Homogenous Catalysts Flashcards
Homogenous Catalysts
Catalysts that are in the same physical state as the reactants
Usual Form of Homogenous Catalysts
An aqueous catalyst for a reaction between two aqueous solutions
Explain how homogenous catalysts work
Work by combining with the reactants to form an intermediate species which then reacts to form the products and reform the product
Enthalpy Profile for a Homogenously Catalysed Reaction
Has two humps in it, corresponding to the two parts/steps of the reaction
Activation Energy
Ea needed to form the intermediates (and to form the products from the intermediates) is lower than that needed to make the products directly from the reactants
What does homogenous catalyst always do?
Explain
Always re-forms.
So it can carry on catalysing the reaction
Describe why the reaction between S2O8 2- and I- is slow
Explain how this effects the rate of the reaction?
The redox reaction is slow because both ions are negatively charged.
The ions repel each other, due to the negative charges.
So the ions are unlikely to collide and react due to the reaction having a high activation energy
Describe Fe 2+ ions effect on the S2O8 2- and I- reaction
Why does this work?
Fe 2+ ions are oxidised to Fe 3+ ions by the S2O8 2- ions.
The newly formed intermediate Fe 3+ ions now easily oxidise the I- ions to iodine, and the catalyst is regenerated.
Because each stage of the reaction involves a positive and negative ion, so there’s no repulsion
Two Equations for the Catalysed Reaction between S2O8 2- and I-
State of all substances
S2O8 2- + 2Fe 2+ –> 2Fe 3+ + 2SO4 2-
2Fe 3+ + 2I- –> I2 + 2Fe 2+
All are aqueous
Autocatalysis Reaction
Where a product of a reaction acts as a catalyst
Explain how an Autocatalysis Reaction Works
As the reaction progresses and the amount of product increases, the reaction speeds up
Describe Mn 2+ ions and their involvement in an autocatalysis reaction
Explain
Mn 2+ is a homogenous catalyst for the reaction between C2O4 2- and MnO4-.
Mn 2+ catalyses the reaction by first reacting with MnO4- to form Mn3+ ions.
The newly formed Mn3+ ions then react with C2O4 2- to form carbon dioxide and re-form the Mn2+ catalyst ions
Two Equations for the autocatalysis reaction involving Mn2+ ions
MnO4- + 4Mn 2+ + 8H+ –> 5Mn 3+ + 4H2O
2Mn3+ + C2O4 2- –> 2Mn2+ + 2CO2
