Labs: Light Emission, Light Absorption, Polarimeter

Question 1

What was the goal of the light emission lab?

How was it achieved?

Answer 1

• 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

Question 2

What are the two mechanisms for light emission?

Answer 2

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

Question 3

What are the 3 kinds of emission spectra?

Answer 3

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

Question 4

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

Answer 4

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

Question 5

Draw the manual spectroscope.

Answer 5

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

Question 6

How is the spectroscope calibrated for the experiment?

Answer 6

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

Question 7

What was the goal of the light absorption lab?

How was it achieved?

Answer 7

• 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

Question 8

What is the formula for absorption?

Answer 8

A = log J₀/J

Question 9

What is the formula for transmittance?

Answer 9

T = J/J₀ x 100%

Question 10

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

Answer 10

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

Question 11

What is the Lambert Beer Law?

Answer 11

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

Question 12

What is the general layout of the spectrophotometer?

Draw it.

Answer 12

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

Question 13

What are the graphs for the light absorption lab?

Answer 13

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)

Question 14

What was the goal of the polarimeter experiment?

Answer 14

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

Question 15

Describe linear polarization.

Draw it.

Answer 15

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

Question 16

What are chiral molecules?

Answer 16

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

Question 17

What are enantiomers?

Answer 17

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

Question 18

What is optical activity?

Answer 18

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

Question 19

Describe how the polarimeter works.

Answer 19

• 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

Question 20

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

Answer 20

α = [α]_D* c * l

α is angle of rotation

[α]_Dis the specific rotation

c is concentration

l is thickness of solution illuminated

Question 21

What is the specific rotation of an optically active substance?

Answer 21

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