Lab2.pdf

Flame tests CH102, lab 2 Goals : Safety : Waste : To safely use a Bunsen burner to observe the emission color of ions. To identify unknown ions through emission color. HCl solutions are corrosive. If contact occurs with the eyes or skin, rinse with lots of water Bunsen burners (next slide) Solutions may be flushed down the sink. Bunsen Burners Review instructions in the lab manual for safe lighting and use. North Carolina Department of Chemistry Chemistry 102 Student Laboratory Manual, 2005, Pearson Custom Publishing, p. 28. http://www.uwplatt.edu/chemep/chem/chemscape/labdocs/catofp/bunsbur/bunsbur2.htm Airflow sleve ? close when lighting. Once lit flame is 3" high, slowly open to give blue flame Gas regulator knob ? open when lighting. After lit, adjust to 3" high. Prelab Assistance Visible light is a form of electromagnetic radiation. All radiation travels at the same speed. c = ??? c = speed of electromagnetic radiation (3.0 x 10 8 m/s) ?= frequency = number of waves that pass a point per second (s -1 ) ?= wavelength = distance from one wave crest to another (nm = 1 x 10 -9 m) Frequency and wavelength are inversely proportional. Light with a high frequency has a short wavelength. Prelab Assistance The energy of electromagnetic radiation is directly proportional to frequency. E = h ? ? E = energy (Joules) h = plank?s constant (6.626 ? 10 -34 J ? s) ?= frequency (s -1 ) Prelab Assistance Atoms absorb electromagnetic radiation when an electron jumps to a higher energy orbital. Atoms emit electromagnetic radiation when an electron falls to lower energy orbital. http://physics.weber.edu/carroll/honors_images/Bohrup2.bmp http://physics.weber.edu/carroll/honors_images/Bohrdown2.bmp Prelab Assistance Atoms absorb or emit 3 main types of radiation: When an atom absorbs a color of visible light, it will appear to be the opposite color on the color wheel (complimentary color). Visible violet blue green yellow red 400 480 540 580 700 Wavelength (nm) http://www.crisp.nus.edu.sg/~research/tutorial/emsp1.gif http://www.cs.berkeley.edu/~sequin/CS184/IMGS/Colorwheel.GIF Lowest energy Highest energy Ultraviolet Infrared Experimental Overview Students will observe the color emitted when ionic solutions are placed in a flame. When placed in water, soluble ionic solids dissolve to form cations and anions. BaCl 2 (s) ! Ba 2+ (aq) + 2 Cl 1- (aq) cation anion Ba(NO 3 ) 2 (s) ! Ba 2+ (aq) + 2 NO 3 1- (aq) cation anion The cations will emit a distinctive color. Please note that elements in Group I generally have a +1 charge. Elements in group II generally have a +2 charge. Experimental Overview " Dip wire loop in HCl for a few seconds. Place loop in hottest part of flame until no color is observed " Dip wire loop in test solution. Place loop in hottest part of flame and observe color. http://jan.ucc.nau.edu/~jkn/Chem%20151%20Manual%20Intro1&2_files/image030.jpg Cl 1- Ba 2+ Ba 2+ Cl 1- NO 3 1- Ca 2+ Ba 2+ Ba 2+ Ca 2+ 3M HCl Sample In-Lab Logic Question Solutions of imaginary ions Y 2+ and R 2+ are placed in a flame. Y 2+ appears yellow and R 2+ appears red. a) Which ion (Y 2+ or R 2+ ) emits the higher energy photons in the visible region? Y 2+ : yellow light is at higher energy than red light. b) Which ion (Y 2+ or R 2+ ) emitted photons with the longer wavelength in the visible region? R 2+ : red light is at lower energy, lower frequency, and therefore higher wavelength. c) Which ion (Y 2+ or R 2+ ) emitted the lower frequency photons in the visible region? R 2+ : red light is at lower energy and therefore lower frequency. Sample In-Lab Calculation The green color in fireworks is due to the emission of green light from Ba 2+ . If the wavelength is 510 nm, what is the frequency of this light? = 510 nm 1 m 1 10 9 nm . . = 5.10 10 -7 m . c = . 3.00 10 8 m/s = 5.10 10 -7 m .. . 5.88 10 14 s = . ? ? ? ? ? Post lab Assistance Example : If a compound appears red, what color is most strongly absorbed? Green is across from red on the color wheel. Example : The H atom emits a broad red band at 6568 ?. m 1 10 10 . . = 6.568 10 -7 m . c = . 3.00 10 8 m/s = 6.568 10 -7 m . .. 4.568 10 14 s = . ? ? ? ? 6568 ? ? a) What is the frequency of this band? b) What is the energy of this band? E = h . ? .. . . . = 6.626 10 -34 J S 4.568 10 14 s -1 = 3.026 10 -19 J http://www.cs.berkeley.edu/~sequin/CS184/IMGS/Colorwheel.GIF