Horrocks L6 Flashcards
(16 cards)
What is an activation control reaction
The rate limiting step is the formation of products from the encounter complexes
What is Kec
Kec is the equilibrium constant for formation of encounter complexes from the reactants
Kec = [AB]/[A][B]
How do we calculate the standard gibbs energy of formation of encounter complexes
∆G ⊖ = −RT lnKec
What is the rate equation for activation controlled reactions
Rate = kr kd/k-d [A][B]
The ratio kd/k-d = Kec
What does kr describe
The rate constant kr describes the rate at which encounter complexes form products
what is transition state theory
A chemical reaction proceeds through a high-energy, unstable arrangement of atoms called the transition state. Reactants must acquire enough energy to reach this transition state before they can be converted into products.
What are the key assumptions in transition state theory
Equilibrium exists between reactants and the transition state.
The transition state is at the energy maximum along the reaction pathway. Once the transition state is reached, it quickly proceeds to form products.
What does activation entropies depend on
molecularity
Steric factors/orientation
Solvation
How does molecularity contribute to activation entropies
two molecules making one transition sate contribute negatively
How does steric factors contribute to activation entropies
Steric factors/orientation requirement also contribute negatively
Describe how solvation contributes to activation entropies
solvation may contribute positively or negatively dependsing on whether solvent molecules are released or taken up by the TS
Describe the entropies of activation in the gas phase for bimolecular and unimolecular reactions
typically bimolecular reactions have large negative ∆S‡
Typically unimolecular reactions have a small negative or even positive ∆S‡
Describe the entropy of activation in solution in terms of charge separation
When charge separation increases
The system becomes more ordered because the separated charges interact more strongly with surrounding solvent molecules
This results in a more negative ΔS‡ (lower entropy), since solvation cages restrict motion and increase order.
Describe the entropy of activation in solution in terms of charge neutralisation
When oppositely charged species come together. The solvent becomes less ordered, since there’s less need to stabilize charges individually. This leads to a positive ΔS‡ (increase in entropy), as solvation shells become looser.
Describe the entropies in activation in solution in terms of charge delocalisation
When the charge is spread out (delocalised) over a larger structure in the transition state The structure becomes more stable and less ordered. This often results in a less negative or even positive ΔS‡, because delocalisation reduces the need for tight solvation and increases flexibility.
Describe the volume of activation
Volume of activation (ΔV‡) describes how the rate of a reaction changes with pressure
