catalyst

catalyst can be divided into heterogeneous and homogeneous catalyst

heterogeneous catalyst includes which are not in phase with reactants.
these kinds of catalyst offers a surface for the reactants to adsorb or adhere on by forming intermediate bonding. these bonding are formed by overlap of outer electrons of reactants and low lying vacant orbitals (usually d-orbital ) of the catalyst(usually d-block elements or compounds). the formation of the intermediate weakens the bond within the reactant molecules by induction of bonding electrons in the intermediate bonding such that the activation energy of the reaction is reduced. the rate of reaction increases.

eg.1)Pt(s) as catalyst
alkene(g) + H2(g) --> alkane(g)

2)MnO2(s) as catalyst
2H2O2(l) --> 2H2O(l) + O2(g)

homogenous catalyst includes those are in phase with reactants.
these catalyst participates the formation of intermediate with higher energetical stability and more or less the activation energy, which in turn same product and the catalyst once involves the intermediate is regenerated. thereby, an alternative mechanism of reaction with lower activation energy is offered to faster the reaction.

eg. Fe3+(aq) as catalyst
original mechanism:
S2O82-(aq) + 2I-(aq) --> 2SO42-(aq) + I2(aq)
new mechanism:
2I-(aq) + 2Fe3+(aq) --> I2(aq) + 2Fe2+(aq)
2Fe2+ +S2O82- +2I- --> 2SO42-(aq) + I2(aq)

2007-05-02 11:19:06 補充：
catalyst can also be divided into positive and negative catalyst.positive one is to provide an alternative mechanism with lower Ea to enhance the rate of reaction.

2007-05-02 11:19:16 補充：
negative one is to provide an alternative mechanism involving hindrance (eg.steric hindrance)factor to increase the Ea of a reaction such that the rate is reduced.

I think it's copying