Chemistry - Lecture 5 Limiting Reactants - Power Point

In a chemical reaction, the reaction stops as

soon as any of the reactants is totally consumed, leaving the excess reactants as leftovers.

CH4 2 HWe 2O have 2.5 mol

+

2O2 We have 6.0 mol

need 3.0 mol We have Less than needed CH4 is limiting reactant

O2 is in excess

CO2

+

2 H2

+

O2

2 H2O

Suppose you have a mixture of 10 mol H2 and

7 mol O2, which reacts to form water. Since the ratio of H2 and O2 is 2:1, the number

of moles of O2 needed to react with H2 is

Moles O2 = (10 moles H2) (1 mol O2) (2 mol H2) = 5 mol O2

Because 7 mole O2 was available at the start of

the reaction, 7 mol O2 – 5 mol O2 = 2 mol O2 will still be present when all the H2 is consumed. The reactant that is consumed in a reaction is

called the limiting reactant or limiting reagent because it determines, or limits, the amount of product formed. 

There are no restrictions on the starting

amounts of the reactants in any reaction. Many reactions are carried out using an

excess of one reagent. The quantities of reactants consumed and the

quantities of products formed, however, are restricted by the quantity of the limiting reactant.

2 H2 + O 2

2

H2O Initial quantities: mol Change (reaction): +10 mol Final quantities: mol

10 mol

7 mol 0

- 10 mol - 5 mol

0 mol

2 mol

10

PRACTICE EXERCISE:  The most important commercial process for converting N2

from the air into nitrogen-containing compounds is based on the reaction of N2 & H2 to form ammonia (NH3): N2 + 3H2

2 NH3

 How many moles of NH3 can be formed from 3.0 mol of N2, and

6.0 mol of H2?

Consider the following reaction:

2Na3PO4 + 3 Ba3 (PO4) 2

Ba (PO4) 2 +

6NaNO Suppose a solution containing 3.50 g of Na3PO4 is mixed with a solution containing 6.40 g of Ba3 (PO4) can be formed?

HOMEWORK: Page 109 3.69 3.70