1. 9.3 Notes
Limiting reagents

2. Learning Targets
Given the quantities of reactants in a chemical reaction, determine the limiting and excess reactants

3. What is a Limiting Reagent?
The coefficients in balanced chemical equations indicate the amount of each substance available to react
N2(g) + 3H2(g) → 2NH3(g)
In this balanced equation one molecule of N2 reacts with three molecules of H2, producing two molecules of NH3
What would happen if two molecules of N2 reacted with three molecules of H2?

4. 2N2(g) + 3H2(g) → 2NH3(g)
Before the reaction takes place, nitrogen and hydrogen are present in a 2:3 ratio.
One molecule of N2 will react with three molecules of H2, to produce two molecules of NH3
At this point, all of the hydrogen has been used up and the reaction stops
One molecule of unreacted nitrogen will remain

5. Limiting vs. Excess Reagents
In this scenario, hydrogen would be considered the limiting reagent
The limiting reagent limits the amount of product that can be formed in a reaction
The reaction occurs until the limiting reagent is used up

6. In contrast, the reactant that is not completely used up is called the excess reagent
In this example, nitrogen is the excess reagent

7. Sample Problems 2Na +Cl2 → 2NaCl
Sodium chloride can be prepared by the reaction of sodium metal with chlorine gas.
2Na +Cl2 → 2NaCl
Suppose that 6.70 mol Na reacts with 3.20 mol Cl2
What is the limiting reactant?
Cl2
How many moles of NaCl are produced?
6.4 moles NaCl

8. The equation for the complete combustion of ethane (C2H4) is
C2H4 + 3O2 → 2CO2 +2H2O
If 2.70 mols C2H4 is reacted with 6.30 mols O2,
Identify the limiting reagent.
Oxygen (O2)
Calculate the moles of water produced
4.2 moles

9. The properties of copper (I) sulfide are very different from the properties of the elements copper and sulfur
2Cu + S → Cu2S
What is the limiting reagent when 80.0 g Cu reacts with 25.0 g S?
Copper (Cu)
What is the maximum number of grams of Cu2S that can be formed?
100.2 g

10. Hydrogen gas can be produced in the laboratory by the reaction of magnesium metal with hydrochloric acid
Mg + 2HCl → MgCl2 + H2
Identify the limiting reagent when 6.00 g HCl reacts with 5.00 g Mg
HCl
What is the maximum number of grams of H2 that can be formed?
0.16 g HCl