[rekk334]

I have an understanding of what makes a reaction zeroth order, but I'm unsure about some other concepts...

I've heard that zeroth order reactions don't really exist they only appear to be zeroth order over certain ranges of concentration. Catalyzed zeroth order reactions are reactions in which rate is dependent on amount of catalyst. (Also, could this be referred to as first order with respect to catalyst?)

I've also heard that MOST zero order reactions are catalyzed reactions (I know there are other types of zero order reactions like photochemical reactions , but I'm not very interested in those). Is this true? Does this only refer to catalytic decomposition reactions or are there other types of catalyzed zero order reactions?

Why do reactions only appear to be zeroth order over a certain range of of concentrations? Is it simply because realistically a reaction never goes to completion and rate will eventually become very small at long periods of time? Or is it more in depth than that?

That looks kind of busy so here's a list of the questions I asked in there...

1. Can catalyzed zero order reactions be referred to as first order with respect to catalyst?

2. Are catalytic decomposition reactions zeroth order?

3. Are there other types of catalyzed zeroth order reactions?

4. Are all catalyzed reactions zeroth order?

5. Why is it that zeroth order reactions only appear to be zeroth order over a certain range of concentrations? What is the cause of this? Is it just because as time gets large rate will asymptotically approach zero? or is there a more complex reason for it?



If anyone can clarify these ideas for me I would really appreciate it.

Edit: I know that zeroth order reactions can result from a surface being saturated and I'm thinking that the same concept could be applied to a catalyst.  I really haven't covered much material about catalysts yet so I'm wondering if this idea makes sense.  Why or why not?

Thanks again.

[fledarmus]

A better question might be, why do some reactions appear to be first or second order? Why would concentration matter for reaction rate?

The answer is that these reactions require a collision between two entities in the rate-determining step of the reaction, and the collisions are driven by statistics. The closer together your reacting components are, the more likely they are to collide with the amount of energy and the geometry that will make the reaction proceed.

So when wouldn't a reaction get faster if there were more molecules colliding? Here are some possibilities:

1) The reaction is occurring within a single molecule. Intramolecular reaction rates do not depend on the concentration of the reactant. If both components of the reaction are attached to the same chain, they are already as close together as they are going to get. Increasing the concentration only increases the chance that you will get an intermolecular reaction instead of an intramolecular reaction. Decompositions, cyclizations, and other types of intramolecular reactions fit into this category.

2) The reaction rate is dependent on something which is external to the molecule, but which is not coming from another molecule. This is where your photochemical reactions come in - if the rate determining step is the absorption of a photon of light to generate an activated reactant (usually a radical), then the only thing that is really going to increase the rate of the reaction is adding more light. Light isn't usually considered as a component in your rate constant.

3) The two components of your reaction are being held in a fixed geometry and close proximity by some third molecule. This is where the catalytic reactions come in. The catalytic species, for example, platinum in a catalytic hydrogenation, will absorb hydrogen and pre-stretch the H-H bond so that it is already weakened. Then it also absorbs your alkene, and the hydrogens are fixed in the appropriate positions to add across the double bond. Enzymes work the same way - the hold the reagents in a perfect geometry to complete the reaction. Adding more of either reagent doesn't increase the rate of the reaction because the reaction rate isn't being determined by random collisions - it is being determined by how well the catalyst can absorb the two reagents and bring them together.

If you think about zeroth order reactions in this way, does it help you to understand your list of questions?

[rekk334]

Thanks for the response.

That definitely gave me a better grasp on reaction order and reactions (especially that first statement), but I still don't fully understand the answers to some of my zeroth order questions.  Could I also get a direct response (and maybe a short explanation) to each question?  I think that would help solidify it for me.  Thanks.

[fledarmus]

The problem is that typical reaction kinetics are solution kinetics - your underlying assumption is that the molecules are uniformly divided in the solution. Catalyst kinetics, especially solid phase catalysts, have almost nothing to do with the solution. Rates of catalytic reactions are determined by the surface area or number of active sites available on the catalyst and the on/off rates or absorption/desorption rates of the compounds. In most cases, the kinetics are a lot more complicated than simply first order, second order, or zeroth order and have a lot more to do with surface chemistry than with solution chemistry.

[rekk334]

oops mistype