Experimental Techniques Flashcards
What are the general requirements for a reaction?
Initiation- how will you start the reaction?
Timing- how will you measure time?
Concentration- how will you measure the concentration?
If initiation is slow, what is required?
Reactants need to be well mixed
You need a mixing process that agrees with the timescale of the experiment
Describe briefly the features of a continuous flow: flow tube reactor
Fast initiation reactions
Reactants are continuously squirted into mixing chamber
[products] will increase as you go down the flow tunnel
Distance related to time by flow speed- concert distance to time and plot T vs [C]
Outline the features of flash photolysis
Fast initiation reaction
- Put precursor (H2O2) in first and mix before reaction
1) mix CH4 +H2O2
2) initiate the reaction with a flash of light- precursor doesn’t react until initiated
3) H2O2 is photolysis to OH
4) OH and CH4 react
What do you need to consider time wise in a slow and fast reaction?
For a slow reaction you can use a manual stop watch
For faster rates you need specialised clock
Time doesn’t need to be accurate but needs to be precise
What do you need to consider with concentration?
Don’t need absolute concentration just something proportional
There are different methods- physical, analytical or optical
How can you measure the change in concentration with physical methods- gases?
[C]= n/v = p/RT
How can you measure the change in concentration with physical methods- pressure?
As the moles increase, the pressure increases
As the pressure increases, the height (volume) of the mercury drops
System kept at constant temperature and volume
How can you measure the change in concentration with physical methods- liquid volume?
The height of the liquid becomes higher as the density decrease (mass stays the same but volume will change hence density changes)
Constant pressure and temperature
How can you measure the change in concentration with physical methods- conductance?
The left hand side= not ionic
Right hand side= ionic
You measure the change in conductance
[A] is determined from change in electrical conductivity (conductance increases)
What are analytical methods?
These methods involve removing samples during a reaction and measuring the concentration of reactants and products
- titrations
- chromatography
- mass spectroscopy
What is a disadvantage of analytical methods?
It is usually a destructive as it uses up some of the compound as a sample is required in the analysis
Ok if volume is large
Bad if volume is small
Outline chemical analytic methods
Titration
1) take sample of reaction every 5 mins
2) quench the reaction
3) tritrate
4) repeat over 30 mins
Outline analytical methods including liquids
Longtime scale means gas chromography can be used
Take samples every 30 mins
Outline analytical methods: smog chamber
See how one pollutants react
Lots of different reactions
What are advantages of optical methods?
Non destructive
Fast- measure photons quickly
Cheap
Robust
What are the two types of optical methods?
Beer lambert law based approaches
Fluorescence based approaches
Describe the beer lambert methods: monochromatic light
Reactant concentrations are determined from light intensity
- absorbs a particular wavelength
A= log I/Io = E c l
Describe the beer lambert methods: broadband light
Different molecules absorb in different regions
Irradiate sample with broad band light
Detect range of wavelength simultaneously
Outline the fluorescence method
Excite a molecule and monitor light emitted to measure concentration
Bond breaking
Collision
Emitting
- see how many photons are remitted by A* to measure concentration
How can you measure a super fast reaction?
Pulse probe experiments Femento second laser pulse= activated species Prove pulse (after 1st pulse) measure the concentration of the activated species
