OZ istu: Percentage concentration & ppm; energy-matter interactions

Question 1

How do you convert from parts per million to percentage composition and vice versa?

Answer 1

ppm → % divide by 10,000

% → ppm multiply by 10,000

Question 2

• What equation describes light as a wave?
• What equation links the particle and wave theories of light?

Answer 2

• c = λν (or c = λf)
• E = hν (or E = hf)

Question 3

List the types of energy change seen in molecules, from lowest to highest energy (and spacing between energy levels).

Answer 3

• Translational energy (overall movement)
• Rotational energy (overall)
• Vibrational energy (of bonds)
• Electronic energy

All these energy changes are quantised, hence “energy levels”

Question 4

Which region of the EM spectrum corresponds to changes in (or the spacing between) a molecule’s ___ energy levels?

1. Rotational
2. Vibrational
3. Electronic

Answer 4

1. Microwave
2. Infrared
3. Visible + UV

Translational energy changes correspond to radio

Question 5

What magnitude of energy change corresponds to changes in the following energy levels?

• Rotational
• Vibrational
• Electronic

Answer 5

Rotational: 1 x 10^-22 J to 1 x 10^-20 J (Microwave)

Vibrational: 1 x 10^-20 J to 1 x 10^-19 J (Infrared)

Electronic: 1 x 10^-19 J to 1 x 10^-16 J (Visible + UV)

Question 6

Which regions of the EM spectrum principally:

• Reach Earth’s atmosphere from the sun’s energy?
• Are emitted by Earth’s surface?

Answer 6

• UV + visible
• Infrared

Question 7

Describe the possible outcomes of a molecule absorbing UV radiation. List these in order of increasing energy.

Answer 7

Excitation of electrons, then one of the following:

1. Release of energy + return to prior state
2. Photodissociation (homolytic bond fission)
3. Ionisation

Question 8

Why is chlorine green?

Answer 8

• Chlorine absorbs light
• Electrons are excited to higher energy levels, then fall back down, emitting photons with E = change in energy level and f = E/h
• Frequency of emitted light corresponds to green visible light, so chlorine is green

Question 9

What is photodissociation?

Answer 9

The breakdown of a molecule into radicals by radiation.

I.e. homolytic bond fission

Question 10

CO₂ absorbs some of the infrared radiation emitted from the Earth’s surface.

1. Explain why CO₂ molecules absorb only certain frequencies of infrared radiation.
2. Explain why absorbing IR makes the atmosphere warmer.

Answer 10

1. Infrared corresponds to vibrational energy. This is quantised, so only frequencies corresponding to transitions between vibrational energy levels are absorbed.
2. When molecules absorb IR, they have higher KE, and they transfer this to other air molecules during collisions.

Question 11

Changing one mole of HCl from the vibrational ground state to the next vibrational energy level requires 32.7 kJ.

Calculate the frequency of radiation absorbed, and state which type of radiation it is.

Answer 11

32.7 kJ mol^-1 = 32,700 J mol^-1

Energy for one molecule = 32,700 / (6.02 x 10²³) = 5.432 x 10^-20

Frequency = E / h = 5.432 x 10^-20 / (6.63 x 10^-34) = 8.19 x 10¹³ Hz

Infrared radiation (x 10^-20 is borderline microwave and IR, but 5.4 x 10^-20 is greater than 1 x 10^-20, so it’s in the IR range)