Uv Spectroscopy

Question 1

Which molecules absorb uv/ visible light

Answer 1

Conjugated pi systems
Longer conjugation= longer wavelength absorption

Question 2

What happens when radiation passes through a sample

Answer 2

Some of the radiation is absorbed by the smoke
Some radiation is reflected/ scattered
Some radiation passes straight through

Question 3

How to minimise the effects of reflection and scattering

Answer 3

Taking the intensity of light (I0) passing through a blank solution and then With a sample solution
Measure the difference

Question 4

Transmittance (amount of radiation which passed through the sample) equation

Answer 4

T= I/Io

Intensity of unknown sample
Intensity of blank

Question 5

What’s does the transmittance depend on

Answer 5

Path length of the cell
Concentration of the absorbing substance
Nature of the substance

Question 6

Effects of Intensity with path length

Answer 6

I decreases exponentially with an increase in path length
I decreases exponentially with an increase in concentration in a fixed path length

Question 7

Relationship between absorbance A and path length

Answer 7

Absorbance is directly proportional to the concentration of absorbing substance and path length

Question 8

Beer- Lambert law

Answer 8

A= acl

a= absorption coefficient
C=1, l=1

Question 9

Molar absorption coefficient

Answer 9

L= cm, c=M
a= molar absorption coefficient (epsilon)

Represents the absorption of a 1 Moldm-3 solution in a cell of path length 1 cm

Question 10

Specific absorbance

Answer 10

L=cm, c= %m/v
Specific absorbance= A(1%, 1cm)

Represents the absorbance of a 1% m/v solution in a cell of path length 1cm

Question 11

luminescence

Answer 11

emission of light by molecules without heat

Question 12

fluorescence

Answer 12

excited molecules rid themselves of the excess electromagnetic energy by emitting the absorbed energy as light of a lower wavelength

Question 13

chemi-luminescence

Answer 13

some product molecules are left in an excited state
light emitted when these molecules return to the ground state is known as chemi-luminescence

Question 14

absorption

Answer 14

excitation to higher electronic and vibrational states

Question 15

vibrational relaxation

Answer 15

through collisions with surrounding molecules, the molecule returns to the v=0 state within the higher electronic state

Question 16

emission

Answer 16

the molecule releases its energy as a photon and returns to the ground state
emits fluorescence

Question 17

stokes shift

Answer 17

emitted light is always of lower energy
because some energy is lost to vibrational relaxation in the excited state
because the molecule can return to an excited vibrational state giving a spectrum of emitted light

Question 18

Kasha’s rule

Answer 18

emission of spectrum is independent of the excitation wavelength
because fluorescence always occurs from the vibrational ground state due to rapid vibrational relaxation

Question 19

reasons for using fluorescence

Answer 19

more selective than absorption
a substance will absorb radiation at once wavelength and emit at another
not all substance fluoresce
more sensitive

Question 19

relationship between chemical structure and fluorescence

Answer 19

rigid structures cannot twist/turn
prevents loss of energy
fluorescent
also large number of delocalised electrons

Question 19

factors affecting the intensity of fluorescence

Answer 19

how much light is absorbed by the sample (more energy absorbed=more energy emitted)
the efficiency with which absorbed light is converted to fluorescence
the experimental setup used to measure the emission

Question 19

equation relating fluorescence and intensity of light

Answer 19

F is directly proportional to (incident intensity- transmitted intensity)

Question 19

quantum efficiency

Answer 19

ratio of number of molecules which fluoresce to number that actually absorb energy
no. of quanta emitted/ no. of quanta absorbed

Question 20

primary inner filter effect

Answer 20

at high concentration, excitation light is mainly absorbed near the entrance face of the cell
creates an uneven distribution of fluorescence

Question 21

secondary inner filter effect

Answer 21

at high concentration, emitted light can also be absorbed
fluorescence intensity can decrease with increasing concentration

Question 22

photodecomposition

Answer 22

intensity of radiation is very high

Question 23

quenching

Answer 23

when fluorescence is decreased by the presence of another substance such as halide or oxygen in the environment