105 Lecture 1 – Flashcards

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question
what is spectroscopy?
answer
the study of the interaction of matter with electromagnetic radiation
question
what is infra-red spectoscopy used to identify?
answer
functional groups
question
what is Nuclear Magnetic Resonance Spectroscopy used to identify?
answer
the CH framework
question
what is ultra-violet and Visible Spectroscopy used for?
answer
to identify the ? system
question
what is mass spectroscopy used to identify?
answer
the mass and formula
question
what is electromagnetic radiation?
answer
it is light that is composed of oscillating electric and magnetic fields that transmit energy through space
question
in relation to the direction of propagation which axis would you find the electric and the magnetic field?
answer

electric field on Y axis

 

magnetic field on X axis

question
why cannot light always be modelled as simply a wave?
answer

because of wave partical duality


thus, we often need a model of light as a corpuscular particle

question
what are particles of electromagnetic radiation called?
answer

photons

 

the energy (E) of a photon is related to the wave properties of 

  • wavelength
  • frequency
  • wavenumber
question
the distance between succesive crests is known as the.....
answer

wavelength

 

 

symbol give : lamba

 

measured in metres

question
waves per second is known as......
answer

frequency

 

symbol given: V

measured in cycles per second = Hz

question
waves per metre as a unit length is known as.....
answer

wavenumber

 

symbol given: V with a line on top.

measured in metres-1

question

2.998 x 108 m-1

is which constant?

answer

the speed of light baby!

 

it is given the symbol C

question

6.63 x 10-34 Js

is which constant?

answer

is it plancks constant

 

 

it is given the symbol 'h'

question
if you were given the frequency, how would you work out the energy of a photon?
answer

E = hv

 

E = plancks constant x frequency

question
if you were given the wavelength how would you work out the energy of a photon?
answer

E = hc/?

 

E= plancks x speed of light / wavelength

question
if you were given the wavenumber, how would you work out the energy of a photon?
answer

E = hcv-

 

E= plancks x speed of light x wavenumber

question
how to work out frequency ?
answer

v = c/?

 

frequency = speed of light / wavelength

question
how to work out wavenumber?
answer

v- = 1/?

 

wavenumber = 1/wavelength

question

photons from different regions of the spectrum have very different energies - their wavelengths and frequencies vary according.

 

name the electro magnetic spectrum from highest energy to the lowest energy (7)

answer

gamma rays, xrays, UV, visible, Infra-red, microwave, radiowave

 

low energy radiowaves have longer wavelengths that high energy gamma rays

question
what wavelengths does visible light have?
answer

400-700nm

 

 

red light has a longer wavelength and lower energy than blue light

question

103 Hz is how many kHz?

106 Hz is how many MHz?

109 Hz is how many GHz?

answer

103 Hz is 1 kHz

106 Hz is 1 MHz?

109 Hz is 1 GHz?

question
when light of a particular wavelength passes into a substance what are the two possible outcomes?
answer

1. no photons are absorbed - the substance transmits light of that wavelength

 

2. some or all of the photons are absorbed - molecules in the sample take up energy

question
if absorbtion occurs, what happens to the energy?
answer

it is always conserved

 

 

hence when a photon is absorbed the molecule undergoes an energy change that is equal to the energy of the photon.

question

true or false?

a molecule can be partially absorbed by a photon?

answer

False

 

a molecule CANNOT be partially absorbed by a photon; It s an all or nothing event

question
what must a photons energy be in order for it to be absorbed?
answer

its energy must be  exactly equal the energy difference between the old and the new molecular energy states.

 

 

the same applies to the emission of a photon when a molecule in an excited state moves to a lower energy state

question
name the two types of molecular energy
answer

internal energy

translational energy

question
define internal energy
answer

energy stored within a molecule from the result of its rotation, vibration or electrical distribution


absortion/emission of a photon changes internal energy of a molecule by an equal amount to the energy of the photon

question
define translational energy
answer

this is the motion of the whole molecule through space


translational energy cannot normally couple with electromagnetic radiation. changes in trasnlational energy result only from collisions between molecules

question

true or false?

translational energy can be coupled with electromagnetic radiation?

answer

false

 

translational energy cannot normally couple with electromagnetic radiation. changes in translation energy result from collisions between molecules.

question

name the 3 ways internal energy can be classified according to how it is stored

 

these forms of energy are quantised what does this mean?

answer

rotational energy

vibrational energy

electronic energy

 

quantised means that the molecules can only lose or gain certain 'well defined' amounts of internal energy. it cannot gain or lose arbitary amounts

 

 

question

the approx energy gap between Rotational energy levels is:

 

?E ~10-19 - 10-22 J

 

which region of the electromagnetic spectrum has energies in this range?

answer

Microwave region have energy in this range

 

? 1- 10-3 m

question
when i say rotational energy which region should pop into your head?
answer
microwave region
question

vibrational energy levels ~ 10-19 - 10-22 J

 

which region has energies in this range?

answer

Infra red

 

 

when  say vibrational energy 

you should think infra red

question
electronic energy levels are related to which region of the electromagnetic spectrum?
answer
UV/ visible light photons
question

here is a easy thing to help you remember the molecular energy levels and their relationship to the elctro magnetic spectrum

 

EU

MR

IV

answer

Electronic energy - UV/visible photons

 

Microwave photons - Rotational energy

 

Infra-red photons - Vibrational energy

question
what do the energy seperations between different quantum levels depend on?
answer
they depend on the molecular structure
question
name the 4 common features of absorbtion spectrometers
answer

a radiation source

a sample cell

a dispersion element

a light intensity detector

 

modern spectrometers use a beam splitter thater than a dispersion device and all wavelengths are collected simultaneously. this > speed and sensitivity

question

what does an absorbtion spectrum show?

 

 

answer

how much light has been absorbed by the sample as a function of the wavelength of the light.

 

the amount of light absorbed at each wavelength is usually displayed as absorbance (A)

 

question
what kind of scale is absorbance measured in?
answer

a logarithmic scale

 

no light absorbed? = absorbance 0

90% light absorbed? = absorbance 1

99% light absorbec? = absorbance 2 etc

question

in IR spectra what is commonly used instead of absorbance?

 

what affect does this have on the graph peaks?

answer

% transmittance is used more commonly that absorbance

 

this is the % of incident light passing through the sample

 

the absorbtion peaks are inverted when showing transmission

question

the internal energy states of molecules are .......... in ways that depend on the structure of the molecule

 

........ gives direct information about the energy seperation between these molecular states because a ....... can only be absorbed if it has ..... the right energy to excite the molecule to a higher .......... energy state. Hence ........ information can be deduced from the energies of the photons that a molecule...........

answer

quantized, spectroscopy, photon, exactly, internal, stuctural, absorbs

 

the internal energy states of molecules are quantised in ways that depend on the structure of the molecule. Spectroscopy gives direct information about the energy seperation between these molecular states because a photon can only be absorbed if it has exactly the right energy to excite the molecule to a higher internal energy state. Hence stuctural information can be deduced from the energies of the photons that a molecule absorbs

question
which neigbouing levels are ~ 1000 more closely soaced than vibrational levels?
answer

Rotational levels

 

vibrational levels are ~1000 more closely spaced than electronic levels

question

in order of closely spacedness name the 3 molecular energy levels.

 

Hint (RVE) Riding is Very Energetic

answer
Rotaional ~ 1000 closely spaced than Vibration that is ~ 1000 more closely spaced than electronic levels
question
a consequence of quantixation is that THERMAL ENERGY in molecular substances is mainly distributed at what?
answer

Translational and Rotational energy. At all ordinary temperatures (0-500K) virtually all molecules are in their lowest (or ground) vibrational and electronic state



any molecules excited to higer vibrational or electroni states by photon absorbtion will have short life times.

question

different spectroscopic techniques are the study of different types of molecular transition

 

what do each of these measure?

 

microwave spectroscopy

infra red spectroscopy

UV/visible spectroscopy

answer

microwave spec - rotational transitions

infra red spec - vibrational transitons

uv/visible spec - electronic transitions

question

name the two ways spectra can be recorded.

 

do they give similar or different information?

 

 

which one is much more widely used?

answer

can be recorded as absorbtion or emission

 

they give similar information but absorbtion spectroscopy is much more widely used

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