Labs: Light Emission, Light Absorption, Polarimeter Flashcards

1
Q

What was the goal of the light emission lab?

How was it achieved?

A
  • to determine the composition of two unknown ion mixtures
  • by measuring the emission spectra of several known metal ions using the manual spectroscope and then comparing the unknowns to this data
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2
Q

What are the two mechanisms for light emission?

A
  1. Thermal Radiation - AKA black body, kinetic energy of particles, higher T = more intense light of lower wavelength
  2. Luminescence - transition of electron from excited to ground state
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3
Q

What are the 3 kinds of emission spectra?

A
  1. **Continuous **- radiation distributed over uninterrupted range of wavelengths
  2. **Band - **closely spaced groups of lines/wide bands along x-axis, characteristic of molecules
  3. **Line **- discrete values along x-axis, single wavelengths, characteristic of atoms/ions
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4
Q

How are single wavelengths separated in order to measure their intensity in spectroscopy?

A

With a monochromator which disperses light into its component wavelengths using either a prism or optical grating

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5
Q

Draw the manual spectroscope.

A

Simplification:

  • two tubes: one with prism system, one with wavelength scale
  • light from flame enters prism system tube, is decomposed and projected to viewer
  • wavelength scale is reflected into path of decomposed light so emission spectrum can be read
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6
Q

How is the spectroscope calibrated for the experiment?

A

Using NaCl solution, because sodium gives off a distinct yellow 590 nm light

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7
Q

What was the goal of the light absorption lab?

How was it achieved?

A
  • to determine the copper ion concentration of an unknown solution
  1. an absorption spectrum was made by measuring absorption of Cu-ligand solution btwn 450 and 570 nm at 20 nm intervals
  2. absorption of several known concentrations of ligand with unknown [Cu] was measured at the determined absorption maximum and the data was plotted as a function of concentration
  3. where the curve of this graph “breaks” can be considered the saturation point, from which can be determined the Cu ion concentration
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8
Q

What is the formula for absorption?

A

A = log J0/J

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9
Q

What is the formula for transmittance?

A

T = J/J0 x 100%

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10
Q

What causes the widening of an absorption spectrum of a dilute solution that would otherwise absorb only at a single wavelength?

A

interactions of solute molecules with solvent molecules slightly alters activation energy of each, so that they do not all absorb at the same wavelength

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11
Q

What is the Lambert Beer Law?

A

for dilute solutions, the absorbance is proportional to the concentration and thickness of the solution through which light is passing

A = log J0/J = ɛ(λ) * C * x

x is thickness

ɛ(λ) is molar extinction coefficient

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12
Q

What is the general layout of the spectrophotometer?

Draw it.

A
  • light is decomposed by monochromator
  • slit in monochromator selects single wavelength
  • monochromatic light shines through sample and is detected on other side
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13
Q

What are the graphs for the light absorption lab?

A
  1. a simple absorption spectrum (absorbance as a function of wavelength)
  2. a graph of absorption as a function of ligand concentration (the edge its plateau indicates the saturation point of the ligand molecules and can be used to determine the ion concentration)
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14
Q

What was the goal of the polarimeter experiment?

A
  1. the identity of an unknown sugar enantiomer was identified by measuring the rotation angle of a solution of known concentration and using it to determine its specific rotation
  2. the unknown concentration of another solution of the same sugar was determined using the specific rotation value from the first experiment
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15
Q

Describe linear polarization.

Draw it.

A

Linear polarization occurs when non-polarized light which oscillates in all directions is restricted to oscillation on a single plane

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16
Q

What are chiral molecules?

A

optically active molecules with a carbon atom connected to four distinct functional groups and thus lack a plane of symmetry

17
Q

What are enantiomers?

A

non-superposable variations of a chiral molecule with different orientations of the groups around the chiral center

18
Q

What is optical activity?

A

the ability to alter the plane of polarization of linearly polarized light

19
Q

Describe how the polarimeter works.

A
  • monochromatic light shines from an Na lamp into the polarimeter
  • a linear polarizer at the end of the polarimeter polarizes the light vertically
  • the polarized light travels through the solution to be measured
  • another linear polarizer near the eyepiece is then turned until it perpendicular to the now-altered plane of polarization (resulting in a dark field-of-view)
  • angle of rotation is read from a scale mounted next to the eyepiece
20
Q

What is the equation for determining the angle of rotation caused by a solution of a chiral substance?

A

α = [α]D* c * l

α is angle of rotation

[α]Dis the specific rotation

c is concentration

l is thickness of solution illuminated

21
Q

What is the specific rotation of an optically active substance?

A

A constant for the substance dependent on temperature and wavelength of light used

has unit of… ° * cm<span>3</span>/g * dm

indicated by [α]D<strong>T</strong>in equations

D is wavelength

T is temperature

22
Q
A