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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 18

ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 18. DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university. CHAPTER 18 ELECTROMAGNETIC RADIATION. ELECTROMAGNETIC RADIATION. - Also known as radiant heat or radiant energy

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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 18

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  1. ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 18 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

  2. CHAPTER 18 ELECTROMAGNETIC RADIATION

  3. ELECTROMAGNETIC RADIATION - Also known as radiant heat or radiant energy - One of the ways by which energy travels through space - Consists of perpendicular electric and magnetic fields Examples heat energy in microwaves light from the sun X-ray radio waves

  4. ELECTROMAGNETIC RADIATION Three Characteristics of Waves Wavelength (λ) - Distance for a wave to go through a complete cycle (distance between two consecutive peaks or troughs in a wave) Frequency (ν) - The number of waves (cycles) per second that pass a given point in space Speed (c) - All waves travel at the speed of light in vacuum (3.00 x 108 m/s)

  5. ELECTROMAGNETIC RADIATION λ1 node amplitude ν1 = 4 cycles/second λ2 peak ν2 = 8 cycles/second λ3 ν3 = 16 cycles/second trough one second

  6. ELECTROMAGNETIC RADIATION Wavelength (m) 10-11 103 Radio frequency FM Shortwave AM Gamma rays Ultr- violet Infrared Microwaves Visible X rays Frequency (s-1) 104 1020 Visible Light: VIBGYOR Violet, Indigo, Blue, Green, Yellow, Orange, Red 400 – 750 nm - White light is a blend of all visible wavelengths - Can be separated using a prism

  7. ELECTROMAGNETIC RADIATION - Inverse relationship between wavelength and frequency λα 1/ν c = λ ν λ = wavelength (m) ν = frequency (cycles/second = 1/s = s-1 = hertz = Hz) c = speed of light (3.00 x 108 m/s)

  8. ELECTROMAGNETIC RADIATION An FM radio station broadcasts at 90.1 MHz. Calculate the wavelength of the corresponding radio waves c = λ ν λ = ? ν = 90.1 MHz = 90.1 x 106 Hz = 9.01 x 107 Hz c = 3.00 x 108 m/s λ = c/ ν = [3.00 x 108 m/s]/[9.01 x 107 Hz] = 3.33 m

  9. THE ENERGY OF PHOTONS Albert Einstein proposed that - Electromagnetic radiation is quantized - Electromagnetic radiation can be viewed as a stream of ‘tiny particles’ called photons h = Planck’s constant (6.626 x 10-34 joule-second, J-s) ν = frequency of the radiation λ = wavelength of the radiation = 1/ λ = wavenumber (m-1)

  10. THE ATOMIC SPECTRUM Transmission - Electromagnetic radiation (EM) passes through matter without interaction Absorption - An atom (or ion or molecule) absorbs EM and moves to a higher energy state (excited) Emission - An atom (or ion or molecule) releases energy and moves to a lower energy state

  11. THE ATOMIC SPECTRUM Excited state Energy Ground state Absorption Emission

  12. ELECTROMAGNETIC RADIATION Molecular Processes Occurring in Each Region 10-11 103 Gamma rays Ultr- violet Radio frequency FM Shortwave AM X rays Infrared Microwaves Visible 1020 104 Electronic excitation rotation vibration Bond breaking and ionization

  13. ABSORPTION OF LIGHT Spectrophotometry - The use of EM to measure chemical concentrations Spectrophotometer - Used to measure light transmission Radiant Power (P) - Energy per second per unit area of a beam of light - Decreases when light transmits through a sample (due to absorption of light by the sample)

  14. ABSORPTION OF LIGHT Transmittance (T) - The fraction of incident light that passes through a sample 0 < T < 1 Po = radiant power of light striking a sample P = radiant power of light emerging from sample Po P

  15. ABSORPTION OF LIGHT Transmittance (T) - No light absorbed: P = Po and T = 1 - All light absorbed: P = 0 and T = 0 Percent Transmitance (%T) 0% < %T < 100%

  16. ABSORPTION OF LIGHT Absorbance (A) - No light absorbed: P = Po and A = 0 - 1% light absorbed implies 99% light transmitted - Higher absorbance implies less light transmitted

  17. ABSORPTION OF LIGHT Beers Law A = εbc A = absorbance (dimensionless) ε = molar absorptivity (M-1cm-1) b = pathlength (cm) c = concentration (M)

  18. ABSORPTION OF LIGHT Beers Law - Absorbance is proportional to the concentration of light absorbing molecules in the sample - Absorbance is proportional to the pathlength of the sample through which light travels - More intense color implies greater absorbance

  19. ABSORPTION OF LIGHT Absorption Spectrum of 0.10 mM Ru(bpy)32+ λmax = 452 nm

  20. ABSORPTION OF LIGHT Absorption Spectrum of 3.0 mM Cr3+ complex λmax = 540 nm

  21. ABSORPTION OF LIGHT Maximum Response (λmax) - Wavelength at which the highest absorbance is observed for a given concentration - Gives the greatest sensitivity

  22. ABSORPTION OF LIGHT Calibration Curve

  23. ABSORPTION OF LIGHT Complementary Colors - White light contains seven colors of the rainbow (ROYGBIV) - Sample absorbs certain wavelengths of light and reflects or transmits some - The eye detects wavelengths not absorbed

  24. ABSORPTION OF LIGHT Complementary Colors λmax 380-420 420-440 440-470 470-500 500-520 520-550 550-580 580-620 620-680 680-780 Color Observed Green-yellow Yellow Orange Red Purple-red Violet Violet-blue Blue Blue-green Green Color Absorbed Violet Violet-blue Blue Blue-green Green Yellow-green Yellow Orange Red Red

  25. ABSORPTION OF LIGHT Complementary Colors

  26. ABSORPTION OF LIGHT Complementary Colors Ru(bpy)32+ λmax = 450 nm Color observed with the eye: orange Color absorbed: blue Cr3+-EDTA complex λmax = 540 nm Color observed with the eye: violet Color absorbed: yellow-green

  27. ABSORPTION OF LIGHT Cuvet - Cell used for spectrophotometry Fused silica Cells (SiO2) - Transmits visible and UV radiation Plastic and Glass Cells - Only good for visible wavelengths NaCl and KBr Crystals - IR wavelengths

  28. ABSORPTION OF LIGHT Single-Beam Spectrophotometer - Only one beam of light - First measure reference or blank (only solvent) as Po b Po P Light source monochromator (selectsλ) sample detector computer

  29. ABSORPTION OF LIGHT Double-Beam Spectrophotometer - Houses both sample cuvet and reference cuvet - Incident beam alternates between sample and reference with the aid of mirrors (rotating beam chopper) b P Light source monochromator (selectsλ) sample detector computer Po reference

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