Practice Problems on Emission and Absorption (H atom) Chemistry 121, Mines

Practice Problems on Emission and Absorption (H atom) Chemistry 121, Mines

1. Consider the energy level diagram of the hydrogen atom according to the Bohr model (right).

(a) Is a photon of light absorbed or emitted when an electron goes from the level n = 4 to n = 3? Give your reasoning.

0 -0.136

4

-0.242

3

-0.545

2

(b) What is the energy of a photon, in J, that is emitted or absorbed when the electronic transition described in part (a) occurs? Note that the energy scale is in units of 10-18 J.

n

Energy (10-18 J)

(c) What is the wavelength, in nm, of the photon that is emitted or absorbed when the transition in part (a) occurs?

-2.179

1

(d) If a mole of electrons were to undergo the transition in part (a), how many photons would be created (or destroyed)? What would be the (combined) energy of those photons, in kJ?

2. (a) Calculate the wavelength of light (in nm) needed to excite an electron from the n = 1 to the n = 4 level in a hydrogen atom. (b) Input this wavelength into the applet shown in class to see if your answer is correct! (c) If an electron keeps getting excited to the n = 4 level, how many different possible "emissive" transitions could occur as the electron "falls" back down to n = 1? Consider all possible "pathways" down, and include "secondary" transitions (i.e., from the n = 3 or n = 2 levels). (d) Do all of these emission events occur while watching the applet (after doing (b) above)?

3 Consider the bluish-green line in the H emission spectrum. Assume you estimate the wavelength reading off a spectrometer to be about 490. nm. Which electronic transition does this correspond to? Show work!

Hints: First determine the energy of the photons that are being emitted to create this "line". Then use the energy values from the diagram from Q1 above to see if you can find a "match" (or near match)--but remember that one line in an emission spectrum is produced by an electronic transition between two energy levels.

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