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Outline

Outline. Final Comments on Titrations/Equilibria Titration of Base with a strong acid End-point detection Choice of indicators Titration Curve method Start Chapter 18 Spectroscopy and Quantitative Analysis. Weak Base titrated with strong acid.

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Outline

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  1. Outline • Final Comments on Titrations/Equilibria • Titration of Base with a strong acid • End-point detection • Choice of indicators • Titration Curve method • Start Chapter 18 • Spectroscopy and Quantitative Analysis

  2. Weak Base titrated with strong acid • Consider a 100 ml of a 0.0100 M base with 0.0500 M HCl • Kb = 1 x 10-5

  3. Initial pH Buffer Region pH after equivalence Dominated by remaining [H+] pH @ equivalence

  4. Electronic SpectroscopyUltraviolet and visible

  5. Where in the spectrum are these transitions?

  6. Where in the spectrum are these transitions?

  7. Light is called electromagnetic radiation

  8. Review of properties of EM! • c=ln • Where • c= speed of light = 3.00 x 108 m/s • l= wavelength in meters • n = frequency in sec-1 • E=hn • or E=hc/l • h=Planks Constant = 6.62606 x 1034 J.s

  9. Where in the spectrum are these transitions?

  10. Beer-Lambert Law AKA - Beer’s Law

  11. The Quantitative Picture • Transmittance: T = P/P0 P0 (power in) P (power out) • Absorbance: A = -log10 T = log10 P0/P How do “we” select the wavelength to measure the absorbance? b(path through sample) • The Beer-Lambert Law (a.k.a. Beer’s Law): A =ebc Where the absorbance A has no units, since A = log10 P0 / P e is the molar absorbtivity with units of L mol-1 cm-1 b is the path length of the sample in cm c is the concentration of the compound in solution, expressed in mol L-1 (or M, molarity)

  12. Absorbance vs. Wavelength Why? • Maximum Response for a given concentration • Small changes in Wavelength, result in small errors in Absorbance A 380 400 420 460 440 Wavelength, nm

  13. Limitations to Beer’s Law “Fundamental” “Experimental” • Not Using Peak wavelength • Colorimetric Reagent is limiting • Concentration/Molecular Interactions • Changes in Refractive Index

  14. Interaction of Light and Matter Start with Atoms Finish with Molecules

  15. Consider Atoms - hydrogen Very simple view of Energy states Assuming subshells have equivalent energies n=6 n=5 Energy n=4 A n=3 n=2 Wavelength, nm n=1

  16. Molecular Spectroscopy

  17. Consider molecules • With molecules, many energy levels. Interactions between other molecules and with the solvent result in an increase in the width of the spectra.

  18. maxwith certain extinction  UV Visible Electronic Spectrum Make solution of concentration low enough that A≤ 1 (Helps to Ensure Linear Beer’s law behavior) UV bands are much broader than the photonic transition event. This is because vibration levels are superimposed. 1.0 Absorbance 0.0 200 400 800 Wavelength, , generally in nanometers (nm)

  19. UV/Vis and Molecular Structure

  20. The UV Absorption process •   * transitions: high-energy, accessible in vacuum UV (max <150 nm). Not usually observed in molecular UV-Vis. • n  * transitions: non-bonding electrons (lone pairs), wavelength (max) in the 150-250 nm region. • n  * and   * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm. Any of these require that incoming photons match in energy the gap corresponding to a transition from ground to excited state.

  21. What are the nature of these absorptions? Example:   * transitions responsible for ethylene UV absorption at ~170 nm calculated with semi-empirical excited-states methods (Gaussian 03W): h 170nm photon  antibonding molecular orbital  bonding molecular orbital

  22. Examples Napthalene Absorbs in the UV

  23. Experimental details • What compounds show UV spectra? • Generally think of any unsaturated compounds as good candidates. Conjugated double bonds are strong absorbers. • The NIST databases have UV spectra for many compoundsYou will find molar absorbtivities  in L•cm/mol, tabulated. • Transition metal complexes, inorganics

  24. Final notes on UV/Vis • Qualitatively • Not too useful • Band broadening • Quantitatively • Quite Useful • Beer’s Law is obeyed through long range of concentrations • Thousands of methods • Most commonly used • Detection Limits ~ 10-4 – 10-6 M

  25. Final notes on UV/Vis (cont’d) • Quant (cont’d) • Cheap, inexpensive, can be relatively fast • Reasonably selective • Can find colorimetric method or use color of solution • Good accuracy ~1-5%

  26. Chapter 5 – Calibration Methods • Open Excel • Find data sheet • Input data table

  27. Uncertainty in Concentration Where: x = determined concentration k = number of samples m = slope n = number of Standards (data points) D = ??

  28. What happens to the absorbed energy?

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