Rate Laws And Arrhenius Flashcards
How do thermodynamics and kinetics differ?
Thermo compares free energy of reactants and products, calculating the driving force IF a transformation occurs. Timescale not considered
Kinetics provide information on how a transformation occurs and on what time scale
How do you work out the rate equation?
Has to be found experimentally, NOT given by the stoichiometric coefficients - unless elementary.
Cannot be found by saying the number of molecules and working it out from that
What is the Isolation method?
If a reaction goes A + B —> P
And B is in vast excess, it can be said that [B]t is constant. ([B]o = [B]t)
This rate equation is then said to be pseudo-first order
What is the initial rate method?
Where the initial rate is measured of a reaction, changing the initial concentration of one reactant to see what effect that has on the initial rate.
Complication; the concentrations and rate of reaction all change with time
What is the integrate rate law for a 0th order reaction?
[R] = [R]o - kt
What is the integrate rate law for a 1st order reaction?
[R] = [R]o exp(-kt)
What is the integrated rate law for a 2nd order reaction?
1/[R] - 1/[R]o = kt
How do you establish if a reaction is elementary?
By experimentally determining the reaction mechanism
How do you find the order of an elementary reaction?
By the stoichiometry, this is not contradictory as one needs to do experiments to see if a reaction is elementary or not
Even if the order does match the stoichiometry, doesn’t mean the reaction is necessarily elementary
If the initial rate of a reaction is v = k’[A]o^n, what is the linearised form?
Log v = log k’ + nlog [A]o
Plot log v against log [A]o
The gradient = n
What is the Arrhenius equation?
What does the Ea/RT of the Arrhenius equation signify?
It is the fraction of encounters (A+B) that have sufficient energy to react
How does temperature affect a small Ea?
A small Ea is HIGHLY temperature dependent
What does the pre-exponential factor A consider?
Collision frequency and orientation term
Individual for each reaction
What is the half life of a zeroth order reaction?
t = [R]o / 2k
What is the half life of a first order reaction?
t = ln2 / k
What is the half life of a second order reaction?
t = 1 / k[R]o
What is the x axis when plotting the Arrhenius equation?
x = 1 / T
TIME DEPENDENT
How do you obtain A graphically?
If on computer, it is the intercept.
If on paper, cannot extrapolate to y axis so obtain A from one data point - best to find from multiple points and give the mean value, with standard error, etc.
What is the gradient when the Arrhenius equation is plotted graphically?
m = -Ea/R
What is the rate of removal of R for a zeroth order reaction?
v = - d[R]/dt = k[R]^0 = k
How do you find the integrated rate equation for a zeroth order reaction?
Rewrite v = - d[R]/dt = k[R]^0 = k as d[R] = -kdt
Integrate d[R] = -kdt to get:
[R]t = [R]o - kt
What is the gradient of an integrated zeroth order reaction?
Gradient = -k
As [R]t = [R]o - kt
What is the rate of removal of R for a first order reaction?
v = -d[R]/dt = k[R]^1
