Question

Please answer these three questions:

1. What are the differences in spectra between molecular and atomic absorption? Explain these differences using the types of transitions.

2. UV-Vis instruments used to measure molecular absorption typically have a broad wavelength light source while atomic absorption instruments use a light source that produces very narrow bands of light. Both are typically in the same UV-Vis wavelength range. Why don’t they use the same source?

3. Alkenes such as n-hexane are used as solvents in UV spectroscopy. The energy difference for the σ → σ* transition is 886 kJ/mol. What wavelength does hexane absorb? Why is n-hexane a good choice as a solvent for UV/Vis spectroscopy?

Answer 1

1. Molecular spectroscopy deals about the interactions of radiatiom with bonding electrons in the outer electronic shell of the molecule. The interactions depend on the structure the molecule. The valence shell electrons are far from the nuclei, relatively small amount of energy is required to excite them.

Atomic spectroscopy deals about the interactionsof radiation and electrons that are independent of bonds. The free electrons are produced by ionisatio of atoms or by breaking of bonds. These require much higher energy.

2. Narrow bands of light ( or sharp) has much higher energy than board bands of light. As atomic spectroscopy requires higher energy than molecular spectroscopy, narrow band of light is used for atomic spectroscopy.

3. The energy difference is 886 kJ/ mole = 886 * 10³ J/ mole = 886 * 10³ / (6.023 * 10²³) J/ molecule

= 1.47 * 10^-18 J/ molecule

Wavelength = Planck constant * speed of light / energy difference = ( 6.625 * 10^-34 J.s * 3 * 10⁸ ms^-1)/ 1.47 * 10^-18 J = 1.352 * 10^-7 m = 135.2 nm

As n-hexane absorbs at a much lower wavelength , it does not interfer with the molecules which absorb in UV-Vis region. For teh reason, n- hexane is a good choice of solvent in UV- Visible experiment.