for Exp. 19 Page 3 of 11 Alscher

4 Continue work for question Report sheet for Exp. 19 Page 4 of 11 Alscher

5 3. The lines of the Lyman series of hydrogen cannot be observed by the naked eye. The transitions are in the ultraviolet region of the EM spectrum. The wavelengths are 91.2nm, 93.8 nm, 95.0 nm, 97.3 nm, nm, and nm. Determine the energy of each A similar series of emissions exists in the infrared region of the hydrogen spectrum. This is the Paschen series. The wavelengths are: 1005 nm, 1094 nm, 1262 nm, and 1875 nm. Determine the energies of these transitions, show your work below, and put your final answers in the chart. Report sheet for Exp. 19 Page 5 of 11 Alscher

6 Lyman Series Wavelength Energy nf ni Paschen Series Report sheet for Exp. 19 Page 6 of 11 Alscher

7 4. What causes light to be emitted from an atom? 5. Why are only certain wavelengths emitted from an atom? (In other words, why do the spectra show lines of light instead of a continuous rainbow of light?) 6. Why does hydrogen emit different wavelengths of light than mercury? 7. For a hydrogen atom, only the series lines from which n f =2 contains frequencies in the visible region. What is the value for n f for which there are frequencies in the visible region for the H-like ions He +, Li 2+, and Be 3+? Report sheet for Exp. 19 Page 7 of 11 Alscher

8 8. The ground state energy of the electron in a He + is eV. Energies such as these are often measured in electron-volts (ev) and converted to joules or kilojoules. a) Calculate the energy in electronics volts of the state of helium ion for n = 5 and n = 2. b) Use the information from a) to calculate the E for the transition n i =5 nf=2 in ev and joules. c) Calculate the wavelength and frequency of the light that is emitted during the transition n i =5 n f =2. d) What region of the electromagnetic spectrum does this line occur? Note this corresponds to the third line of the Balmer series of the He + ion. 1/λ =RZ 2 (1/n f 2 1/n i 2 ), X10-19 J = 1 ev Report sheet for Exp. 19 Page 8 of 11 Alscher

9 9. Scientists in the 19th and early 20th century were unable to explain why no electrons are made when light shines on the metal surface in the frequency of light below minimum, threshold value, although light of the higher frequency causes the emission of electrons. They were also unable to understand the observation that the greater frequency light is, above the threshold frequency, the greater the speed, and therefore, the kinetic energy of the emitted electrons. Einstein first successfully explained the photoelectric effect in He proposed that there was a minimum amount of energy necessary to eject electron from a metal surface. This minimum energy (the work function of the metal) is a characteristic property of the metal. If a photon of the light striking the metal has energy less than the work function of the metal, no electrons are emitted. If a photon of the light used has energy greater than the work function of the metal, the kinetic energy of the emitted electron equals the difference in the energy between energy of the photon and work function of the metal. a) If a yellow light of wave length nm is shined on a potassium surface, the emitted electrons have a kinetic energy of 5.77 X10-20 J/electron. Calculate the value of the work function and the threshold frequency for potassium. b) If UV light of wavelength nm is shined on a potassium surface what will the speed of emitted electron be? Report sheet for Exp. 19 Page 9 of 11 Alscher

10 10. The energy diagram for hydrogen like ion is given on the next page. a) Identify the ion by determining its atomic number Z. [Hint 1: E gs H = 13.60eV, and you know the E gs for He. Hint 2: don t be fooled by the numbers, sometimes a cigar is just a cigar.] b) On the diagram draw an arrow to indicate the transition that gives rise to the second line in the Lyman series for this ion. c) Calculate the frequency and wavelength of the radiation emitted when the transition you drew occurs. d) At what region of the electromagnetic spectrum does the line occur? e) Using only the energy level diagram how much energy is required to remove the electron from its nucleus? f) Write an (chemical) equation for the reaction that occurs when the electron is removed. This might require you looking into chapter 7 ahead of time. Report sheet for Exp. 19 Page 10 of 11 Alscher

11 0-3.4 ev -4.89eV ev ev eV eV Report sheet for Exp. 19 Page 11 of 11 Alscher