General

Question 1

What is meant by electronic, vibrational and rotational spectroscopy?

Answer 1

Vibrational- how molecules vibrate

Rotational- how molecules rotate

Electronic- how electrons move between orbitals

Question 2

What links frequency and wavelength?

Answer 2

V = C / wavelength

Remember-: mr speed frequents the club ‘wave’

Question 3

How do you covert wavelength into wavenumber?

Answer 3

Wavenumber= 1/ wavelength x 100

Question 4

How do you find the energy of one photon?

Answer 4

E= hv

Remember- E hates V

To find one mole, multiply by Na

Question 5

Different types of molecular transition involve absorption of energy from different parts of EM spectrum (use different spectroscopy)
- which energy transitions do UV/visible light, IR, microwave and radio waves correspond to?

Answer 5

Low energy UV or visible light corresponds to electronic energy levels
IR corresponds to vibrations of bonds
Microwave corresponds to rotation
Radio waves corresponds to electron spin

Question 6

What is wave particle duality?

Answer 6

Radiation can appear to be waves or particles

• used interchangeably

Waves= oscillating electric and magnetic field 
Particles= photons

Question 7

How do atomic spectra arise?

Answer 7

Atomic spectra arise from transitions between electronic energy levels

Question 8

What are the units of c, wavelength, v, wavenumber, h, Na and k

Answer 8

C is ms-1
Wavelength is m
V is s-1
Wavenumber is cm-1 (number of waves in 1cm) 
H is Js
Na is mol-1
K is JK-1

Question 9

What does quantum theory tell us about quantisation?

Answer 9

A molecule is allowed to exist only one of several discrete energy levels
Only have discrete amounts of electronic, vibration, translational and rotational energy
Molecules can only have certain energies

Question 10

What does a transition involve and what does it need to happen?

Answer 10

A transition involves the intake or release of a discrete amount of energy
For a transition to occur, the energy provided must correspond to the energy gap
Light must have the v=^E/ h
This means a photon will only be absorbed if it’s energy matches the gap

Question 11

When light is emitted, what is the energy released and what is the frequency?

Answer 11

The energy released is ^E= hv

The light emitted will have v= ^E/ h

Question 12

What graphs will be plotted?

Answer 12

Absorbance, emission or scattering strength versus wavelength, wavenumber or frequency

Question 13

What are the roles of radiation source, monochromatic and detector in a spectrometer?

Answer 13

Radiation source- to cover required wavelength region
monochromatic- to select one wavelength to send to sample
Detector- to measure the intensity of the transmitted beam

You measure the intensity of absorption or transmission at different wavelengths

Question 14

What determines position and size of spectral peak?

Answer 14

The position is determined by the energy of the transition

The size depends on the number of absorption or emission events that take place

Question 15

What does spectral intensity depend on?

how many molecules are absorbing or transmitting

Answer 15

• conc of sample
• path length
• how many molecules have the correct energy at this frequency
• how likely a transition is

Question 16

Transmittance is the amount of light that goes through a sample, what is the equation?

Answer 16

T = I/ Io

For percentage x100%

Question 17

What is the equation for absorbance?

Answer 17

A= log(Io/I)= log(1/T)

Question 18

What determines band intensity?

Answer 18

Selection rules
Population of energy levels
Number of molecules

Question 19

What are selection rules and how do they influence band intensity?

Answer 19

Quantum theory tells us that not all possible transitions can occur
- there are rules that state which are allowed and which are forbidden
ALLOWED= strong band
FORBIDDEN= weak or no band

Question 20

How does population of energy levels influence band intensity?

Answer 20

The greater the number of molecules in the level from which the transition starts, the stronger the band from this transition

• more likely a photon will be absorbed

Question 21

How does the population of the upper and lower level change with spacing?

Answer 21

If ^E is is small (closely spaced) then the population of the upper and lower state is similar
- transition can occur from many state = many bands
If ^E is large (large spaces) then there are fewer molecules in the upper state
- transition occur from mainly one state= one band

Question 22

For an absorption and emission peak, which population level affects band intensity?

Answer 22

For an absorption peak, the population of the lower level in the transition affects the intensity of the peak
For an emission peak, the population of the upper level in the transition affects the intensity of the peak.

Question 23

How does the number of molecules influence band intensity?

Answer 23

The greater the number of molecules the light encounters, the stronger the band.

Question 24

How can you increase the number of molecules encountered by the light?

Answer 24

Increase path length or increase the concentration.