20.1.Convert the following to frequencies (Hz): (i) 2 10 –7
m,(ii)500 nm, (iii)1000 cm–1.
20.2.Complete the table below (the first entry has been com-
pleted).
Definition Word
Gain of energy of an atom or absorption
molecule by collision with a photon
Loss of energy by an atom or molecule by
the giving out of light
The logarithm of the ratio of the incident to
transmitted intensities
The production of light by chemical reaction
The absorbance of a sample at a wavelength
at unit pathlength and unit concentration
20.3.Calculate the energy of 1 mol of photons of light with
an equivalent wavelength of 1000 nm.
20.4.The energy gap between two energy levels in a molecule
is 30 kJ mol–1. What is the frequency of the light required to
be absorbed or emitted in a transition? To which part of the
electromagnetic spectrum (UV, visible, infrared or microwave)
does this radiation belong?
20.5.(i)In a spectrometer, 84% of the incident light at 240
nm is absorbed by a solution. What is the absorbance of the
solution?
(ii)The molar absorption coefficient of a dye at 450 nm is
1000 mol–1m^2. Calculate the percentage of light transmitted
through exactly 1 cm of dye solution of concentration 1.00
10 –5mol dm–3at the same wavelength. By how much will the
dye solution need to be diluted to give a solution with an
absorbance of 0.010?
20.6.The (first) standard ionization energy of Na(g) is 494 kJ
mol–1. What wavelength of light would need to be absorbed in
order to ionize sodium atoms?
20.7.The energy E (in kJ mol 1 ) of the electronic levels in
the hydrogen atom are easily calculated using the equation1310
E= – ——–
n^2wherenis the principal quantum number. (Test this formula
by applying it to n= 2. As Fig. 20.7 shows, E= –328 kJ mol–1.)
(i)Calculate the energy of the n= 50 level.
(ii)What frequency of light will be emitted in the transition
n= 50 →n= 3?20.8.(i)The sun consists of an incredibly hot centre (above
10 6 K) surrounded by a relatively cool layer at about 5800 K.
If the sun is examined by a hand spectroscope, the solar spec-
trum is seen to contain some dark lines, Fraunhofer lines,
superimposed upon a continuous blend of ‘rainbow’ colours.
What causes the dark lines?
(ii)The following diagram shows the simplified electronic
energy levels of an atom or molecule:
Draw in all the possible transitions corresponding to absorp-tion and emission at room temperature. Use this diagram to
explain why the emission spectra of atoms and molecules
generally contain more lines than their absorption spectra.20.9.(i)Sketch the expected absorption spectrum for a red
filter.
(ii)Study the following UV–visible spectrum of a compound
and predict the colour of its solution.20 · LIGHT AND SPECTROSCOPYproduction of carbon-based solids in plants from a carbon-containing gas was
something of a mystery to early scientists, who referred to photosynthesis as ‘car-
bon fixation’ – a phrase that is still in use today. Because of the widespread occur-
rence of plants using photosynthesis on land and sea, the scale of carbon fixation
is enormous, with about 10^11 tonnes of carbon being taken out of the atmosphere
per year.●Photosynthesis consumes CO 2 and also returns O 2 to the atmosphere. Photo-
synthesis is the most important way in which CO 2 levels (increased by the burn-
ing of fuels and by respiration) are reduced, and in which oxygen levels in the air
are replenished.396
Revision questions