Ellesmere AQA GCSE Chemistry

10.03.05 The Limiting Reactant (HT)

Syllabus

In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used.
The reactant that is completely used up is called the limiting reactant because it limits the amount of products.
Students should be able to explain the effect of a limiting quantity of a reactant on the amount of products it is possible to obtain in terms of amounts in moles or mass

What does this mean?

The Limiting Reactant

Example 1

If we react 10 moles of Carbon with 10 moles of Oxygen we should make 10 moles of CO2

C + O2 --> CO2

But if we react 10 moles of Carbon with 5 moles of Oxygen we'll make 5 moles of CO2

Not all of the Carbon will react because there isn't enough Oxygen.

The Oxygen is the Limiting Reactant - the reaction stops when it runs out.

Example 2

It would take 20 moles of Hydrogen to react with 10 moles of Oxygen and 20 moles of water would be made.

2H2 + O2 --> 2H2O

If we mixed 8 moles of Hydrogen with 6 moles of Oxygen we would make 8 moles of water.

Why?

6 moles of Oxygen needs 12 moles of Hydrogen, but we only have 8 moles of Hydrogen.

The Hydrogen is the Limiting Reactant.

1 mole of Hydrogen makes 1 mole of water.

So the 6 moles that react will make 6 moles of water.

Moles and the limiting Reactant.

Example 1

If we react 10 g of Carbon with 10 g of Oxygen how much CO2 is made?

C + O2 --> CO2

We can't use the ratio in grams - it doesn't work.

So, convert both masses to moles.

Moles (C) = mass/Mr = 10/12 = 0.8333

Moles (O2) = mass/Mr = 10/32 = 0.3125

the ratio is 1:1

So the Oxygen will run out - is the limiting reactant.

0.3125 moles of CO2 will be made.

Mass = Moles x Mr = 0.3125 x 44 = 13.75

Example 2

If we mix 10g of Hydrogen with 10g of Oxygen how much water is made?

2H2 + O2 --> 2H2O

Moles (H2) = mass/Mr = 10/2 = 5

Moles (O2) = mass/Mr = 10/32 = 0.3125

the ratio is 2:1

So, 0.3125 moles of Oxygen only uses 0.615 moles of Hydrogen

So the Oxygen will run out - is the limiting reactant.

0.3125 moles of water will be made.

Mass = Moles x Mr = 0.3125 x 18 = 13.75 = 5.625g

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