uv vis spec

Question 1

What is spectroscopy?

Answer 1

The study of the interaction of matter and electromagnetic radiation

Question 2

What is electromagnetic radiation (EMR)?

Answer 2

• EMR is radiant energy that displays the properties of both particles and waves
• There are different types of electromagnetic waves that make up the
electromagnetic spectrum (EMS)

Question 3

An example of electromagnetic radiation

Answer 3

Visible light

Question 4

The electromagnetic spectrum

Answer 4

gamma 
x
uv
visi
ir
radio

Question 5

Gamma rays

Answer 5

are emitted from the nuclei of some radioactive elements and because of their high energy can severely damage biological organisms.

Question 6

X-rays

Answer 6

are less harmful than gamma except in high doses.

Question 7

UV light

Answer 7

is responsible for sunburn. Repeated exposure leading to skin cancer by damaging DNA molecules in skin cells.

Question 8

Visible light

Answer 8

electromagnetic radiation that we see.

Question 9

Infrared radiation

Answer 9

We feel IR as heat

Question 10

Radio waves

Answer 10

have the lowest energy (frequency) Radio, TV. Radio waves are also used in NMR and in magnetic resonance imaging MRI

Question 11

•Electromagnetic radiation has 2 components:

Answer 11

an electric field and a magnetic field which are in planes at right angles to each other.

Question 12

A KEY Difference between electromagnetic waves and other types of waves,

Answer 12

(e.g. sound, water and seismic waves which exist only by virtue of the medium in which they move), EM waves can travel through a vacuum

Question 13

Frequency,

Answer 13

n = the number of crests of the wave that pass a point per second. The SI unit of frequency, n, is the reciprocal second, s-1, which is also called the hertz, Hz.

Question 14

Wavelength,

Answer 14

lama = distance between two adjacent crests or troughs of the wave. The SI unit of wavelength, , is metres, m, though multiples such as nm
(10-9 m) are frequently used.

Question 15

Velocity,

Answer 15

in a vacuum all electromagnetic waves travel with the same velocity: c 8 -1
= 2.998 x 10 ms .

Question 16

speed of light eqn

Answer 16

c=lambda x v

Question 17

photon

Answer 17

A particle of electromagnetic radiation is called a

Question 18

The energy, E, of a photon is given by

Answer 18

E = hv (h is Planck’s constant, h = 6.626 x 10-34 J s)

Question 19

Energy of the photon (wave) is directly proportional to

Answer 19

FREQUENCY

Question 20

Energy of the photon (wave) is inversely proportional to

Answer 20

WAVELENGTH

Question 21

The SI unit of energy

Answer 21

is the joule, J (although kJ mol-1 is often quoted)

Question 22

wavenumber

Answer 22

E = hc (1/lambda) = hc v

Question 23

The SI unit of v (vibrational frequency)

Answer 23

is m-1

Question 24

c speed of light

Answer 24

2.998 x 10^8 m s^-1

Question 25

energy kj to kj/mol

Answer 25

multiply by Avogadro's number 6.023 x 10^23 mol^-1

Question 26

how does uv/visible spec work

Answer 26

``` UV/Vis Light source to monochromator to cell ( Io incident light) to I (light not absorbed by sample) to the detector ```

Question 27

monochromator

Answer 27

The monochromator selects a single wavelength from the wide range provided by the light source.

Question 28

cell

Answer 28

The cell (cuvette) can be glass or plastic for visible light but must be quartz for UV light (glass and plastic absorb UV).

Question 29

What factors control the amount of absorbance?

Answer 29

1. The thicker the sample, the more absorption takes place. 2. More absorption is seen for more concentrated samples (more molecules present to absorb the light). 3. Some molecules are better than others at absorbing a particular wavelength; it is an inherent property of the molecule.

Question 30

The Beer-Lambert law - at a single wavelength

Answer 30

A= Ecl

Question 31

Absorbance, A, of a sample depends on:

Answer 31

1. concentration, c, of the absorbing species. mol L^-1 2. length of light path, l, through the cell cm 3. molar absorption coefficient, E. L mol^-1 cm^-1

Question 32

Molar absorption coefficient, E:

Answer 32

some molecules are better than others at absorbing a particular wavelength; it is an inherent property of the molecule. e is also known as the molar absorptivity and the extinction coefficient in older textbooks.

Question 33

transmitted light is equal to

Answer 33

I = Io 10^(-elc)

Question 34

Absorbance is equal

Answer 34

log(Io/I)

Question 35

Transmission, T

Answer 35

Absorption data can also be reported as transmission T= Io/I A=-log(T)

Question 36

Visible light

Answer 36

400nm (violet) to 750nm (red), UV 200-400 nm

Question 37

two types of transition

Answer 37

n to π* and π to π*

Question 38

transition

Answer 38

A change between energy levels is called a transition

Question 39

vibrationally excited

Answer 39

The lowest energy transition moves an electron between the HOMO and LUMO orbitals. Generally the excited state will also be vibrationally excited.

Question 40

Excited states last for

Answer 40

only last for very short periods of time (1 to 10 nanoseconds), because the higher energy state is energetically unstable.

Question 41

relaxation processes

Answer 41

The extra energy is lost through relaxation processes such as emission of light or heat.

Question 42

n to π* is ________ energy than π to π*

Answer 42

n to π* is less energy than π to π*

Question 43

Effects of conjugation

Answer 43

1. Conjugation decreases the energy gap between HOMO and LUMO. 2. Hence less energy is required for electronic transitions. 3. Therefore transitions occur at longer wavelengths. 4. If a compound has enough double bonds it will absorb visible light and the compound will be coloured, e.g. beta-carotene which is orange and is found in carrots and tomatoes has lamda max= 455nm

Question 44

chromophore

Answer 44

A molecule or part of a molecule that can be excited by absorption is called a chromophore.