9.3 Notes

Learning Targets Given the quantities of reactants in a

chemical reaction, determine the limiting and excess reactants

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?

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

Limiting vs. Excess Reagents In this scenario, hydrogen would be

considered the limiting reagentThe limiting reagent limits the amount of

product that can be formed in a reactionThe reaction occurs until the limiting reagent

is used up

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

reagent

Sample Problems 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

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

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

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