# Moles, excess and limiting reagents

### Moles, excess and limiting reagents

#### Lessons

• 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.
• Introduction
a)
Recap on mole/mass/volume/concentration equations

b)
Assumptions we made in previous videos

c)
Definition of terms in chemical reactions.

d)
Why one reagent is the most important.

• 1.
Discuss
Study the reaction: 2H$_2 +$O$_2$→2H$_2$O
a)
50 g of O$_2$ gas and 50 g of H$_2$ gas were reacted together.
Which reagent is the limiting reagent?

b)
What mass of the other reagent is in excess?

• 2.
Study the reaction: 2C$_6$H$_{14} +$19O$_2$→12CO$_2 +$14H$_2$O
a)
If 120 g of O$_2$ and 150 g of C$_6$H$_{14}$ are reacted together, what is the limiting reagent?

b)
How many grams of the excess reagent are present in excess?

• 3.
500 g of Fe$_2$O$_3$ is reacted with 750 g of C in the reaction: 2Fe$_2$O$_{3\;(s)} +$3C$_{\;(s)}$→4Fe$_{\;(s)} +$3CO$_{2\;(g)}$
a)
What mass of Fe is produced?

b)
What is the limiting reagent in this reaction?

c)
How many extra grams of this reagent are in excess?

• 4.
45 g of Ca$_3$(PO$_4$)$_2$ is reacted with 36 g C and 85 g SiO$_2$ according to the reaction:
2Ca$_3$(PO$_4$)$_2 +$6SiO$_2 +$10C→P$_4 +$6CaSiO$_3 +$10CO
a)
What mass of P$_4$ can be made from these quantities?

b)
What is the limiting reagent?

c)
Find the excess mass of both excess reagents.