Lab 7- Spectrophotometry And Beer’s Law Flashcards
What is the purpose of this lab?
To learn the principles of operation of a spectrometer, to measure the absorption spectrum of a solution of a colored material, and to do a spectrophotometric definition of the amount of copper in a sample.
What is the general note in this lab?
Spectroscopic methods, such as those described in this experiment, are probably the most valuable tools available in modern chemistry for studying the properties of chemical substances and for measuring amounts of various substances in some material of interest. Unfortunately, few textbooks give spectroscopy the attention it deserves. You will be carrying out other experiments using spectroscopic methods during the course. Refer back to the material in the section below.
What are the notes regarding Light, Absorption of Light, and Color?
Light can be described as an oscillating electrical and magnetic wave-electromagnetic radiation- with properties characterized by its wavelength. Note that we use the term “light” to refer to a far greater range of wavelengths than in the visible light to which our eyes are sensitive.
When various wavelengths of light are absorbed by a molecule, the molecule can rewind by changing in some way. Absorption of light in the infrared region (for example, the radiation from a heat lamp) typically causes a molecule to vibrate more rapidly; absorption of microwave radiation (for example, the radiation from a microwave oven) causes molecules to rotate more rapidly; and absorption of light in the visible or ultraviolet region causes electrons in the molecule to move into higher energy levels.
Define Light
Light can be described as an oscillating electrical and magnetic wave-electromagnetic radiation-with properties characterized by its wavelength.
How does absorption of light in the Infrared Region affect a molecule?
It typically causes a molecule to VIBRATE more rapidly. IV
How does absorption of microwave radiation affect molecules?
It typically causes molecules to ROTATE more rapidly. MR
How does absorption of visible light and ultraviolet light affect molecules?
It causes electrons in the molecules to move into HIGHER energy levels. VE
How will something appear colorless?
A solution that deos not absorb the light i the visible region of the spectrum will appear colorless.
What happens if a solution absorbs one or more wavelengths from the visible spectrum?
If a solution absorbs one or more wavelengths in the VISIBLE spectrum, the result will be a colored solution.
The color of the solution perceived by yours eyes will be the _____ of the color of the light absorbed by the solution.
Complement
If a solution absorbs yellow-green light to yellow light, the solution will appear _____?
VIOLET
What wavelengths does permanganate strongly absorb and what color characteristic does it have?
Absorbs strongly at about 520-550 nm; violet.
How can one predict the color of a solution?
They can do so from the wavelengths it absorbs, or the wavelengths a solution is likely to absorb from its color. USING THE COLOR WHEEL.
The wheel is arranged such that the complementary color is found on the opposite side of the wheel from the absorbed color.
Define the Absorption Spectrum
Lights of different wavelengths is absorbed to differing extents by a solution.
A graph of the amount of light absorbed by a solution is called the absorption spectrum.
Define Beer’s Law
If a solution is colored, the intensity of the color is a measure of the concentration of the solution. For example, by looking at the color of a cup of coffee, you can tell if the coffee is weak or strong (that is, dilute or concentrated). The more light absorbed by a solution, the higher the concentration of the solution.
What factors deos the fraction of light absorbed by a solution depend on?
1) The chemical species present in the solution
2) The wavelength
3) The concentration of the absorbing species
4) The length of the light path through the sample (a 10-cm deep cup of coffee will absorb more light and appear darker than a 1-cm deep cup if the concentrations are the same.
CW-CL. (Chemical Species-Wavelength-Concentration (of absorbing species)- Length of Light path through the sample).
What is the unit for C (Solute concentration)?
Mole/Liter
What are sample tubes called? What is the inside diameter called?
Cuvet-The Path length for the light
What is a general rule for I0 and I?
If light intensity I0 shines on the sample, then the intensity of light leaving the sample will be some SMALLER value of I.
What is the equation for Beer’s Law? What does each thing stand for?
A= ebC
A= Absorbance of the solution (where A=log (I0/I))
B= Path length of the solution in CM
C= concentration of the solution in mole/liter
E (epsilon)= Molar absorptivity of the substance at the wavelength at which the measurement is made, with units liter/mole-cm. E is essentially a proportionality constant
How de we measure the absorbance?
Using an instrument called the spectrometer.
How do spectrometers measure absorbance?
Most spectrometers do not measure the intensity I directly, but measure the absorbance (A=log I0/I) or transmittance (T=I/I0) of the sample. The instrument you will be using, an ocean optics spectrometer, can measure both the absorbance and the percent transmittance %T (100 times the transmittance).
What are the different equations for A?
A=log (1/T)
A=-logT
A= -log (%T/100)
What is the combined equation for A?
-log(%T/100)=ebC
Define Deliquescence
When a substance takes up so much water from the atmosphere that it dissovles.
Define Efflorescence
When a hydrate loses water upon standing.
Define Dessicant
An anhydrous compound that will absorb water from the air or other sources.
