Horrocks L3 Flashcards
(29 cards)
What is the Arrhenius law
The Arrhenius Law describes the relationship between the rate constant k of a chemical reaction and the temperature T
Describe the Arrhenius law graph
Plot of lnk against 1/T
The gradient of this like is -Ea/R
The intercept on the graph is ln A
Straight line with negative gradient
What do two molecules need when they collide
when two molecules collide they need an energy Ea to overcome the activation barrier
What is the activation energy supplied by in colliding molecules
the activation energy is supplied by the relative relocate of the colliding molecules
1/2mvrel^2 >/= Ea
Describe how many molecules have sufficient kinetic energy
only a small fraction of molecules have enough kinetic energy according the the maxwell Boltzmann distribution
What does the number of collisions per second depend on
Depends on the number density of reactants
The mean relative speed of reactants
The collision cross section σ
What is the collision density of AB denoted by
ZAB
What are the units for collision density
m-3s-1
Define collision density
The collision density is the number of molecular collisions per unit volume per unit time.
What is the basic collision theory expression for the rate
rate = amount of A:B collisions per second x chance of a reactive collision
What equation do we use to estimate the rate of reaction for a bimolecular reaction. Also known as collision theory equation
rate = Pσ √(8KbT/πµ) Na[A][B] x e^(-Ea/RT)
what is P
Steric factor
What can collision theory not predict
the activation energy is not predicted by collision theory
What does collision theory correctly predicted
correctly predicts the form of the temperature dependence of rate constants. The pre-exponentially factor is also predicted by collision theory
Why do we need to use a steric factor
we need to take into account how the geometric requirement affect the rate. The steric factor assumes only a fraction of collisions occur with the correct geometry
Describe the harpoon mechanism in the reaction:
K + Br2 -> KBr + Br
The K atom is on a path to miss the Br2 molecule but manages to transfer an electron as it passes. This forms K+ and Br2- which then attract electrostatically pulling them into a reactive encounter
How do we calculate the change in energy upon electron transfer in the harpoon mechanism
∆E = I - Ea - (e^2/4πϵ0R)
I- ionisation energy
Ea - activation energy
R* - electron transfer distance
How do we calculate the steric factor P
P = σ∗/σ = e^4/(4πϵ0d(I-Ea))^2
σ∗ - reactive cross section
d - collision diameter
What is the problem with unimolecular reactions
Unimolecular gas phase reactions often obey Arrhenius’ Law. We explained Arrhenius’ Law in terms of the requirement to obtain an activation energy by collision. If there’s a collision, why is the reaction not second order?
what is the solution to the problem of unimolecular reactions
partial solution - lindemann mechanism
Explain the lindemann mechanism
explains why some reactions, expected to be unimolecular actually exhibit second-order kinetics in certain conditions. The presence of a third body molecule, which helps activate the reactant, makes the reaction rate depend not just on the concentration of the reactant but also on the concentration of the third body.
What does the higher the value of p (steric factor mean)
a greater fraction of molecular collisions have the correct orientation to lead to a successful reaction
How do we calculate the unimolecular rate constant at energy E
K1(E) = (1- (E/E)) ^(S-1)
E - threshold energy needed to break a bond
S - number of vibrational degrees of freedom
What is a potential energy surface
A Potential Energy Surface is a tool that maps the potential energy of a system as a function of its atomic positions.
