Catalysis Flashcards
What is a catalyst?
A catalyst speeds up a reaction without affecting the equilibrium position
In practice, they can degrade over time
What are some expectations of catalysts?
Stable
Cheap
Not corrosive
Active in low quantities
Recoverable and reusable
Minimal toxic waste
Soften the reaction conditions
Increase yield
High purity
High selectivity
What is the Eyring equation?
k = ((boltzmann constant x T) / h) x exp(-delta G / RT)
How can we plot the Eyring equation in a line where y=mx+c?
We can plot y = ln(k/T) and x = 1/T
What does delta H dagger tell us about in a reaction?
It yields information about the bond strength
What does delta S dagger tell us about in a reaction?
It yields us information about the changes in order
What is the equation for turnover frequency (TOF)?
TOF = number of cycles (turnovers) / number of centres x time
What is the equation for turnover number (TON), and what factors does this take into account?
TON = Moles of product / moles of catalyst
This takes into account deactivation, and reflects the maximum yield attainable from a catalyst
What are the positives of heterogenous catalysts?
Easy to regenerate, recover (filter) and reuse
Thermodynamically and mechanically stable
Often metals or metal oxides
Activity is linked to surface area
What are the negatives of heterogenous catalysts?
Selectivity is often low
Reaction control problems due to diffusion and heat transport
Can degrade over time due to leaching, poisoning, swelling or sintering
Reproducible preparations can be difficult
Hard to control the shape or size of the catalyst at the molecular level
What are the positives of homogeneous catalysts?
Very active leading to high rates
Selective and easily modifiable
Mild working conditions
Often acids, bases or transition metal complexes
Ligand and solvent variations can be used positively
Mechanistic information is easier to obtain
What are the negatives of homogeneous catalysts?
Low thermal stability
Hard to separate
Recovery of catalyst can be costly
What are the positives of enzyme catalysis?
Classified by function
Highly specific
What are the negatives of enzyme catalysis?
Subject to regulation by inhibitors and activators
Catalyse biological transformations
Sensitive to the environment e.g. pH
What is specific acid catalysis?
The hydroxonium ion is the catalyst, with the H+ being fully transferred before the RDS
Rate is proportional to [RH+]