Understanding Light Absorption in Dyes

Which of the following best explains what happens as photons of visible light are absorbed by dye molecules?

• Certain electrons in the dye molecule move to a higher energy level, with the difference in energy between the lower and higher energy levels being the same as the energy of the absorbed photons.

• Certain chemical bonds in the dye molecules begin to bend and stretch, with the difference in energy between the lower and higher vibrational states being the same as the energy of the absorbed photons.

• The dye molecules begin to rotate faster in certain modes, with the difference in energy between the lower and higher rotational states being the same as the energy of the absorbed photons.

• Certain covalent bonds in the dye molecules begin to break and re-form, with the bond energies of the bonds being the same as the energy of the absorbed photons.

One type of organic molecule can be converted to another type of organic molecule through an oxidation-reduction process, as represented in the diagram above. Which of the following best explains why infrared spectroscopy is an appropriate method to confirm that the product contains a carbonyl?

• The absorption of infrared radiation causes the nonbonding electrons on oxygen to transition into a higher electronic energy level.

• The absorption of infrared radiation leads to an increase in molecular vibrational level associated with the stretching of the carbonyl.

• The absorption of infrared radiation causes the electrons in the pi bond to transition to a higher bonding energy level.

• Infrared radiation leads to an increase in the molecular rotational level associated with the carbonyl as the axis of rotation.

The absorption spectrum of a certain red dye is shown above. If a student analyzing the same concentration of this dye neglected to wipe fingerprints off the cuvette before placing it in the spectrophotometer, how would the absorption curve be affected?

• The peak of the curve would be higher because more light would be absorbed.

• The peak of curve would be lower because less light would be absorbed.

• The peak of the curve would be shifted to the left because less light would be absorbed.

• The peak of the curve would be shifted to the right because more light would be absorbed.

A student uses a spectrophotometer to analyze a solution of blue food dye. The student first rinses a cuvette with distilled water. Then the student adds the blue dye solution to the cuvette, forgetting to rinse the cuvette with the blue dye solution first. The student places the cuvette in the spectrophotometer and measures the absorbance of the solution. Assuming that some distilled water droplets were still in the cuvette when the blue dye solution was added, how would the measured absorbance be affected?

• The measured absorbance would be too low, because the distilled water left in the cuvette would slightly dilute the solution.

• The measured absorbance would be too low, because the distilled water would change the optimum wavelength of absorbance.

• The measured absorbance would be too high, because the water would allow more light to pass through the cuvette.

• The measured absorbance would be too high, because the distilled water would also absorb some light.

• The measured absorbance will be lower than actual absorbance

• The measured absorbance will be higher than actual absorbance

• The measured absorbance will remain unchanged.

The frequency and energy ranges of photons in some parts of the electromagnetic spectrum are given in the table above. Which of the following could be the energy of a photon in the visible range?

(a)  

Electromagnetic radiation with a maximum wavelength of 540 nm is needed for the study of the photoelectric effect in potassium atoms. What is the approximate frequency that corresponds to this wavelength?

(a)