Friday, August 11, 2017 0

Limiting reagents is a concept that is easy to understand in one context… but tricky to understand in another. Before I expand on this statement, what do I mean by limiting reagents?

Well, unless you have some super precise mass balances, when carrying out a chemical reaction you will always add too many atoms / molecules / particles of one substance and not enough of the other. The one that you have added too much of is the reagent that is referred to as ‘in excess’ (not to be confused with the 90’s Australian rock band, INXS) and the substance that gets used up is the limiting reagent.

It makes sense that the limiting reagent is the substance that has the least number of particles, which should lead you into thinking that it has the least number of moles.

For example, if carbon is reaction with oxygen as follows:

C + O2 –> CO2

And you have 0.681 mol of C and 0.791 mol of O2, it is quite clear that C is the limiting reagent as there are the least number of moles of it, which O2 is in excess as it has the greatest number of moles.

However, what would happen if you had hydrogen and oxygen reacting to make water:

2H2 + O2 –> 2H2O

If you had 0.488 mol of H2 and 0.366 mol of O2, the O2 would still be in excess. How is this the case I hear you ask? Clearly, there are fewer moles of O2 so it should be the limiting reagent.

The answer lies in the mole ratio. For every one mole of O2, two moles of H2 are needed. Which means that 0.366 mol of O2 could potentially react with 2 x 0.366 mol of H2 (or 0.732 mol of H2). There is not 0.732 mol of H2 but only 0.488 …. Which leads to the conclusion that H2 is the limiting reagent.

By No machine-readable author provided. Miles~commonswiki assumed (based on copyright claims). [Public domain], via Wikimedia Commons

Why is it important to know about this concept? In school experiments, it probably does not matter but in the industrial scale, it does. For example, if you reactants are costly, you don’t want to waste them or one of the reactants may be poisonous or toxic – you don’t want that one left over at the end of the reaction.

How do you tackle the problem of limiting reagents or reagents in excess? Do you have any tips you want to share with regards to determining which reagent is in excess or which is limiting?