Sciencemadness Discussion Board

How much is excess?

mdahan - 11-6-2013 at 03:45

I always wondered how much to add into the solution when excess is considered. I know more pushes the reaction to equilibruim but there must be an amount you can put or use that would make you not use that much of the excess as well as pushing the reaction to its maximum potential.

Hockeydemon - 11-6-2013 at 04:34

Quote: Originally posted by mdahan  
I always wondered how much to add into the solution when excess is considered. I know more pushes the reaction to equilibruim but there must be an amount you can put or use that would make you not use that much of the excess as well as pushing the reaction to its maximum potential.


The excess component is that which is left over after the other component has been consumed.

X + 2Y --> XY

Y would be in excess & X the limiting reactant. If you continue to add X until it exceeds Y than you have changed which reactant is the limiting reactant, and which is in excess.
3X + 2Y --> 2XY

I'm not sure I understand the second part of your question though? Excess & limiting reactant are just terms to describe which product will be consumed first, and which will be left over. They aren't inherent to any particular reactant in an equation.

Lambda-Eyde - 11-6-2013 at 05:42

Hockeydemon: If I understand his question correctly, I think he's perfectly aware of what an excess is. What I think he wants to know is exactly how much excess is meant when an article states "reagent A was added in excess".

Hockeydemon - 11-6-2013 at 05:49

Quote: Originally posted by Lambda-Eyde  
Hockeydemon: If I understand his question correctly, I think he's perfectly aware of what an excess is. What I think he wants to know is exactly how much excess is meant when an article states "reagent A was added in excess".


That makes a bit more sense haha. But would it not just mean that you have to add more than the stoichiometric amount of the limiting reactant? Outside of that how would you discern any actual quantity?

mdahan - 11-6-2013 at 06:43

Thats correct. I just dont know if it is dependent on the order of the reaction or if its just done off of statistical data.

Hexavalent - 11-6-2013 at 07:41

It is almost invariably decided based on the reaction being performed on the substrates being used; this is usually to accommodate for any material which may be destroyed, used in side reactions, sublimes, boils, etc.. To this extent, the quantity you expect to be lost is replaced initially as the stoichiometric excess.

Nicodem - 11-6-2013 at 08:08

Quote: Originally posted by mdahan  
Thats correct. I just dont know if it is dependent on the order of the reaction or if its just done off of statistical data.

It does not have anything to do with the order of the reaction or statistical data (whatever you mean by that). The "excess" phrase only has a connection to the reaction stoichiometry. Whatever amount is above the stoichiometric value is its excess. For example, a 10% excess of a reactant means that this is present in a 1.10-fold amount when compared to the stoichiometric value. There is nothing special about this terminology. It is as simple as it sounds and if a procedure calls for an excess without defining how much of an excess, it usually only means that the person skilled in the art knows what that excess value is by understanding the context.

A specific example: Let's say that a procedure calls for a precipitation of AgCl from 100 mL of a 0.10 M AgNO3(aq) solution using an excess of a 1.00 M NaCl(aq) solution. In such a context a person skilled in the art would know that he can use anything reasonably more than 10 mL NaCl(aq) to achieve the same goal. Reasonable means that it makes no sense, or is even detrimental, to use 100 mL. Equally, you would know that it is unreasonable to use a substoichiometric volume (like 5 mL). So you would use an excess that would compensate any uncertainty in the initial assay and/or concentration determinations. In this case where the concentration is given to the +/- 0.01 M precession you could even use only 2% excess and still be certain that you did use an excess. But usually the precision of volumetric determinations is lower than that (e.g., by using graduated cylinders), so a more reasonable excess would be something like 10-20% (1-2 mL).

amazingchemistry - 11-6-2013 at 21:13

This only addresses part of the question as I understand it. If I'm correct in interpreting the question, OP is also asking how much beyond the stoichiometric amount has to be added to things like reversible reactions, where we know that an excess of reactant will drive the reaction forward, before a point of diminishing returns is reached (where adding extra reactant doesn't make sense considering the amount of extra product you'd get)

Nicodem - 12-6-2013 at 06:30

Quote: Originally posted by amazingchemistry  
If I'm correct in interpreting the question, OP is also asking how much beyond the stoichiometric amount has to be added to things like reversible reactions, where we know that an excess of reactant will drive the reaction forward, before a point of diminishing returns is reached (where adding extra reactant doesn't make sense considering the amount of extra product you'd get)

If you formulate his question this way, then the question already contains the answer. I can only resume what I already said: A person skilled in the art generally has some expectation on what the limits for a reasonable excess in a specific system are. I might be excessively unspecific, but that's only because the original poster is so unreasonably general.