Limiting Reactants

…or what runs out first stops the reaction…

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Analogy• Recipe for a ham sandwich: 2 slices of bread,

2 slices of ham, 1 slice of cheese.• How many complete sandwiches can be made

with 4 slices of bread, 5 slices of ham, and 4 slices of cheese?

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Answer:

Limiting ingredient:

2 sandwiches

Excess ingredients:

Bread

Ham and cheese

=

+

+ +

+

=

In a chemical reaction, when one reactant runs out, the reaction stops

• The reactant that runs out first is the Limiting Reactant.

• The reactants that are left over are the Excess Reactants.

http://wps.prenhall.com/wps/media/objects/165/169519/blb9ch0307.html

2H2 (g) + O2 (g) 2H2O(g)

2H2 (g) + O2 (g) 2H2O(g)

10 mol H2 x 1 mol O2 = 5 mol O2 needed

2 mol H2You have 7 mol O2, you need 5 mol O2 , thus, 7 mol O2 – 5 mol O2 = 2 mol O2 left over, so… O2 is the excess reactant, you have 7 mol O2, but you only need 5 mol O2, therefore….

H2 is the limiting reactant

You can start with either reactant (it doesn’t matter). Let’s start with H2 …

2H2 (g) + O2 (g) 2H2O(g)

7 mol O2 x 2 mol H2 = 14 mol H2 needed

1 mol O2

Thus, you have 10 mol H2, not the 14 mol H2 you need, so…

H2 is the limiting reactant

O2 is the excess reactant by default

Or you could start with O2 and get the same result …

The Concept

• You must use the limiting reactant to determine the theoretical yield, NOT the excess reactant.

• Since we usually deal with grams in lab, not moles, we will need to add some additional steps to convert grams to moles…

The MechanicsMost problems start with grams, so:

• Step 1: Convert the grams of each reactant to moles.

• Step 2: Use the moles of either reactant to calculate the moles of the other reactant needed.

• Step 3: Compare what you have to what you need to determine the limiting reactant and excess reactant.

• Step 4: Complete the stoichiometry calculations using the limiting reactant.

• Optional: You can also detemine how much of the excess reactant is used up with a mole ratio and subtract from what you have to get the left over reactant.

It’s not as difficult as it sounds. Let’s try one together:

The most important commercial process for converting N2 and H2 to ammonia, NH3, is the Haber Process:

N2 (g) + 3H2 (g) 2 NH3(g)

What mass of NH3 can be formed from 12.0 g H2 and 84.0 g N2?

Ok, now you try:

A strip of zinc metal weighing 2.00 g is placed in an aqueous solution containing 2.50 g of silver nitrate.(a) Which reactant is limiting? (b) How many grams of Ag will form? (c) How many grams of the excess reactant will be left at the end of the reaction?