lecture 4 - chemical kinetics Flashcards
arrhenius equation
k = A exp(- Ea / RT )
heating a reaction causes it to
go faster
increases its rate
temp has an exponential effect on k
what k in arrhenius equation
rate constant - varies with temp
what is A in arrhenius equation
pre exponential factor
same units as k
what is Ea
activation energy
J mol-1
what is R
gas constant
what is T
temperature
in K elvin
what is the arrhenius equation based on
empirical observations
another equation for arrhenius
k= A exp (-Ea/kB T)
Ea now has J units - Ea of an individual molecules, not a mole
activation energy
energy barrier that must be overcome when molecules collide in order for a reaction to occur
Ea units
kJ/mol
kinetics depends on
the Ea
not how exo/endothermic smt is
area under the curve
amount of particles
area under the curve after the Ea point
probability of a particle having the Ea.
shows the amount of particles that have the Ea or more
if temp stays the same but the Ea increases, what happens to the amount of molecules that have the Ea
it decreases
if the temp changes what happens to the graph
it changes
the distribution of energy changes
low temp graph
tall and skinny
high temp graph
wide and short
when temp is increased, what happens to the amount of molecules that have the Ea
it increases
exp (-Ea/RT) is proportional to what part of the curve
area under the curve past the Ea bit
this is where the temp dependence of the rate constant comes from
what 2 things does the pre exponential factor depend on. A
frequency of collisions (determined using collision frequency)
probability of correct geometry of collisions for the reaction to occur
eg: frequency and orientation
probability a collision will occur with the correct geometry is also known at the
steric factor
what value do steric factors usually have
a very small number or 1
what does steric factors having a small value mean/ represent
a very specific geometry is required
what does a steric factor having the value of 1 mean
that any collision will lead to a reaction if it has the Ea required.
orientation/geometry doesnt matter
what do we do to the steric factor and collision frequency to get the pre exponential factor, A
we multiply them together
some reactions have multiple steps and the Ea and A correspond to 1 individual step however for k
may be a combo of individual rate constants ( combo of Ks)
or correspond to the K of the slowest step
ln(k) = ln(A) - Ea/RT
plotting ln(k) against 1/T
gives straight line
gradient = -Ea/R
Ea = gradient x -R
intercept = ln(A)
then calculate A and Ea
in some reactions, Ea changes with
temperature
due to changes in reactant structure + quantum mechanical tunnelling
temp dependent Ea equation
Ea = -R d(ln((k)) / d(1/T)
what does the arrhenius equation define
the rate constant ks temperature dependence
