Lecture 1 - Reaction Profile And Measurements Of Reaction Rates Flashcards
What factors affect the rate of a reaction?
Concentration - increasing conc increases rate
(More molecules, more likely to have collisions, successful collisions etc)
Temp - high temp increases rate
Molecules gain more energy, move faster, more energy to get over energy barriers etc (RT = energy Jmol-1)
Rate does scale lineary with temo and not always with conc
Reaction profiles - an intro
A + B —> C + D
The sequence of steps A + B undergo on the way to form C + D is known as the reaction pathway
Progress along reaction pathway is followed through a reaction coordinate (x axis)
Reaction pathway has an energy barrier, known as the activation energy. Reactants must overcome this to form the products
As molecules cross the energy barrier, they proceed via a transition state
Reaction may proceed by a number of steps, via. A number of intermediate molecules
Where do transition states and intermediates occur at during a reaction pathway
How to tell if its exo or endo
TS - occur at energy maxima (unstable species that cannot be isolated) they will want to however be at the lowest energy possible
Int. - occur at energy minima ( can be detected and, under certain circumstances, can be isolated)
If products are lower in energy than reactants - exo reaction
How to measure reaction rates: how is reaction rate characterised?
It is the rate of the disappearance of the conc of a reactant, or appearance of a product
For t ( x axis) against [reactant] y axis it is a negative exponential curve (ie goes from all reactants to (almost) no reactants
For [product] against t it is another exp. Curve except it goes from 0 to all product
How can you measure reaction rates? (General answer)
You monitor the conc of a reaction of product as a function of time
Eg gas pressure, pH, conductivity, various spectroscopic methods
Absorbable is another way - it measure the fraction of light absorbed by a sample and absorbance is prop to conc
Reaction rates - stop watch method
Mix reactants in vessel and start a stopwatch
• Measure concentration of product or reactant at some
instant in time
–Continuous monitoring of absorption of light
–Remove a sample for analysis (titration, etc.) (reaction must be slow enough and analysis must be quick for this to be reliable)
• ‘Classic’ way of measuring rates of reaction
Measuring the rate of faster reactions: continuous flow
• Two reagents are mixed and passed along a flow-tube.
• Observe product formation in the flow-tube – concentration is probed at different points along the tube.
• Reaction time depends upon distance from mixing pointand flow-rate.
Plot of [reactant] or [product] as function of time
Wasteful in reagents!
How visualise: 2 syringes which pump into the start of a long tube (where mixing will begin). The distance can give you time as you know flow rate. So where you are on the tube is kind of like a tube frame
A + B —> C
Measuring reaction rates - stopped flow
• Two reagents are mixed and passed via an observation cell into a third ‘receiving’ or ‘stopping’ syringe.
• Reagents flow through until the receiving syringe is full and hits an end-stop, starting the data acquisition period.
• Transmission of light through the observation cell monitored as a function of time. Plot of [reactant] or [product] as function of time (detector will plot Abs (y axis) against t
• Much less wasteful in reagents.
• Fast mixing times can be achieved ~ms.
A + B —> C
Measuring reaction rates - relaxation methods - works for time scales in micro seconds (us) which is x10-6
Diagram on slide 19
– disturb equilibrium and monitor changes in concentration spectrophotometrically
– e.g. T- jump: increase the temperature by passing electrical current through the sample.
(It heats up reactants very quickly and then measure relaxation after)
Measuring reaction rates - flash photolysis
Diagram on slide 22
Consider a chemical reaction brought about by light : a photochemical reaction.
Compound A absorbs UV light and converts into compound B, which is highly coloured.
[B] is determined by measuring its characteristic absorption band (as a function of time) as concentration is prop to absorbance.
Once generated, B decays back to A following a simple rate law.
Lasers can generate flashes of light of <10-15 s !
A —hv—> B hv = photon
B—k—> A
(Lamp)——> {sample}——>(light detector) I(t)
SUMMARY
• Chemical kinetics measures rates of reactions.
• Chemical dynamics leads on to atomic/molecular level
understanding of reaction mechanisms.
->reaction pathway, activation energy, transition state,
intermediates.
• Reaction rates measured by monitoring the change of
concentration of reactant or product over time.
• Need a measurable property dependent on the concentration of either product or reactant.
• Challenge is often to follow faster and faster reactions.
• Stopped flow, relaxation methods and flash photolysis are
useful methods.
