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Catalysis

I understand that, because of their variable oxidation states, transition metals are good catalysts, but why does having variable oxidation states make them good catalysts?

Reply 1

Beverooni

Catalysts remain unchanged after a chemical reaction and lower the activation energy needed for the reaction. This must involve low levels of energy otherwise the system would need to recover the energy spent in electron movement. Therefore, if the transitional state is of a low energy then the recovery of this energy is minimal and does not disrupt the system. If you think about the d & s orbitals and the energy difference (3d < 4s) there is very little energy difference between them. This minimal difference allows reactions to become more spontanious. Also, there are 5 x d orbs (all redundant ie same energy) so again - movement and/or excitement of electrons is minimal. Hybridisation of orbitals id covered in EdExcel but not other boards. This may help you understand 'energy' and the conservation/consumption of it: http://en.wikipedia.org/wiki/Orbital_hybridisation

this is a diagram of 1st ionzation energies - the 5 x d orbs produce a really flat curve with very little energy difference between them. Hope this helps!! :smile:

first ionization energy graph.png18.0KB

Reply 2

username1445490

Imagine two reacting species which are both negatively charged and they are going to react in a redox reaction.
eg A + B --> C + D
Because the have the same charge and they repel each other they need a lot of energy to collide and so this reaction has a high activation energy.
However if we used say Fe2+ as a catalyst then particle A can react with the Fe2+ in a redox reaction and form C + Fe3+. Then B reacts with the Fe3+ and reduces it back to Fe2+ and forms D.

The transition metal is acting as a go between giving an alternative path which has a lower activation energy. One of the species oxidises the transition metal ion and the other species reduces it back to what it was at the start so the transition metal ion is not permanently changed.