Kinetics Flashcards
What are the stoichiometric coefficients
Reactants = -ve no. moles
Products = +ve no. moles
What is the rate of reaction equation
Vc = 1/Vi + dni/dt
What is the equation for the rate constant
Vc = k c1^m1 c2^m2 c3^m3 …
What is the molecularity
How many molecules
are involved as reactants
in an elementary reaction step
What is a unimolecular reaction
One molecule reacts
in essential reaction step
What is a bimolecular reaction
Two molecules react
in essential reaction step
What is a trimolecular reaction
Three molecules react
in essential reaction step
What is the integrated rate law for a first order reaction
c(t) = co exp(-kt)
What is the half life of a first order reaction
Time when c(t) = 1/2 co
t1/2 = ln2/k
What is the mean lifetime of a unimolecular reaction
Time when c(t) = 1/e co
𝜏 = 1/k
What is the integrated rate law of a bimolecular reaction if cAo = cBo
1/c(t) = 1/co + kt
What is the integrated rate law of a bimolecular reaction if cAo ≠ cBo
ln(cB(t)/cA(t)) = ln(cBo/cAo) + (cBo - cAo)kt
When is a bimolecular reaction pseudo-first order
If A in great excess
[A] doesn’t noticeably change
When is a trimolecular reaction pseudo-second order
M is an inert collision partner
often [M] doesn’t change noticeably
What are the options for a recombination reaction
1 trimolecular step
Sequence bimolecular steps
When is bimolecular recombination possible
If A* lives long enough
to experience stabilising collision w/ M
Examples of recombination reactions
Bi preferred: polyatomic & radicals
Tri preferred: atoms
A* molecule too short lived
What is the equation for reversible reactions
Vc(t) = -d[A]/dt = ka[A] - kb[B]
What is the integrated rate law for unimolecular reversible reaction
ΔcA(t) = ΔcAo exp(-(ka + kb)t)
What is the relaxation time of a reversible unimolecular reaction
Time where ΔcA = 1/e ΔcAo
𝜏R = 1/ (ka + kb)
How is the thermodynamic equilibrium constant worked out
Equilibrium => Vc = 0
K = ka/kb = [B]eq/[A]eq
What is a zeroth order reaction
Rate of reaction is constant
Vc = k
What is the Beer-Lambert law from spectroscopic detection
I = Io exp(-σcl)
∴ A = σcl
What are the flow methods
Stirring reactor
Flow tube
Stopped flow
What is the disadvantage of stirring reactor
Thorough mixing => time
not suitable for studying fast reactions
How does a flow tube work
Mixture flows with velocity through the tube
reactants mix in tube
What is the equation for a flow tube
l = v t
How does stopped flow work
Two syringes drive reactants into mixing chamber
Mixture passes spectrometer
Stops at stopping syringe
What are the relaxation methods
Shock wave tube
Flash photolysis
What is a relaxation method
Mixture is in equilibrium
sudden perturbation creates deviation
return to equilibrium monitored
How does a shock wave tube work
Mixture contained at room temp in high pressure
separated by membrane (eg. Al foil)
from low pressure compartment
Membrane bursts
mixture travels at shock wave
into low pressure compartment
New equilibrium establishes
concentration measure by light absorption
as wave passes through light beam path
How does flash photolysis work
Homogeneous mixture
Light flash causes reaction to start
Reaction monitored by spectroscopy
What does two consecutive t1/2 values show
Reaction is first order
How can >2 sets of initial rates be used to calculate the reaction order
Plot ln(Vco)/ln[Ao]
reaction order = gradient of slope
What is the Arrhenius equation
ln(k) = ln(A) - Ea/RT
What are the units of A from the Arrhenius equation
Same units as rate constant
What is the role of M in a reaction
Inert collision partener
What is the pre-equilibrium
Activation & deactivation steps
in thermodynamic equilibrium
not disturbed by the reaction step
What is the effect of a fast pre-equilibrium on the rate constant equation
k(x)[A*] = k(x)k[A]
How can you work out the temperature dependance of keff
keff = k(x) exp(-ΔE/RT)
What is the steady state approximation
d[X]/dt ≈ 0
What are the stages of a chain reaction called
Initiation
Chain propagation
Inhibition/retardation
Branching
Termination
What is the role of I⋅
Chain carrier
What is a linear chain reaction
Chain reaction w/out branching
No. chain carriers doesn’t increase
What is a branched chain reaction
Chain reaction w/ branching
No. chain carriers increases
When is the Rice-Herzfeld consistent with stoichiometry
When k1 is negligible
How can the rate constant of a bimolecular reaction be estimated
k = σ vrel
What is the bimolecular rate constant with regards to temperature
k(T) = (8(kb)T/πμ)^1/2
