Horrocks L5 Flashcards
(16 cards)
What is an encounter complex
An encounter conplex is a short-lived intermediate formed when two reactant molecules come into contact or close proximity in solution often before a chemical reaction occurs
What does an encounter complex represent
it represent the first step in a molecular reaction in solution
What happens after the encounter complex is formed
[AB] is the encounter complex which made either dissociate into free reactants or proceed to form products if the reaction happens quickly enough
What is a diffusion controlled reaction
A reaction is said to be diffusion-controlled when the rate-limiting step is the diffusion of reactants together, not the chemical transformation itself.
When do diffusion controlled reactions happen
occurs when the actual chemical reaction is very fast nearly instantaneous.
The rate of product formation is therefore limited by how fast the reactants can encounter each other through random motion (diffusion).
What is Kd
Kd is the diffusion - controlled rate constant, it is related to the diffusion coefficient of the reactant A and B
Rate = Kd[A][B]
What is the difference between an encounter complex and transition state
No covalent bonds are made/broken in the formation of the encounter
complex. It is not the same as the transition state
What is a solvent cage
A solvent cage is formed by solvent molecules that trap or confine reactants, products, or intermediates in close proximity for a short period of time due to solvation and intermolecular interaction
Describe fick’s first law of diffusion
fick’s first law applies when the concentration of particles does not change with time
J = -D δc/δx
What can we interpret from fick first law of diffusion
Particles diffuse in the direction that reduces concentration gradients. The flux is proportional to the steepness of the gradient
What is fick’s second law of diffusion
If the gradient δc/δx changes at a position x the flux towards x will be different from the flux away from x. The concentration will then change with time
δc/dt = D δ^2c/δx^2
Give examples of diffusion controlled reactions
proton transfer
Recombination reactions
Quenching of excited states
Describe how proton transfer reactions can be diffusion controlled
proton transfer to/from hard atoms (O,N) which is thermodynamically favoured is usually diffusion controlled
Describe how recombination reactions can be diffusion controlled
reactions which do not require any bond breaking may have low activation energies and large rate constants.
How can quenching of excited states be diffusion controlled
physical quenching of excited states is often diffusion controlled
KQ ~ kd
What is the stern volmer equation
I0/I = 1 + TkQ [Q]
I - fluorescence intensity in the presence of quencher Q
I0 - the intensity in the absence of quencher
T - lifetime of the excited state in the absence of the quencher
