A couple kinetics questions

[apoptosis123]

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Say you have the mechanism for a reaction, and the slowest step isn't the first step. When you write the rate equation for the slow step, you have an intermediate in it, maybe like this: rate=k[intermediate][reactant]

...don't you have to do the substitution thing where you assume the previous step is in equilibrium so that you can have a rate law that doesn't include an intermediate? One of my content review books doesn't even mention it and has the correct answer to a few questions including an intermediate in a rate equation.

Second question: Can't catalysts increase the frequency of collisions between molecules? One of my books says they don't. I know enzymes do this (in addition to other ways for lowering the Ea), but I thought metal catalysts do this as well?

Thanks

 

[vsl5]

Say you have the mechanism for a reaction, and the slowest step isn't the first step. When you write the rate equation for the slow step, you have an intermediate in it, maybe like this: rate=k[intermediate][reactant]

...don't you have to do the substitution thing where you assume the previous step is in equilibrium so that you can have a rate law that doesn't include an intermediate? One of my content review books doesn't even mention it and has the correct answer to a few questions including an intermediate in a rate equation.

Second question: Can't catalysts increase the frequency of collisions between molecules? One of my books says they don't. I know enzymes do this (in addition to other ways for lowering the Ea), but I thought metal catalysts do this as well?

Thanks

1) Yes normally you don't write it in terms of an intermediate but I wouldn't say its wrong, but if you see 2 answers and one has the same rate law but in terms of the equilibrium of the step forming the intermediate, choose that over just having the intermediate there

2) I think this depends on the catalyst, most of them increase the steric factor so that collisions that do occur have the proper collision geometry to cause a reaction, but I think the number of collisions is dependent on temperature and pressure only, at least in terms of total collisions. For enzymes, if an active site "traps" molecules in a small space, I think thats how they get the collision frequency of specific molecules to increase (but not all of them).