Kinetics Flashcards
Reaction rate is
Always positive
Elementary reaction
Single step reactions
occur atatomic level
where order is = to molecularity
How to decide the order
The relationship between the values
If there’s no increase in rate with conc change it’s 0
If it’s proportional 1
If it like x4 then it’s 2
typical units for liquids/solutions for reaction rate
moldm-3s-1
What are the problems with initial rate experiments
often inaccurate
wasteful - using loads resources to ignore more of the data
old fashion
What happens in a half life experiment
determine the half life from the data by discovering how long it takes the rate to half
the greater the gradient the higher the k value
inform me of the integrated rates laws for first order reactions
the k constant is found out by putting the reaction into the form the reaction = k x concentration of reactant to the power of the order of that reaction = d[A]/dt.
d[A]/dt is negative if the [A] is a reactant.
this is then rearranged to move all the [A] relating factors together
then intergrated
Try to put this into a linearized form
first order reaction integrated laws put in nicer form
[A]t = [A]0e to the power of -kt
this is because for 1st order [A] follows an exponential decay
how can the k constat be found using the half life method
[A]t = [A]0 e^-kt
rearranged to form k = ln(2)/t (at time 1/2)
or t at half life point = ln(2)/k
rate law equation for second rate reactions
-d[A]/dt = 2k[A]^2
linearised integrated form of 2nd rate reaction
1/[A]t = 2kt + 1/[A]0
indicates second order as gradient is timed by 2
what is a common problem with second order reaction
in cases where there are 2 reactants if [A] is not equal [B] difficult to interpret
to conteract this keep one reactants concentration constant by using a large excess of the unwanted reactant so its effectively constant
this makes its pseudo first order
therefore in the rate equation you can equate that reagent to Keff in which its K x the conc of that reagent
After obtaining the Keff value that then be then divided by the conc of that reactant to get the k value
what are typical units for a second rate reaction
dm3mol-1s-1
2nd order half life equation
t1/2 = 1/2k[A]0
0th order half life equation
t1/2 = [A]0/2k
what is a lifetime in kinetics
time to reach 1/e of initial rate
lifetime equation of 1st order
t = 1/k
lifetime equation of 2nd order
r = e-1 /2k[A]0
lifetime equation of 0th order
t = [A]0/k x e-1/e
what is the Arrhenius equation
k = Ae to power of -Ea/RT
linearised form of arrhenius equation
lnK = lnA - Ea/RT
where the gradient is -Ea/R
How do you know if a plot is arrhenius like
if the plot of lnK vs 1/T is linear
How to handle complex reactions
isolate them and basically complete pseudo first order reactions separately for each variable
what is the portor’s flash photolysis experiment used for
studying of fast reactions
1. premix excess reactant (B) and precursor (A2)
2. radicals (A radical ) genrated in flash
