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Chem. 31 – 4/7 Lecture

Chem. 31 – 4/7 Lecture. Announcements. AA Lab Report – due April 14 th Additional Problem (read note to determine which complex reactions need to be considered) and Quiz on Wednesday Today’s Lecture The Systematic Method Spectroscopy Theory Beer’s Law. The Systematic Method 2 nd Example.

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Chem. 31 – 4/7 Lecture

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  1. Chem. 31 – 4/7 Lecture

  2. Announcements • AA Lab Report – due April 14th • Additional Problem (read note to determine which complex reactions need to be considered) and Quiz on Wednesday • Today’s Lecture • The Systematic Method • Spectroscopy • Theory • Beer’s Law

  3. The Systematic Method2nd Example • An aqueous mixture of CdCl2 and NaSCN is made • Initial concentrations are [CdCl2] = 0.0080 M and [NaSCN] = 0.0040 M • Cd2+ reacts with SCN- to form CdSCN+ K = 95 • HSCN is a strong acid • Ignore any other reactions (e.g. formation of CdOH+) • Ignore activity considerations • Determine the concentrations of all species

  4. The Systematic Method3rd Example • A student prepares a solution that contains 0.050 mol of AgNO3 and 0.0040 mol NH3 in water with a total volume of 1.00 L. The AgNO3 is totally soluble, NH3 is a weak base, and Ag+ reacts with NH3 to form Ag(NH3)2+. Assume the Ag+ does not react with water or OH-. Go through the first 5 steps of the systematic method.

  5. The Systematic MethodStong Acid/Strong Base Problems • When do we need to use the systematic approach? • examples: 4.0 x 10-3 M HCl. 7.2 x 10-3 M NaOH • Key point is the charge balance equation: • for strong acid HX, [H+] = [X-] + [OH-] • If [X-] >> [OH-], then [H+] = [X-] • for strong base NaOH, [H+] + [Na+] = [OH-]

  6. The Systematic MethodGeneral Comments Effects of secondary reactions e.g. MgCO3 dissolution Additional reactions increase solubility Secondary reactions also can affect pH (CO32- + H2O will produce OH- while Mg2+ + H2O will produce H+) Software is also available to solve these types of problems (but still need to know steps 1 → 5 to get problems solved)

  7. Chapter 17 - Spectroscopy A. Introduction 1. One of the main branches of analytical chemistry 2. The interaction of light and matter (for purposes of quantitative and qualitative analysis) 3. Topics covered: - Properties of Light - Absorption of Light - Electromagnetic Spectrum - Beer’s Law - Spectrometers

  8. Spectroscopy • Fundamental Properties of Light • Wave-like properties: λ λ = wavelength = distance between wave crests n= frequency = # wave crests/s = wave number = # wave crests/length measure c = speed of light (in vacuum) = 3.00 x 108 m/s Relationships: v = λ·n and = 1/λ

  9. SpectroscopyFundamental Properties of Light 1. Other wave-like properties - diffraction, interference 2. Particle-like properties a) Idea of photons (individual entities of light) b) Energy of photons E = hn = hc/l

  10. SpectroscopyAbsorption vs. Emission • Absorption - Associated with a transition of matter from lower energy to higher energy • Emission - Associated with a transition from high energy to low energy A + hn→ A* A* → A + hn Excited State Energy Photon out Ground State Photon in

  11. SpectroscopyRegions of the Electromagnetic Spectrum Many regions are defined as much by the types of transitions occurring (e.g. outer shell electron) as by the frequency or energy of the transitions Outer shell electrons Bond vibration Nuclear spin Short wavelengths Long wavelengths Gamma rays X-rays UV + visible Microwaves Radio waves Infrared High Energies Nuclear transitions Inner shell electrons Molecular rotations Low Energies Electron spin

  12. SpectroscopyInterpreting Spectra • Major Components • wavelength (of maximum absorption) – related to energy of transition • width of peak – related to energy range of states • complexity of spectrum – related to number of possible transition states • absorptivity – related to probability of transition A* DE dE Ao A dl l (nm)

  13. SpectroscopySome Example Questions • A nuclear magnetic resonance (NMR) spectrometer absorbs light at a frequency of 750 MHz. This is in the radio frequency and Hz = s-1. What is the wavelength of this light? • An infrared absorption band occurs at a wavenumber of 812 cm-1. What is the wavelength (in mm) and energy (J/photon) of that light? • What type of light involves transitions of inner shell electrons? • Two compounds in water are observed to absorb light. Compound X has narrow absorption peaks while compound Y has broad absorption peaks. What can be said about compound X? • It has transitions of greater energy • Its transitions are more likely • Its transitions occur between more defined energy states • It has a less complex set of energy states

  14. SpectroscopyBeer’s Law Transmittance = T = P/Po Absorbance = A = -logT sample in cuvette Light source Absorbance used because it is proportional to concentration A = εbC Where ε = molar absorptivity and b = path length (usually in cm) and C = concentration (M) Light intensity in = Po Light intensity out = P b ε = constant for given compound at specific λ value

  15. SpectroscopyBeer’s Law Question • Half of the 284 nm light is absorbed when benzoic acid at a concentration of 0.0080 M is in a cuvette with a path length of 0.5 cm. What is the molar absorptivity of benzoic acid at this wavelength?

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