AP Chem 3.11 - 3.13

Molecules that absorb infrared radiation undergo…

• bond vibrational movement.

• rotational movement.

• electron energy promotion (electron excitement).

Tags

SAP-8.C

Molecules that absorb microwaves undergo …

• rotational movement.

• electron energy promotion.

• bond vibrational movement.

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SAP-8.C

Molecules that absorb UV radiation ....

• experience rotation or spin.

• experience electron promotion (electron excitement).

• experience bond vibration.

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SAP-8.C

The diagrams above show the ultraviolet absorption spectra for two compounds. Diagram 1 is the absorption spectrum of pure acetone, a solvent used when preparing solutions for an experiment. Diagram 2 is the absorption spectrum of the solute for which the absorbance needs to be measured to determine its concentration. When the student reads the absorbance of the solution at 280 nm, the result is too high. Which of the following is most likely responsible for the error in the measured absorbance?

• The student added too little solute to the acetone before measuring its absorbance

• The student rinsed the cuvette with the solution before filling the cuvette with the solution

• The student forgot to calibrate the spectrophotometer first by using a cuvette containing only acetone

• The wavelength setting was accidentally changed from 280 nm to 300 nm before the student made the measurement

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SAP-8.C

Infrared spectroscopy is a useful tool for scientists who want to investigate the structure of certain molecules. Which of the following best explains what can occur as the result of a molecule absorbing a photon of infrared radiation?

• The energies of infrared photons are in the same range as the energies associated with changes between different electronic energy states in atoms and molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of electronic transitions within the molecules.

• The energies of infrared photons are in the same range as the energies associated with different vibrational states of chemical bonds. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the types and strengths of different bonds in the molecules.

• The energies of infrared photons are in the same range as the energies associated with different rotational states of molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of transition between different rotational energy states of the molecules.

• The energies of infrared photons are in the same range as the total bond energies of bonds within molecules. Chemical bonds can be completely broken as they absorb infrared photons of characteristic wavelengths, thus revealing the energies of the bonds within the molecules.

Answer explanation

Infrared spectroscopy is based on molecules absorbing radiation at frequencies that match their vibrational frequencies.

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SAP-8.A

Which statement correctly compares what occurs when molecules absorb photons in the microwave region with what occurs when molecules absorb photons in the infrared region?

• Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause the molecules to increase their vibrational energy states.

• Microwave photons cause electrons in the molecules to increase their electronic energy states, whereas infrared photons cause the molecules to increase their rotational energy states.

• Microwave photons cause the molecules to increase their vibrational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states.

• Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states.

Answer explanation

Infrared radiation has a higher frequency (shorter wavelength) than microwave radiation has. Since the energy of a photon is directly proportional to its frequency (and inversely proportional to its wavelength), infrared radiation has a higher energy per photon than microwave radiation has. Molecular vibrations are studied using infrared radiation. Molecular vibrations require more energy than molecular rotations, which are studied using microwave radiation.

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SAP-8.A

Beta-carotene is an organic compound with an orange color. The diagram above shows the ultraviolet spectrum of beta-carotene. Which of the following statements is true about the absorption bands in the spectrum?

• The absorption band between 250 and 320 nm is due to transitions in electronic energy levels, and the absorption band between 380 and 520 nm is due to transitions in molecular vibrational levels.

• The absorption band between 250 and 320 nm is due to transitions in molecular vibrational levels, and the absorption band between 380 and 520 nm is due to transitions in molecular rotational levels.

• The two main absorption bands are associated with transitions in electronic energy levels. The band in the region corresponding to shorter wavelengths shows a lower absorbance than the band in the region corresponding to longer wavelengths.

• The two main absorption bands are associated with transitions in molecular vibrational levels. The band in the region corresponding to shorter wavelengths shows a lower absorbance than the band in the region corresponding to longer wavelengths.

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SAP-8.A

The diagram above represents the photoelectric effect for a metal. When the metal surface is exposed to light with increasing frequency and energy of photons, electrons first begin to be ejected from the metal when the energy of the photons is 3.3×10⁻¹⁹ J.

Which of the following is closest to the frequency of the light with photon energy of 3.3×10⁻¹⁹ J ?

Answer explanation

The energy of a photon is related to the frequency v of the electromagnetic wave according to Planck’s equation (E=ℎv). If the energy E is divided by Planck’s constant ℎ, the frequency is 5×10¹⁴s⁻¹.

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SAP-8.B

A beam of light has a wavelength of 506 nanometers. What is the frequency of the light? (nm = 10^-9)

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SAP-8.B

A beam of light has a frequency of 4.615E14 s^-1. What is the color of this light?

• orange

• blue

• green

• purple

• yellow

Tags

SAP-8.B