Developing Metals 2: Catalytic Activity Flashcards
Definition of catalyst
- Substance which increases the rate of a chemical reaction without undergoing any permanent chemical change its self
- lowers activation energy and provides an alternative route/pathway
Definition of heterogeneous catalyst
Catalyst is in a different physical state to the reactants
eg. Solid catalyst with the reactants in gas liquid or aqueous phase
Definition of homogeneous catalyst
Catalyst is in the same physical state to the reactants
eg. All in aqueous solution
Explain how transition metals can act as heterogeneous catalysts ( 3 marker )
- Transition metals can use d and s electrons
- To form weak bonds with reactants
- Providing a rate of lower activation energy
What property of transition metals allows them to act as a homogeneous catalysts
- ability to have more than one oxidation state
-act as an intermediary in a redox reaction
speed up reactions involving transfer of electrons i.e. redox reactions
Explain how a transition metal is able to increase the rate of a redox reaction ( I.e. how acts as a homogeneous catalyst ) ( 4 marker )
- Transition metal ion reacts with one of the reactants ( transition metal reacts to form an intermediate )
- Oxidation state of the transition metal ion changes
- New iron (intermediate )then reacts to reform ( regenerate ) original transition metal ion
- Activation energy for this reaction is lower compared to if the reaction were to happen without the transition metal ion
Give examples of day-to-day situations that involve transition metals acting as heterogeneous catalysts
- Harber process = iron is catalyst
2. Car exhausts = platinum , rhodium & palladium catalysts
What property allows transition metals to act as heterogeneous catalysts
Unfilled d orbitals allows them to accept electrons
Describe the process of heterogeneous catalysis in terms of transition metals ( 4 + 2 internal steps in step 2 )
- gases adsorbed onto surface of transition metal
- Transition metal ions form weak bonds with reactants
= reactants in closer proximity when adsorbed =increase concentration of reactants = increase rate of reaction
= Bonds that form onto the catalyst may withdraw electron density from the molecules internal bonds= weakening them
- Products deadsorbed from catalyst surface
- Leading active sites available for more gasses ( reactants )
Why is the strength of adsorption essential
If bonds to strong
= products remain fixed to catalyst
=prevent further catalysis
= catalyst poisoning
In what other AS topic does homogeneous catalysis occur
Ozone = Cl• = catalyst O3 depletion = reactants and catalysts all in gas state = homogeneous
Cl• = regenerated = catalyst
In O3 + Cl• —> ClO• + O2
ClO• + O ——> Cl• + O2
= O3 + O —> 2 O2