Home > Chemistry > Chemistry 1A Part Deux: Lecture 34 Notes

Chemistry 1A Part Deux: Lecture 34 Notes

Overview

I’m going to double up for the next few days in an attempt to finish up this series.  It seems that there was a shooting at Berkeley in the day before this lecture.  This lecture is 50 minutes.

Details

Spectroscopy is a powerful tool for learning about atoms and molecules.

She spends some time on a graph of the solar emission spectrum of intensity versus wavelength.

Two Types of Spectroscopy

Emission spectroscopy:

Hot sample glows, analyze energy coming out.  Measure the intensity of the emitted light as a function of wavelength.

Absorption spectroscopy:

White light source .  See where the sample absorbs.  Measure absorption versus wavelength.  High absorption, no light gets through.

Flame Emission Spectroscopy

Fuel CH4, oxidant of air, liquid same

1 Aspiration of a liquid sample into flame
2 Evaporation of water, drying
3 Atomization – generate individual atoms
4 Absorbtion of energy by the atoms in the flame
5 Emission of the absorbed energy as light
6 Spectral analysis of the emitted light
7 Light intensity is proportional to concentration

Different Flame Colors

CH4 + 2O2 = CO2 + 2H2o  -818 kJ/mol = dGo rxn

Orange from blackbody radiation of carbon particles.  Emission of molecular fragments show as CH from blue.  As the air/methane ratio increases, the flame becomes blue.  Thus there is more O2.

Intensity = k C

All metals yield a different flame.  Na is orange.  K is purple.  Li is red.  Ba is white.  Use a calibration curve to take known samples to find intensity, then find the unknown.

Infrared Absorption Spectroscopy

…for compound identification and structure determination.  Relative transadmittance on the y axis, wavenumbers in cm-1 on the x axis.  C-H stretches occur at 3000 wavenumbers, C-H bends at 1500 wavenumbers.  The CH stretch is higher energy.  After hitting the sample with a frequency sweep, there are certain areas with no transadmittance.    C-O stretch at 1000 wavenumbers.

Nuclear transition – Gamma rays/core electron transition x rays/valence electron   UV/molecular vibrations  IR/molecular rotations  Microwave/nuclear spin flip  FM/TV

Light Detector Phototube

light strikes the cathode, electrons are emitted and current is detected.

Kinetic Energy of Ejected Electrons

The KE of the ejected electron varies linearly with the energy of the impinging light.

KE = 1/2 mv2

KE electron = E photon – Ionization Energy

E = hc/w = h v

They spend the last 12 minutes doing “the big C demonstration.”   This starts at 38 minutes in.  WTF is this obsession with football?

Wrap Up Points

Flame emission spectroscopy can be used to measure concentrations of metal ions in solution.

IR absorption spectroscopy can be used to ID composition of complex mixtures of organic compounds.

The energy of incident photons if above a critical value will cause electrons to be ejected from the metal surface with a specific KE, E photon = IE + KE photon

Review

P = mv

U = PE = mgh

KE = 1/2 mV2

PV = nRT

G = H-TS

dE  = dEo – .59/n  ln Q

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