[qw098]

Hi guys,

I have a small question regarding reaction intermediates/writing the overall equation which I don't understand. Let's say I have the hypothetical equation shown below:

A+B<--> 2X   (In equilibrium)
X-->C           (Slow)

If I were to write the overall reaction of this reaction, my professor said I would write:

A+B-->2C   Because my professor said the stoichiometric number of step two would be two so I would get rid of the intermediate.

I however wrote the overall equation to be:
A+B--> X+C   I argued... if you form two molecules of a species in the first step, why can one of them not react with another species to form a product, while the other one stays as a product. My professor didn't explain himself at all and it made no sense why that couldn't happen.

I then told him, let's say I have the following mechanism:

A+B-->X
2X--->C

The first step in the mechanism would have to be done twice because my reactant is 2X in the second step, so I would have to form two products in step one so the overall equation would then be: 2A+2B-->C

I guess my question is, why can I not form two molecules of a species in my first step, then one of the those molecules act as an intermediate to form another product, and the other molecule stay as a product of the reaction like my proposed mechanism of:

A+B<--> 2X   (In equilibrium)
X-->C           (Slow)

With me saying the overall reaction is A+B-->X+C

Any help would be GREATLY appreciated! Thanks!

[Caustikola]

A+B<--->2X
X---->C
is your original equation; and i'm assuming the equilibrium of this system lies to the far right. Now, the overall equation can't be
A+B---->X+C as you speculated because
1. X could be activated complex which is itself unstable and can either decompose back to A & B or favorably, to C. This specie in a real sense may not even last up to 10^-15 seconds... so it can be ridiculous to include it in the equation of reaction
2. Even if X isn't an activated complex; the fact that  X---->C  suggests that the conversion of X to C is practically complete.
X<---->C is not the same as X---->C as you know, so going by your equations
I'd say
X--->C for the reaction to be complete;
2X--->C  presents a clearer view so that the overall equation will be
A+B--->C
Intermediate X has been successfully eliminated from the equation as you can see. An intermediate cannot be part of the overall equation or else it isnt an intermediate