Chem.25-26: Sept.24-3 Experiment 2 Session 2 Preparation Pre-lab prep and reading for E2, Parts 3-5 Experiment 2 Session 2 Electrons and Solution Color Three hour lab Complete E2 (Parts - 5) Prepare discussion presentation Prepare team report. Give team report to GSI at the end of lab or turn in by grace deadline to GSI s atrium level mailbox. Background Information: Energy and electrons Electrons will move from a lower to an available higher energy level if the provided energy = exactly that needed for a possible energy level transition If electrons move from a higher to a lower energy level, the difference in energy will be released. DEMO
Visible Light and Energy Light is a form of energy λ 4 Violet - Blue - Green - Yellow - Orange - Red λ 8 The shorter the wavelength (λ), the greater is its energy The shorter the wavelength (λ), the higher is its frequency Wavelength and Energy The shorter the wavelength, the greater its energy DEMO. Expose a balloon containing H 2 and Cl 2 to red light ( λ 65). 2. Expose a balloon containing H 2 and Cl 2 to blue light ( λ 45). Balloon containing H 2 and Cl 2 Light source Part 3. Solution Color and Light Interaction Plot an absorption spectrum for each team assigned salt solution Determine if salt solution color is predictable based on the cations interaction with visible light versus: - placement of the cation s element in the periodic table? - the cation s electron configuration? - the cation s ionic radius? 2
A H s IIA IIIA IVA VA VIA VIIA 3 Li 2s Na 3s 9 K 4s Pre-transition. 37 Rb 5s 55 Cs 6s 87 Fr 7s 4 Be 2s 2 2 Mg 3s2 IIIB IVB VB VIB VIIB VIIIB VIIIB IB IIB 2 Ca 4s 2 38 Sr 5s 2 56 Ba 6s 2 88 Ra 7s 2 2 22 23 24 25 26 27 28 Sc Ti V Cr Mn Fe Co Ni 3d 4s 2 3d 2 4s 2 3d 3 4s 2 3d 5 4s 3d 5 4s 2 3d 6 4s 2 3d 7 4s 2 3d 8 4s 2 39 4 4 42 43 44 45 46 Y Zr Nb Mo Tc Ru Rh Pd 4d 5s 2 4d 2 5s 2 4d 3 5s 2 4d 5 5s 4d 5 5s 2 4d 7 5s 4d 8 5s 4d 57 La* 5d 6s 2 Color versus Periodic Table Position Compare the color data of salt solutions containing cations from transition and pre and post transition families Transition Post-transition VIIIA 2 He s 2 5 6 7 8 9 B C N O F Ne 2s 2 2p 2s 2 2p 2 2s 2 2p 3 2s 2 2p 4 2s 2 2p 5 2s 2 2p 6 3 4 5 6 7 8 Al Si P S Cl Ar 3s 2 3p 3s 2 3p 2 3s 2 3p 3 3s 2 3p 4 3s 2 3p 5 3s 2 3p 6 29 Cu 3d 4s 3 Zn 3d 4s 2 3 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 4s 4p 4s 4p 2 4s 4p 3 4s 4p 4 4s 4p 5 4s 4p 6 47 Ag 4d 5s 48 Cd 4d 5s 2 49 In 5 Sn 5 Sb 52 Te 53 I 54 Xe 5s 5p 5s 5p 2 5s 5p 3 5s 5p 4 5s 2 5p 5 5s 5p 6 72 Hf 73 Ta 74 W 75 Re 76 Os 77 Ir 78 Pt 79 Au 8 Hg 8 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn 5d 2 6s 2 5d 3 6s 2 5d 4 6s 2 5d 5 6s 2 5d 6 6s 2 5d 7 6s 2 5d 9 6s 6s 2 6p 6s 2 6p 2 6s 2 6p 3 6s 2 6p 4 6s 2 6p 5 6s 2 6p 6 5d 6s 5d 6s 2 89 4 5 6 7 8 9 + Element synthesized, Ac # + + + + + + but no official name assigned 6d 7s 2 6d 2 7s 2 6d 3 7s 2 6d 4 7s 2 6d 5 7s 2 6d 6 7s 2 6d 7 7s 2 Beer-Lambert Law A λ = ε c l at λ = absorptivity factor concentration path length Concentration and path length are held constant while taking the sample s absorption spectrum (Part 3) Recording a spectrum Calibrate ( absorbance and % transmission) the spectrophotomer with the blank every time you change the λ before taking the absorbance of your sample Use the same sample and holder (cuvette) so that sample concentration and path length are constant 3
Absorption Spectrum (Part 3) Record the solution color and the wavelength and color of the wavelengths of absorption and transmission max Absorption λmax Transmission λmax Light vs. Transmission ABSORBANCE = -LOG TRANSMITTANCE Abs = % light transmitted Abs = % light transmitted reading values = to. for minimal error vs. Transmission Spectrum The identity of a solution can be determined from its absorption (or transmission) spectrum. 4
Solution Color and Light Interaction Q. What wavelength colors will a solution of NiSO 4 transmit? DEMO Spectrum of. M differences across wavelengths are due to?. Differences in the absorptivity coefficient ( ε ) 2. Differences in the concentration of the sample. 3. Differences in the path length of the sample holder. 4. All the above. Violet Blue Green Yellow Orange Red.7.6.5.3.2. 4 45 5 55 6 65 7 Wavelength λ (nm) Which statement below is correct?. Color of Abs λ max = blue-purple. 2. The sample is green. 3. ε is greater at λ 5 than at λ 4. 5
Beer-Lambert Law and Path Length Path length and light absorbance are directly proportional at a fixed wavelength and concentration. DEMO Different spectrophotometers have different path lengths! Don t change spectrophotometers in the middle of an analysis (Parts 4 and 5)! Path Length and Absorption Spectra Fig. Absorption spectra at different sample path lengths Path length changes will result in a proportional change in absorbance values if sample concentration is constant. readings will alter proportionately across all wavelengths; the spectrum pattern will not alter. Concentration and Absorption Spectra Fig. Absorption spectra at different sample concentrations Sample concentration changes will result in a proportional change in absorbance values if path length is constant. readings will alter proportionately across all wavelengths; the spectrum pattern will not alter. 6
Part 4. Concentration and Light Plot a calibration curve for your team assigned sample - Write a mathematical expression to express the pattern between the concentrations and the absorbance values of your team assigned sample Successfully use the calibration curve to determine the unknown concentration of your team assigned sample (Part 5) Beer-Lambert Law A λ = ε c l at λ = absorptivity factor concentration path length Wavelength and path length are held constant when producing a calibration curve (Part 4) Beer-Lambert Law and Concentration.2.8.6.2 2 3 4 5 6 7 8 Concentration (mm) is proportional to concentration at a constant wavelength (λ )and constant path length DEMO 7
and Path Length readings for a calibration curve will alter if the path length is altered 2.5 2..5..5....2.3.5 [Plastocyanin], mm = /2path length and Path Length Path length must be fixed for a calibration curve or absorbance readings will be in error. Q. How do you hold the path length constant? Answer: Preparation of Calibration Curve Prepare a set of solutions of known and accurate concentration by diluting the team prepared and assigned. M solution M V = M 2 V 2 8
Preparation of Calibration Curve Refer to the samples absorption spectrum to choose a wavelength for the calibration curve The wavelength of max absorbance is typically chosen. Why?. Calibration Curve Wavelength.2 2.5.8. 25 35 45 55 65 75....2.3.5 Wavelength (nm) [Plastocyanin], mm Spectrum of.6 mm Plastocyanin 2..5..5 Calibration curve at 6nm Q. Will the slope of the linear line of a calibration curve produced at 55 nm be greater or less than the slope of the line of the above calibration curve produced at 6 nm? Wavelength of Calibration Graph?.2 Purple Blue Green Yellow Orange Red.8 Absorption.8.6.2 Absorption.7.6.5.3.2. 4 45 5 55 6 65 7 λ (nm)..2.3.5 [M + ] (Molar) Q. A M solution of M + has the absorption spectrum on the left. Circle the wavelength of its calibration graph: 425 5 55 6 65 9
Preparation of Calibration Curve Calculate the slope of the linear line of your calibration curve.2.8.6.2 2 3 4 5 6 7 8 Concentration (mm) Calibration Curve Slope Q. What does the slope of the calibration curve represent in the Beer-Lambert expression A λ = εlc? Answer:.2.8.6.2 2 3 4 5 6 7 8 Concentration (mm) Unknown concentration determination Q. A sample of unknown concentration of XY(aq) has Abs >.2 at a λ of 6nm, What is its concentration (mm)?.2.8.6.2 2 3 4 5 6 7 8 Concentration (mm) Slope =.5Abs/mM Fig. Calibration Curve of XY(aq) at 6 nm
Beer-Lambert and Calibration Curve Determine the concentration of a diluted sample of the unknown if the unknown s absorbance reads above an Abs = or outside the linear line of the calibration curve Fig. Calibration Curve Deviation The Beer-Lambert law only applies at low concentrations! Do NOT extrapolate the linear line of a calibration curve! Unknown concentration determination Eyeball the graph ONLY for an approximate concentration..2.8.6.2 2 3 4 5 6 7 8 Concentration (mm) Use the slope and Beer-Lambert law to determine an exact concentration What is the unknown concentration? The diluted sample has an absorbance at λ of 6nm =.57..2.8.6.2 2 3 4 5 6 7 8 Concentration (mm) Fig. Calibration Curve of XY(aq) at 6 nm Abs λ6 = elc = c Slope =.5Abs/mM
What is the unknown concentration? Q. A diluted sample of unknown concentration = 3.8 mm. You prepared the diluted sample by adding 6. ml of water to 2. ml of the sample of unknown concentration. What is the unknown concentration (mm)? a) 3.8 mm b) 7.2 mm c).4 mm d) 5.2 mm Questions? Contact nkerner@umich.edu 2