In a chemical reaction, the reaction stops as

soon as any of the reactants is totally

consumed, leaving the excess reactants as

leftovers.

CH4 + 2O2 CO2 + 2 H2OWe have

2.5 mol

We have

6.0 mol

need

3.0 mol

We have Less than needed

CH4 is limiting reactant

O2 is in excess

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.

The other reactants are sometimes called

excess reactants or excess reagents.

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 + O2 2 H2O

Initial quantities: 10 mol 7 mol 0 molChange (reaction): - 10 mol - 5 mol

+10 mol

Final quantities: 0 mol 2 mol 10 mol

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?

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