lect11_reel1_2pg - Ohio REEL Project Research Experiences...

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Unformatted text preview: Ohio REEL Project Research Experiences to Enhance Learning Partner Institutions University of Akron (UA) Bowling Green State University (BGSU) Capital University (CU) Central State University (CtlSU) University of Cincinnati (UC) Cleveland State University (CSU) Columbus State Community College (CSCC) University of Dayton (UD) Kent State University (KSU) Miami University of Ohio (MU) Ohio University (OU) University of Toledo (UT) Wright State University (WSU) Youngstown State University (YSU) The Ohio State University (OSU) http://ohio-reel.osu.edu/ Supported by the National Science Foundation How does a research module differ from a “normal” lab • Research exploration – Outcome is not known in advance • Involves all phases of the research process – Form a hypothesis – Conduct experiments to test hypothesis – Interpret & report results – Modify hypothesis • Tackles problems of societal interest – Chemistry plays a central role in many challenges facing society • Builds on previous experiments – The details of the experiment evolve from year to year 1 Research Timeline Week of Mon/Tue Lab Wed/Thur Lab Wed/Thur April 28 REEL-I,II & III REEL-I,II & III May 5 REEL-III & IV REEL-IV May 12 REEL-IV REEL-IV REEL-I: Use X-ray fluorescence (XRF) and X-ray powder diffraction (XRPD) to REELidentify an unknown salt REEL-II: Use UV-Visible (UV-Vis) spectroscopy to probe the electronic structures REELof transition metal complexes in solution REEL-III: Use solid state reactions to prepare pigments, characterize the REELcomposition, crystal structure and electronic structure of the pigments using XRF, XRPD and UV-Vis methods. REEL-IV: Build on the ideas developed in REEL-III to prepare and characterize REELinorganic pigments of your own design. Logistics • Research will be conducted in teams – Students will work in teams (~4 students per team) • Research will be pursued collaboratively – Dr. Woodward & Dr. Stoltzfus – REEL Lab Coordinator (Harry Seibel) – Teaching Assistants – Peer Mentors • Research presentation – Each research group will present their results at one of three REEL poster presentations (May 20,21,22) • Research documentation and reporting – Each student will prepare a report in the form of a scientific paper to describe their research findings 2 Peer Mentors Back Row: Lana Alghotani, Sachin Sharma, Jen Scherer, Alex Paraskos, Eric Smith, Sarah Watson, Ashley Doles, Derek Heimlich, David Albani, Front Row: Jalpa Patel, Sam Karnitis, Gina Aloisio, Stephen Smith, Brittany Thompson, Ravi Rajmohon, Ken Verdell, Amy Ullman, Amy Tucker, and Kristen Brandt Experimental Methods • Synthesis – Direct solid state reactions • Characterization – X-ray powder diffraction – X-ray fluorescence – UV-Visible Spectroscopy Ocean Optics UV-Visible Spectrometer (~$7,000) X-ray Powder Diffractometer (~$65,000) 3 Pigments Pigment: Coloring matter used to make paint. Pigments work by selectively absorbing a portion of the visible light while the remaining visible light is reflected. For more info see http://webexhibits.org/pigments/ Causes of Color • Emitted Light – – – • Steering and/or Interference Effects – – – • Blackbody Radiation, Incandesence (light bulb, flame) Gas Discharges/Excitations (neon lights, aurora borealis) Luminescence (LED’s, fluoresecent lights, chemluminescence) Dispersive Refraction (rainbows, prisms) Scattering (blue sky) Interference & Diffraction (butterflies, beetles, opals, CD’s) Absorbed Light – – – – Intra-atomic excitations (Complex ions, gemstones) IntraMolecular Orbital Excitations (Chlorophyll, organic dyes) Band to Band Transitions in Semiconductors (CdS, SnS2, HgS) Interatomic (charge transfer) excitations • Oxoanions (i.e. CrO42−, MnO4−), Pigments (Prussian blue, chrome yellow), gemstones (sapphire) For more info see http://webexhibits.org/causesofcolor/ 4 The Electromagnetic Spectrum Violet Blue Green Yellow Orange Red The Color Wheel UV Violet Blue Green Yellow Orange Red Near IR 100-400 nm 400-425 nm 425-492 nm 492-575 nm 575-585 nm 585-647 nm 647-700 nm 10,000-700 nm Energy in Joules E= hc λ 12.4 - 3.10 eV 3.10 - 2.92 eV 2.92 - 2.52 eV 2.52 - 2.15 eV 2.15 - 2.12 eV 2.12 - 1.92 eV 1.92 - 1.77 eV 1.77 - 0.12 eV ( 6.626 ×10 = −34 )( J ⋅ s 2.998 × 108 m / s ) λ Wavelength in meters 1 eV = 1.602 × 10−19 J 5 Absorption of Light & Color If absorbance occurs in one region of the color wheel the material appears with the opposite (complimentary color). – a material absorbs violet light → Color = Yellow – a material absorbs green light → Color = Red Absorption of Light & Color If absorbance occurs in multiple regions of the color wheel the material generally takes on a color in the middle of the colors that are not absorbed. – a material absorbs violet, blue and green light → Color = Orange – a material absorbs violet and red light → Color = Yellow-Green 6 UV-Visible Spectroscopy Monochromatic light (light of a single wavelength) is passed through the sample and the amount of light absorbed by the sample is measured. Color and Cu2+ complexes Color [Cu(H2O)4]2+ [Cu(NH3)4]2+ 7 Electronic excitations and Absorbed Light • Intra-atomic excitations Intra– – • Molecular Orbital Excitations – • Conjugated organic molecules Band to Band Transitions in Semiconductors – • Transition metal ions, complexes and compounds Lanthanide ions, complexes and compounds Metal sulfides, metal selenides, metal iodides, etc. Interatomic (charge transfer) excitations – – Ligand to metal (i.e. O2− → Cr6+ in SrCrO4) Metal-to-Metal (i.e. Fe2+ → Ti4+ in sapphire) When a molecule absorbs a photon of ultraviolet (UV) or visible radiation, the energy of the photon is transferred to an electron. The transferred energy excites the electron to a higher energy atomic or molecular orbital. Because atoms and molecules have quantized (discrete) energy levels light is only absorbed when the photon’s energy corresponds to the energy difference between two orbitals. Intra-atomic (localized) excitations Intra [Ni(NH3)6]2+ NiSO4 Cu3(CO3)2(OH)2 CuSO4 Malachite In these complexes the color comes from absorption of light that leads to excitation of an electron from an occupied d-orbital to an empty (or ½-filled dorbital). The energy separation between d-orbitals depends upon the interaction between the d-orbitals and the ligands. This is the main cause of color in most compounds containing transition This transition metal elements with cations that contain d-electrons. d- 8 Molecular Orbital (HOMO-LUMO) excitations In these complexes the color comes from absorption of light that leads to excitation of an electron from an occupied molecular orbital to an empty molecular orbital. The orbitals are generally pi-orbitals. Chlorophyll See also the following discussions in your text: The Chemistry of Vision (p.342, BLB) & Organic Dyes (p.353, BLB). This is the main cause of color in organic molecules containing alternating single and double bonds (conjugated molecules). Band to Band Transitions HgS (Vermillion) CdS (Cadmium Yellow) Energy – Wide band gap semiconductors Empty Conduction Band “Cation band” Eg In these complexes the color comes from absorption of light that leads to excitation of an electron from a filled valence band to an empty conduction band. These excitations can be considered a subset of charge transfer excitations because the filled valence band has more anion character while the empty conduction band has more “cation” character. Filled Valence Band “Anion band” This is the main cause of color in metal sulphides, selenides and iodides. sulphides, 9 Interatomic (charge transfer) excitations Cr PbCrO4 In these complexes the color comes from absorption of light that leads to excitation of an electron from one atom to another. The charge transfer in the CrO42− is from oxygen to chromium. Charge transfer excitations absorb light much more strongly than intraatomic excitations. This is very attractive for pigment applications. This is the main cause of color in compounds containing oxoanions where the transition metal ion has a d0 electron configuration (i.e. MnO4−, CrO42−, VO43−) History of Yellow and Red Pigments • Ancient Pigments – – – – – – Red Ochre: Fe2O3 (O2− to Fe3+ charge transfer) Yellow Ochre: Fe2O3·H2O (O2− to Fe3+ charge transfer) Red Lead: Pb3O4 (O2− to Pb4+ charge transfer) Lead-Tin Yellow: Pb2SnO4 (O2− to Sn4+ charge transfer) LeadVermillion: HgS (band to band transition, S2− to Hg2+) Orpiment: As2S3 (band to band transition, S2− to As3+) • Synthetic pigments – – – – 1797, Chrome yellow: PbCrO4 (O2− to Cr6+ charge transfer) 1800, Indian yellow: C19H16O11Mg·5 H2O (Mol. Orb. Transition) Mg· 1807, Lemon yellow: SrCrO4 (O2− to Cr6+ charge transfer) 1818, Cadmium Yellow: CdS (band to band transition, S2− to Cd2+) 10 Indian Yellow Euxanthic acid (Mg salt) C19H16O11Mg·5 H2O “The Milkmaid” by Johannes Vermeer Synthesis Procedure Derived from urine of cows that had been fed mango leaves. The cow urine is then evaporated and the resultant dry matter formed into balls by hand. Finally the crude pigment is washed and refined. Synthetic Pigments and Art Synthetic “Wheatfield with Crows” by Vincent van Gogh “Christ in a Storm” by Rembrant van Rijn The traditional yellow and red ochres are earthy hues which tend to make the paintings darker. Note the difference between Rembrant who painted before synthetic pigments were discovered and van Gogh who in his later years extensively used CdS and PbCrO4. 11 Pigments & Toxicity Emerald Green was one of the favorite pigments of many impressionist painters (van Gogh, Cezanne, Monet) the chemical formula of this pigment is Cu(CH3COO)2 · 3 Cu(AsO2)2 However, Emerald green is quite toxic. It is also called Paris Green because it was used to kill rats in the sewers of Paris. It has also been used as an insecticide. The health problems of some of the impressionist painters (van Gogh’s mental illness, Monet’s blindness, Cezanne’s diabetes) have been linked to the use of toxic pigments. Claude Monet The Japanese Bridge 1899 12 ...
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