SpectroscopyLight in chemistry

The Nature of Light• Acts as both a

particle and a wave

• Photoelectric effect only explained through waves

• Interference, diffraction, polarization only through particles

Light!

• As a wave...

• it has an amplitude, frequency, speed (speed of light), and length

• different lengths=different colors

• low-energy waves have long wavelengths; high-energy waves have shorter ones.

Low-energy v. High-energy

Electromagnetic Spectrum

Types of Spectra• Continuous

• light is composed of a continuous range of energies-colors

• Discrete

• emission- bright lines at very distinct energies

• absorption- dark lines

Continuous Spectra

• heat radiation through light

• broad range of wavelengths- continuous range of energies/colors

• YOU emit heat radiation through light

• waves are too long to be seen (infrared)

Discrete Spectra- Emission

• Quantum Mechanics: Electron clouds can only have specific energies

• each element has its own set of possibilities- identifiable

• Atoms tend to settle to ground state- requires the least amount of energy

Emission Line Spectra• When an atom from a higher

energy level settles to a lower level, it gets rid of energy through the emission of light

Emission Line Spectra

• Smaller energy jumps=smaller energy photons emitted=longer wavelengths

• Larger energy jumps=higher energy photons emitted=shorter wavelengths

But sometimes...

• THEY GET EXCITED!

• and they jump to a higher energy level, rather than settling down.

• atoms can absorb that energy-specific photon

• ABSORPTION line spectra

Dark-line absorption spectrum

So what does this have to do with anything?

You will be identifying compounds using a

spectrometer!

• Beam of light separates into its “component wavelengths” through a diffraction grating (or a prism)

• Each single beam is then split into two equally intense beams by the half-mirror contraption

UV-Visible Spectroscopy

UV-Visible Spectroscopy• One beam (sample) goes

through the cuvette containing the compound being studied in transparent solvent

• Light absorption- I

• Reference beam passes through reference cuvette only with solvent (control)

• Light absorption- I0

UV-Visible Spectroscopy• If the compound doesn’t

absorb light, then I=I0

• If the compound absorbs light, then I<I0

• Absorption can be presented as transmittance (I/I0) or as absorbance (log I0/I)

• Absorbance of 0= 100% Transmittance

• Absorbance of 2= 1% Transmittance

UV-Visible Spectroscopy• The absorbance of a sample is

proportional to its molar concentration in the sample cuvette

• When comparing different compounds, you consider a molar absorptivity value

• Absorbance/ (concentration)(length of light path through cuvette)

How to use the spectrometer

• Wear gloves or be very careful with the cuvettes- fingerprints affect the results of the experiment!

• Set the spectrometer to the desired wavelength, insert reference cuvette, and hit “set zero” button to calibrate

• Insert the sample solution to measure its absorbency (if you do this multiple times, make sure that you zero the spec. with the reference cuvette in between each sample)

Get inspired!