Moles, excess and limiting reagents - Stoichiometry

Moles, excess and limiting reagents


• For a chemical reaction to happen, all of the reactants must be present and available to react.

• Chemical reactions proceed until one of the chemical reactants runs out. When one of these chemicals runs out, the reaction stops and no more products can be made.

• The chemical that you have the least amount of, or that runs out first is called the limiting reagent because it limits the reaction from happening any longer.

• All the other reagents (reactants) involved that are not limiting reagents are excess reagents, or are “in excess”. We call it this because when the reaction stops there will still be some of these reagents left over, unreacted and unable to react.

• To find out the limiting reagent, you need to find the amount of product that can be made, with respect to each reactant involved. The reactant that would produce the smallest amount of product is the limiting reagent.

• To find the mass of excess reagent, find the amount of the excess reagent that reacts based on the amount of limiting reagent. Then, subtract that from the total amount of excess reagent available.

• Knowing your limiting reagent is important because a limit on the amount of reagents available puts a limit on the amount of products formed too!

In this lesson, we will learn:
• To fully understand the language used to describe chemical reactions.
• To identify by calculation the limiting reagents in a given chemical reaction.
• To calculate quantities of excess reagents.
  • Intro Lesson
  • 1.
    Study the reaction: 2H2+_2 + O2_2 →2H2_2 O
  • 2.
    Study the reaction: 2C6_6H14+_{14} + 19O2_2 →12CO2+_2 + 14H2_2O
  • 3.
    500 g of Fe2_2O3_3 is reacted with 750 g of C in the reaction: 2Fe2_2O3(s)+_{3\;(s)} + 3C(s)_{\;(s)} →4Fe(s)+_{\;(s)} + 3CO2(g)_{2\;(g)}
  • 4.
    45 g of Ca3_3(PO4_4)2_2 is reacted with 36 g C and 85 g SiO2_2 according to the reaction:
    2Ca3_3(PO4_4)2+_2 + 6SiO2+_2 + 10C→P4+_4 + 6CaSiO3+_3 + 10CO
Teacher pug

Moles, excess and limiting reagents

Don't just watch, practice makes perfect.

We have over NaN practice questions in Chemistry for you to master.