Limiting Reagents and Percent Yield Lecture 1

Applied Physics and Chemistry

Limiting ReagentPicture making sandwiches:

2 pieces bread + 3 slices meat + 1 slice cheese sandwich

What if you had 20 slices of bread, 24 slices of meat and 12 slices of cheese?

How many sandwiches could you make?What would be left over?

Limiting reagentDefinition:

The reactant that runs out first and limits the amount of products that can be formed

How do we know what’s limiting?Stoichiometry!

CalculationsRequires ability to determine moles from

mass and mass from moles!Requires balanced equation!

ExampleSuppose we have 25 kg of nitrogen gas and

5 kg hydrogen gas. We mix them and heat to react and form ammonia. What amount of ammonia can we form?

First: write and balance the equationN2 + H2 NH3

N2 + 3H2 2NH3

Next: Find the moles from grams!25 kg N2 = 25 000 g5 kg H2 = 5000 g

Example continued25 000 g N2 : find moles!

892 moles N2

5 000 g H2 : find moles!2480 moles H2

Next: Determine which reactant is limiting by using mole ratios

892 moles N2 x 3 moles H2 = 2680 moles H2

1 mole N2

Do we have that much H2?

No! H2 is limiting reagent!

N2 is in excess.

Example continuedNext, use the LIMITING REAGENT to find the

amount of product formed.2480 mole H2 x 2 mole NH3 = 1650 mole

NH3

3 mole H2

Then convert moles of NH3 to mass!

1650 mole NH3 x 17.03 g NH3 = 28 100 g NH3

1 mole NH3

Steps!Write and balance the equationConvert masses to molesUse mole ratios to determine which

reactant is limitingUse limiting reagent to determine the

amount of product (moles)Convert from moles of product to mass