Collision Theory Flashcards
What assumptions does collision theory make?
- assumes minimum activation energy
- assumes reactant molecules are hard spheres
- molecules must collide with a certain minimum kinetic energy along their line of approach for a reaction to occur (overcome Ea)
k= Ae^-Ea/RT
What is A related to?
What is e^-Ea/RT related to?
A is related to the frequency of collisions
e^-Ea/RT is the fraction of collisions with energy greater than Ea- so essentially successful collisions
Describe what the areas under the curve of a Boltzmann distribution mean
The area under each curve is proportional to the number of collisions
The area under each curve above Ea is proportional to the number of collisions that leads to reactions
Are all collisions equal?
No not all collisions are equal
Molecules will have different energies
There is a steric factor where the molecules have to hit in the correct orientation for a reaction to occur
What is the equation for the pre exponential factor A?
A is the frequency of collisions
A= kcollision x steric factor(P)
What is the equation for the reaction cross centre, sigmaAB?
SigmaAB= {8kb x T/ pi x RM of AB}
{} means square root
What is negative temperature dependence?
A negative temperature dependence means that the overall reaction is slower when the temperature is increased
Can you have a reaction with no temperature dependence?
Yes, this is where the activation energy equals zero
Which reactions generally show a negative temperature dependence?
Termolecular reactions (3 reactants)
This is because it’s a multi step reaction where the temperature sensitivity of the forward and back reactions are different
All reactions are faster but back reaction is faster than forwards
What can the Arrhenius equation be applied to?
The kinetics of a reaction is governed by the rate determining step
The Arrhenius equations applies to the rate constant for this step
How does Arrhenius apply in complex reactions?
The rate constant of several reactions contribute to the overall rate constant
The temperature dependence of the overall rate constant does follow the Arrhenius equation
A plot of lnk vs 1/T gives a straight line with a gradient -Ea/R
Rate constants follow the Arrhenius equation
