Spectroscopy (College Board AP® Chemistry): Flashcards
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Define electromagnetic spectrum.
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Define electromagnetic spectrum.
The full range of frequencies (and wavelengths) of electromagnetic radiation, from low-energy radio waves to high-energy gamma rays, organised by frequency, wavelength and energy.
True or False?
Higher frequency electromagnetic radiation has a shorter wavelength and carries more energy.
True.
Frequency and wavelength are inversely proportional (c = λν), and frequency is directly proportional to energy — so higher frequency means shorter wavelength and greater energy.
What type of molecular motion is stimulated when a molecule absorbs microwave radiation?
Rotation. Microwave photons promote molecules to an excited rotational state, changing the orientation of the molecule.
When molecules are exposed to infrared radiation they ..........; when exposed to ultraviolet or visible radiation, their .......... jump between energy shells.
When molecules are exposed to infrared radiation they vibrate; when exposed to ultraviolet or visible radiation, their electrons jump between energy shells.
True or False?
Different types of electromagnetic radiation travel at different speeds.
False.
All electromagnetic waves travel at the same speed — the speed of light (c = 2.998 × 108 m s-1). Only their frequencies and wavelengths differ.
List the regions of the electromagnetic spectrum in order of increasing energy.
Radio waves → Microwaves → Infrared → Visible → Ultraviolet → X-rays → Gamma rays.
Energy increases as frequency increases and wavelength decreases.
Define excited rotational state.
The state a molecule reaches when it absorbs a microwave photon and begins to rotate more rapidly, changing its orientation in three-dimensional space.
Absorption of .......... radiation stimulates molecular rotation, while absorption of .......... radiation stimulates molecular vibration.
Absorption of microwave radiation stimulates molecular rotation, while absorption of infrared radiation stimulates molecular vibration.
True or False?
Infrared spectroscopy exploits changes in molecular vibrations to identify compounds.
True.
Each bond vibration mode has a characteristic frequency. Infrared spectroscopy measures which IR frequencies a sample absorbs to identify functional groups and compounds.
Why are ultraviolet and visible photons able to cause electronic transitions, while infrared photons are not?
UV and visible photons carry enough energy to promote electrons from lower-energy orbitals to higher-energy orbitals. Infrared photons have lower energy, sufficient only to stimulate bond vibrations rather than electronic transitions.
Define excited vibrational state.
The state a molecule reaches when it absorbs an infrared photon and its bonds vibrate more energetically — the bonds behave like springs stretching, compressing, or bending at a characteristic vibrational frequency.
True or False?
An electronic transition caused by UV or visible light absorption is always accompanied by changes in rotational and vibrational states.
True.
Electronic transitions are typically followed by changes in the rotational and vibrational states of the molecule, because all three types of energy are quantised and interconnected.
A microwave oven heats food by transferring energy to water molecules. What type of molecular transition does this illustrate, and why is microwave radiation — rather than infrared — used?
It illustrates excitation to a rotational state: microwave photons match the rotational energy levels of water molecules, causing them to rotate and generate heat through molecular friction rather than stimulating the bond vibrations that infrared radiation would produce.
Define photon.
A discrete particle (quantum) of light. The photoelectric effect demonstrated that electromagnetic radiation behaves as particles as well as waves — each particle of light is called a photon.
True or False?
When an atom absorbs a photon, an electron jumps to a lower energy shell.
False.
Absorbing a photon promotes an electron to a higher energy shell (excited state). The electron returns to the ground state — a lower energy shell — when a photon is emitted.
What experimental observation led scientists to conclude that light behaves as a particle?
The photoelectric effect — certain metals emit electrons when light is shone on them. The results could only be explained if light arrived as discrete packets of energy (photons) rather than as a continuous wave.
The energy of a photon is given by E = h.........., where h is .......... and has a value of 6.626 × 10-34 J s.
The energy of a photon is given by E = hν, where h is Planck's constant and has a value of 6.626 × 10-34 J s.
A photon has a frequency of 6.0 × 1014 Hz. Calculate its energy. (h = 6.626 × 10-34 J s)
E = hν = 6.626 × 10-34 × 6.0 × 1014 = 3.98 × 10-19 J
True or False?
In the hydrogen emission spectrum, electron transitions to n = 1 release photons in the ultraviolet region.
True.
Transitions to n = 1 (the Lyman series) release the most energy, corresponding to UV photons. Transitions to n = 2 produce visible light, and transitions to n = 3 produce infrared radiation.
How are the equations E = hν and c = λν used together to find a photon's wavelength from its energy?
Rearrange E = hν to find frequency: ν = E/h. Then substitute into c = λν and rearrange: λ = c/ν = hc/E. This links energy directly to wavelength.
Define absorbance.
A dimensionless measure of how much light a sample absorbs at a given wavelength. It is directly proportional to molar absorptivity, path length, and concentration, as expressed by the Beer-Lambert law: A = εlc.
The Beer-Lambert law states that A = .........., where ε is molar absorptivity, .......... is path length, and c is concentration.
The Beer-Lambert law states that A = εlc, where ε is molar absorptivity, l is path length, and c is concentration.
True or False?
Absorbance has units of mol-1 dm3 cm.
False.
Absorbance (A) is dimensionless — it has no units. Molar absorptivity (ε) carries the units (mol-1 dm3 cm-1 in some conventions), but A itself is unitless.
In most Beer-Lambert experiments, why is absorbance treated as proportional to concentration alone?
Because molar absorptivity (ε) and path length (l) are held constant throughout the experiment — the cuvette size is fixed and the same wavelength is used — so A = εlc simplifies to A ∝ c.
Define calibration curve in the context of Beer-Lambert experiments.
A graph of absorbance vs. concentration plotted using standard solutions of known concentration. The resulting linear relationship is used to determine the concentration of an unknown sample from its measured absorbance.
True or False?
A calibration curve for a Beer-Lambert experiment should show a linear positive correlation between absorbance and concentration.
True.
Because A = εlc and ε and l are constant, absorbance increases linearly with concentration. A line of best fit through the standard data points allows unknown concentrations to be read off directly.
A student uses a calibration curve with the equation A = 60c − 0.02. The unknown sample has an absorbance of 0.275. What is the concentration of the unknown sample?
Rearrange: c = (A + 0.02) / 60
c = (0.275 + 0.02) / 60 = 0.295 / 60 = 4.92 × 10-3 M
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