C3 Photoelectric effect PDF

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assignment for chem 141 (intro chem)...

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Ch Che em 14 141 1 FFaall 20 2015 15 TH THE E PHO HOTTOE OELE LE LEC CTRI TRIC C EEFFE FFE FFEC CT Group Name ______________ Background Science was at its pinnacle of understanding the natural world as of 1900. Only three things had yet to be explained. Scientists assumed once blackbody radiation, photoelectric effect and atomic spectra were explained that the study of physics would be complete. The assumption was wrong and led to a new branch of science known today as quantum mechanics. The photoelectric effect is the emission of electrons from a surface when illuminated with light of a certain frequency. The first insight to understanding this phenomenon was presented in 1900 by Max Planck. His formula, E = h, (where h is Plank’s constant) related the energy of a photon to its frequency. Albert Einstein extended this idea of quantized photonic energy to a stream of photons (electromagnetic radiation) and explained the photoelectric effect. Part 1 Objective - What is the relationship between the energy of a photon and its frequency? Qualitatively graph photonic energy versus frequency on the axes to the right.

Part 2 Objective - What determines if electrons are ejected from the surface? 1. Go to http://phet.colorado.edu/simulations/sims.php?sim=Photoelectric_Effect and launch the simulation. 2. Make the following adjustments to the simulation once it has launched.  Increase the intensity to 50%  Check the box “electron energy vs light frequency”. Once these adjustments have been made you should notice the ejection of elections from the surface.

3. Increase the wavelength of the light until electrons are no longer ejected. Record the wavelength in the table below and complete the calculations. 1 electron-volt (eV) = 1.6 X 10-19 J 4. Repeat the above step for each of the metals under the pull down menu. Wavelength Frequency Energy Metal (nm) (Hz) (J) Sodium 545 5.50  1014 3.64  10-19 Zinc 293 1.02  1015 6.78  10-19 Copper 265 1.14  1015 7.55  10-19

Energy (eV) 2.28 4.24 4.72

5. The minimum frequency of a photon that can eject an electron from a surface is called the threshold frequency, t. What is the threshold frequency, t, for Metal Threshold Frequency (Hz) each of the metals? Sodium 5.50  1014 Zinc 1.02  1015 Copper 1.14  1015

Part 3 Objective - What determines the energy of the ejected electrons? Work Function Metal 6. The minimum amount of energy required for an electron to (J) escape from a metal is called the work function, W, and is given Sodium 3.64  10-19 by the equation W = ht. Calculate the work function for each of Zinc 6.78  10-19 the metals in joules and electron-volts using the threshold Copper 7.55  10-19 frequencies for each metal.  h = 6.63 X 10-34 Js or h = 4.14 X 10-15 eVs 7. When an electron is ejected from the surface, what type of energy does the electron possess? Kinetic energy 8. Below is a graph of electron energy vs light frequency for platinum. Record and identify the following on the graph.  The threshold frequency. 1.5  1015 Hz  The range of frequencies that give an electron a kinetic energy greater than zero. 1.5  1015 to higher  The range of frequencies that do not eject an electron. 1.5  1015 and lower Part 4 Objective - What affects the number of ejected electrons? 9. Make the following additional adjustments to the simulation.  Check the box “current vs light intensity”.  Select the metal platinum. 10. Adjust the frequency of the incident light slightly above the threshold frequency. 11. Vary the intensity of the light and observe any changes in the number of ejected electrons. Increased intensity = increased # of electrons and therefore increased current 12. Increase the frequency of the incident light until it is well above the threshold frequency. 13. Vary the intensity of the light and observe any changes in the number of ejected electrons. Increased intensity = increased # of electrons and therefore increased current Part 5 Objective - What determines if the photoelectric effect occurs? 14. What’s the relationship between the frequency of the incident photon, threshold frequency and the ejection of electrons? The incident light must have a minimum frequency, the threshold frequency, or no electrons are ejected

15. What’s the relationship between the energy of the incident photon, the work function and the ejection of electrons? The incident phton must have at least as much energy as the work function to eject an electron

16. What’s the relationship between the kinetic energy of the ejected electrons, the energy of the incident photon and the work function? Energy of the incident photon – work function = kinetic energy (or: the kinetic energy is the amount of energy left over after releasing the electron form the metal. 17. What’s the relationship between the intensity of the incident light and the average kinetic energy of the ejected electrons? None

18. What’s the relationship between the intensity of the incident light and the number of the ejected electrons? Direct, assuming the light is of sufficient energy to release the electrons. Greater intensity = greater # of electrons released 19. One of the photochemical reactions that occurs in the thermosphere is the dissociation of nitrogen gas by incoming ultraviolet light: N2 + h 

2N

(NOTE: On a “microscopic scale”, the symbol “h” represents one photon.) The bond energy of the N—N (triple) bond is 946 kJ/mol in N2. Calculate the minimum frequency (in Hz) of the light that is capable of breaking the N—N bond in N2. (

1 𝑚𝑜𝑙𝑒 946 𝑘𝐽 1000 𝐽 )( )( ) = 1.57 𝑥 10−18 𝐽/𝑝ℎ𝑜𝑡𝑜𝑛 𝑚𝑜𝑙𝑒 1 𝑘𝐽 6.022 𝑥 1023 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 E = h therefore n – 2.37  1015 Hz

20. Explain why the frequency you calculated in Exercise 20 is the minimum frequency. You can have greater energy that is converted to kinetic energy but if the photon does not have that minimum amount it cannot break the bond 21. The photoelectric effect was one of the main effects used by Einstein to develop the photon theory of light as mentioned in the intro. Use what you have learned with this simulation and any other sources necessary to explain in 30 words or fewer how conceiving of light as photons instead of waves explained the photoelectric effect. Classical physics says energy can ‘build up” in any amount in matter, so low energy light should gradually increase the energy of the system until the electron is released. There would also be a time lag as this building up happened. However, this was not seen (no time lag and no low energy light). Einstein proposed that you had to have a packet of light, a photon, of sufficient (or greater) energy strike the metal to release the electron—the photon had to have enough energy to completely remove the electron and not build it up. Further, one photon releases one electron and as intensity is related to # of photons this explain the above observation that increasing the intensity of the light increased the # of electrons released if you are above the threshold. No matter how bright, low energy light cannot release electrons....