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Achem 315 > Light And Absorbance > Flashcards

Light And Absorbance Flashcards

1
Q

What is frequency

A

The amount of waves (occilations) per second

1 oscillation per second is Hz

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2
Q

Relation between frequency and wave length equation

A

V(wavelength)=c

C is speed of light
V= frequency

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3
Q

What is a photons energy given by

A

The symbol E

E=hv
Or E=hc/v
H is plancks constant

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4
Q

What is light

A

Electromagnetic radiation

(Also frequncy)

It can be quantized as packet:units of energy (photons)

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5
Q

What can light do when is interacts with matter

A

Can be absorbed by the atom/molecule (its energy is taken in and the colour is not shown)

Can scatter off the molecule (the light changes direction in diff way when it interacts) this scattering can be elastic or inelastic

Can be transmitted through the material (light interacts with objects then passes through the object)

Can also be emitted by atoms/molecules (it realesse stored E and we see the light)

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6
Q

What happens to E in elastic scattering?

A

E does not change during elastic scattering

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7
Q

What happens to the molecules to account for the energy inputs for

X rays

Uv

Infrared

Microwave

A

X rays: the bonds between molecules break and ionize

Uv: the electrons get excited and jump up in energy

Infrared: molecules vibrates

Microwave: molecules rotate

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8
Q

When light hits atoms, what happens to the electrons

This is atomic absorbance

A

If absorbed, The electrons are energized and promoted to a higher level energy orbital/level (promoted to excited state)

If the atom emits the photon the energy is lowered

This is atomic absorbance

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9
Q

In molecular absorbance, usually where is the electronic transition going to occur

A

The electronic transitions are in the molecular orbitals

A HOMO to LUMO transition that involves sigma and pi orbitals

If a larger wavelength, the change in orbitals is also larger

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10
Q

In organic molecules, where are most of the electron trantstions happened

A

In the UV wavelength,

Most organic molecules absorb the light in the uv range

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11
Q

What is special about longer conjugated pi systems in regard to molecular absorbance

A

The have a larger area over which the electrons can move

And the electrons require less energy to transition from pi to pi anti binding orbital

This means they have a larger range of wavelength they can absorb (move from uv to visible range)

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12
Q

What are chromophores

A

Molecules that absorb light a a certain wavelength and emits colour as a result

Most chromophore of molecules are aromatics or conjugated systems

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13
Q

In a molecule, Because each electron transition corresponds to a very specific wavelength that is being introduced to molecule, what do we notice

A
  1. If monochromatic light of that specific wavelength the the molecule absorbs is sent through the sample, the lights intensity will dim (because the light got absorbed by the sample
  2. If polychromatic light (many diff colours/wavelengths) is sent in to that sample, that wavelength that matches the absorption wavelength will be reduced but other wavelengths won’t

Ex. If blue absorbed we see orange, if red and blue absorbed, we see yellow

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14
Q

What does P and P0 stand for

A

Irradiance

Also called power out and power in

Usually p< or equal to p0

If p is lower that power in, that mess the photons lost were absorbed and more light was lost meaning more of the molecule is absorbing light

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15
Q

1 photon absorbed equals

A

1 electron transition

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16
Q

What is transmittance

A

The fraction of the original light that passes through the sample

If the sample is not absorbing, it’s 100% T (blank)

If the sample is absorbing everything, it’s 0% T (dark blank)

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17
Q

What is absorbance

A

How much the sample is absorbing

If p=P0 the absorbance is 0 since that power out was the same as in

Log scale so if it went from A=1 to 2 that means the sample absorbed 10x more light

A is always less or equal to 2

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18
Q

What is beers law

A

It describes the relation between absorbance and transmittance

Specific to molecule and wavelength

It’s not specific to the instrument/set up (means we can compare across other labs)

b= path length (length of cuvette)
e=molar absorptivity
c=concentration of the sample

If e is small less light absorbed, if big more light

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19
Q

How do you make an absorbance spectrum

A

Since absorbance is diff at diff wavelengths, you measure a range of wave lengths and their absorbances

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20
Q

At low concentrations of a compound was happens to the absorbance

A

We lose some details on the absorbance spectrum graph, it gets blurred

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21
Q

What are the minimun components of a spectrophotometer

A

A light source (should be reliable and give right wavelength)

A monochromator or a filter to select the wavelength (selects a narrow band of wavelengths to pass through from the light source so all the light is absorbed by the sample)

A sample holder (cuvette)

A detector: which converts the light intensity that passes through the sample to a signal that we can read and use

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22
Q

What should you be careful of with a cuvette during spec

A

The material is made of needs to allow the wavelength range that we’re interested in to pass through

It should also be compatible with the solvents/sample you’re using

Not made of glass because glass blocks uv light, need quartz for uv

Cuvette that are exposed to IR wavelengths need to be made off Kbr or Nacl after 84000/cm wavelength

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23
Q

Why do we use different shapes of sample holders for measurements

A

The shape depends of what we want to measure

Longer or shorter path lengths for more or less dilute solutions

Microcells for if the sample is small amount

Flow cuvette for watching the reactions inside it progress without cooling out the samples

Thermal for if you want to keep the temp controlled

24
Q

Gasses a dilute and have low absorbances, what can we do to increase the signal for gaseous samples

A

Increase path length (m/km, but not actually a km just by reflection)

Make sure light is collimated so it doesn’t get lost in long distance of the cell (b)

Increase the pressure to increase C

25
Q

What are the downside to increasing pressure for a gaseous sample to increased its C

A

It could become more reactive, pressure broadening

26
Q

What are single beam instruments

A

They have only one light path

You measure a blank solution to get P0

Then replace the blank with the analyte solution and get P

27
Q

What are double beam instruments

A

Splits light evenly between the analyte and blank cuvette so you can get p and p0 without moving anything

It reducers the error of moving the cuvette

28
Q

What is the first error that affect absorbance

How do we reduce it

A

Scatter from particles/bubbles in solution which direct light away from the detector and reduces P more that it should be

Absorbances gets increased from true value

Reduced by filtering the samples, using clean solutions, and lids to keep dust out

29
Q

What is the second error that affect absorbance

A

Reflection off the cuvette walls

This reduces P by losing light (can be reduced by using same cuvette for P and P0)

Caused by cuvette position/angle (align the cuvette carefully)

A is increased since P decreases

30
Q

What is the third error that affect absorbance

A

Stray light can enter if the lid isn’t closed

The extra light from the outside will be included into the P value.

A will be decreasing since P is increased and we can tell where light came from or went

31
Q

What determine what analyte concentration we use to measure absorbances

A

We need an analyte concentration that gives an absorbance between 0.1-1

Outside the range is more error prone

32
Q

What does is mean if absorbance is too low

A

P is very close to P0

Any variation in intensity can cause fluctuations in A

33
Q

What does is mean if absorbance is too high

A

The P is very low which may be due to electronic noise in the detector

This may influence the reading you do (cause uncertainty)

Affects like shadowing can happen (another molecule in solution doesn’t get to absorb any light)

34
Q

What is the slope of the calibration curve for absorbance

The y int

A

eb from beers law

Ideally zero

35
Q

What does e in beta law depend on

A

Analyte and wavelengths

36
Q

How do we correct for path length changes throughout labs when measuring absorbance

A

Subtract A value from blank from all sample A values and itself

This brings the initial absorbace to zero and cancels out the cuvette scattering light and any small A from the solvent

37
Q

Calculation of mass and concentration of unknown using eb and A and linear equation

A

Slide 25

38
Q

What is non radiative decay

A

When the light is absorbed by something the energy from it is dissipated through collisions with other molecules as kinetic (heat) energy

39
Q

What is luminescence

A

After light that has been absorbed, the energy from it is released as a photon from the excited state of the molecule

In this category is phosphorescence and fluorescence

40
Q

What are the characteristics of fluorescence compared to absorbance

A

absorption is 10^-15 seconds (Fast) but fluorescence is slower than this, the exited state lasts for 10^-8 to 10-4 seconds

The emmision of a photon is at a lower energy meaning it comes in at a longer wavelength

Singlet electron transition (electron keeps its original spin)

41
Q

What is stokes shift

A

The emmision of a photon is at a lower energy meaning it comes in at a longer wavelength

Losing energy

Absorption is at low wavelength and fluorescent is at a higher

42
Q

Explain the first step in how a photon travels during fluorescence

A
  1. The photon is absorbed and excites an electron from and electron pair in the molecule from ground state (S0) to excited state (s1)

One electron from the pair stays in the ground state

43
Q

Explain the second step in how a photon travels during fluorescence

A
  1. The electron in the excited state (s1) undergoes vibrational relaxation

it becomes less excited loses energy and moves down a few energy levels (non radiative decay, no light emitted )

44
Q

Explain the third step in how a photon travels during fluorescence

A
  1. Emmision: the electron undergoes radiative decay (emits light) to any vibration level in S0

The energy from this decay is released/emitted as a photon

The electron then relaxes and goes back to ground state and keeps the spin pairing

45
Q

Explain what internal conversion And inter system crossing is

A

Internal: the nonradiative transition of the electron that keeps is original spin

Intersystem: the nonradiative transition of the electron that changes its orginal spin

46
Q

Explain phosphorescence

A

It involves an intersystem crossing to a triplet (T1) excited state (the spin changes)

Long time in the excited state (10^-4 to 10^-2 seconds

Has a stokes shift larger than fluoresces because of the 2 relaxations. Losing more energy so the wavelength is longer

47
Q

Explain the first step in how a photon travels via phosphorescence

A

The electron gets excited, goes to s1

then does nonradiative decay to T1 while changing its spin

This is how it does intersystem crossing

48
Q

Explain the second step in how a photon travels via phosphorescence

A

After the intersystem crossing, the flipped electron does another non radiative relaxation to a lower level in T1

Is stays there for longer because the electron now has the same spin as the other one so it won’t go to the ground state as readily

49
Q

Explain the third step in how a photon travels via phosphorescence

A

The electron that has just relaxed in T1 (triplet excited state) relaxes radiatively

This releases the photon after a longer waiting time

The electron converts its spin again (to singlet) and goes back to ground state and relaxes

50
Q

What is chemiluminescenes

What things use this

A

When we get emmision of photons even through no light has hit the sample

A energy of a chemical reaction produces the excited state electron and the light that is emitting

Luminol and glow sticks

The amount of light out give the concentration

51
Q

What is internal conversion

External

A

Conversion to a lower energy electron within the same molecule

Conversion to a lower energy electron by losing energy to other molecules

52
Q

Slide 31 comparison of absorption and emmision

A

Ok look at it actually

53
Q

What is included in a flourecense spectra

A

Both excitation and and emission spectra

54
Q

What is an excitation spectrum

A

It’s made by measuring a varied range of excitation wavelengths while keeping a the emmited light at a consant wavelength

55
Q

Slide 32

A

Achem 315 (13 decks)

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