Catalysis (AQA A-Level Chemistry): Revision Notes
6.2.7 Catalysis
Catalysis by Transition Metals
Transition metals and their compounds are highly effective catalysts due to their variable oxidation states, which allow them to easily gain or lose electrons. This enables them to catalyse redox reactions either as heterogeneous or homogeneous catalysts.
Heterogeneous Catalysts
In heterogeneous catalysis, the catalyst is in a different phase from the reactants. The reaction takes place on the catalyst's surface at specific active sites.
Mechanism of Action:
- Adsorption: Reactants are adsorbed onto the catalyst's surface.
- Bond Weakening: Bonds within the reactant molecules weaken, facilitating the reaction.
- Favourable Orientation: Reactants are held in an optimal orientation for the reaction.
- Desorption: Products are released, freeing up active sites. Support Medium: To increase efficiency, catalysts are often spread over a support medium, maximising surface area and reducing costs.
Example: The Contact Process
Catalyst: Vanadium(V) oxide ()
Reaction:
Mechanism:
Step 1:
Step 2:
In the Contact Process, reduces to and is then regenerated, facilitating continuous conversion of to
Catalyst Poisoning
Impurities in reactants can poison a heterogeneous catalyst by blocking its active sites. This reduces the catalyst's efficiency and increases replacement costs.
Homogeneous Catalysts
A homogeneous catalyst is in the same phase as the reactants, usually in solution. These catalysts work by forming an intermediate species that reacts to form the products and regenerates the catalyst.
- Intermediate Formation: The catalyst reacts with one reactant to form an intermediate.
- Product Formation: The intermediate then reacts with another reactant to form the product and regenerates the catalyst.
Example: Catalysis by Ions
Reaction:
This reaction is slow due to the repulsion between the negatively charged ions.
Adding ions accelerates the reaction as follows:
Step 1:
Step 2:
Explanation:
and facilitate electron transfer between and , speeding up the reaction.
Autocatalysis: Self-Catalysis by a Product
In autocatalysis, one of the reaction products acts as a catalyst.
Example: Autocatalysis
Reaction:
Initially slow due to repulsion between and , the reaction speeds up as ions are produced:
Step 1:
Step 2:
Explanation:
The product catalyses its own formation by accelerating the reduction of and oxidation of
This effect demonstrates autocatalysis, where the reaction rate increases as more accumulates, helping to overcome initial ion repulsion.