Limiting Reactant

In a chemical reaction, reactants interact with each other to form products. The reactant that is completely consumed is the limiting reactant, and the reactant present in a quantity greater than necessary to react completely with the limiting reactant is the excess reactant.

A recipe analogy is helpful to understand these concepts. Here, one cup of flour, 2 eggs, and 3 tablespoons of sugar make five waffles. If there are 3 cups of flour, 4 eggs, and 8 tablespoons of sugar, how many waffles can be made?

There is enough flour to make 15 waffles and enough sugar to make 13 1/3 waffles. However, there are only enough eggs for 10 waffles. Here, egg is the limiting reactant because it makes the least amount of waffles, while the flour and sugar are in excess.

Now, consider the combustion reaction between methane and oxygen to produce carbon dioxide and water. Recall that the coefficients of a balanced equation represent the stoichiometric amounts of the reactants and products.

Therefore, the stoichiometric mole ratio of methane to carbon dioxide is one to one, and that of oxygen to carbon dioxide is two to one.

Suppose there are 80 grams of methane and 128 grams of oxygen. Which is the limiting reactant, and how much carbon dioxide will form? First, the masses of the reactants must be converted to moles using their molar masses.

Stoichiometrically, 5 moles of methane produce 5 moles of carbon dioxide, while 4 moles of oxygen produce only 2 moles of carbon dioxide. Since oxygen produces the least amount of carbon dioxide, it is the limiting reactant, while methane is the excess reactant.

Knowing the limiting reactant, the number of product moles can be converted to grams. Therefore, 88 grams of carbon dioxide can be produced.

But how much methane is unreacted? The mole ratio of methane to oxygen indicates that four moles of oxygen would completely react with 2 moles of methane. So, 3 moles of unreacted methane are in excess.