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Limiting Reagents

Determining which reactant runs out first

Video tutorial
A chemical equation describes what happens when a chemical reaction takes place. It uses chemical formulae to tell us what happens when the reactants (left) react to form the products.

When glucose is burnt in the open air, there is enough oxygen in the air to ensure that all of the glucose reacts. The oxygen is said to be in excess meaning that there is more than enough of it. The reaction stops when the glucose runs out. When this happens, there is still oxygen left. Glucose is the limiting reactant as the amount of water and carbon dioxide is controlled by how much glucose there is to start with.

If we mix the exact number of moles of each reactant that is needed, they are said to be stoichiometric amounts.

In the glucose example, it was easy to spot that glucose is the limiting reactant since the amount of oxygen in the air is effectively unlimited. In other cases, it may not be so obvious. If there is still some reactant left at the end of a reaction then it is present in excess. In many reactions, such as when there are many reactants or they are colourless gases or solutions, determining the limiting reagent is much more difficult. We then need to work out the number of moles of each reactant and write down the balanced chemical equation:
The limiting reactant is the one that produces the least amount of product.

When working out limiting reagents, moles rather than masses must be used.
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