Limiting Reactant

A limiting reactant is a substance that stoichiometrically limits the amount of product(s) that can be formed.

In chemical reactions, if the reactants are present in amounts that will be used up entirely in the reaction, it is said that stoichiometric amounts of reactants are present. However, it is often the case that stochiometric amounts of reactants are not present. In this scenario, one of the reactants will “run out” before the other. This reactant is said to limit the reaction. In other words, the reactant that is consumed first limits the amounts of products that can form from the reaction.

In any stoichiometry problem, where reactants are NOT mixed in stoichiometric quantities, it is essential to determine which reactant is limiting in order to calculate correctly the amounts of products that will be formed.

For example, suppose a lemonade recipe calls for 1 cup of sugar for every 6 lemons. You have 12 lemons and 3 cups of sugar. Which ingredient is limiting, the lemons or the sugar? How many recipes of lemonade can you make?

The ratio of lemons to sugar that the recipe calls for is 6 lemons to 1 cup of sugar. We can calculate the number of lemons required to “react with” the 3 cups of sugar as follows:

6 lemons 3 cups sugar   18 lemons 1 cup sugar

Thus, 18 lemons would be required to use up 3 cups of sugar. However, we have only 12 lemons, so the lemons are limiting. The number of recipes of lemonade that can be made depends upon the number of lemons present.

1 recipe of lemonade 12 lemons   2 recipes of lemonade 6 lemons

Steps for Solving Stoichiometry Problems Involving Limiting Reactants

1. Write and balance the equation for the reaction. 2. Convert known masses of reactants to moles. 3. Using the numbers of moles of reactants and the appropriate mole ratios, determine which reactant is limiting. 4. Using the amount of the limiting reactant and the appropriate mole ratios, compute the number of moles of the desired product. 5. Convert from moles of product to grams of product, using the molar mass (if this is required by the problem). 2

1. Nitrogen gas can be prepared by passing gaseous ammonia over solid copper(II) oxide at high temperatures. The other products of the reaction are solid copper and water vapor. How many grams of nitrogen gas are formed when 18.1 g of ammonia are reacted with 90.4 g of copper(II) oxide? 3

2. Methanol (CH3OH), also called methyl alcohol, is the simplest alcohol. It is used as a fuel in race cars and is a potential replacement for gasoline. Methanol can be manufactured by combination of gaseous carbon monoxide and hydrogen. Suppose 68.5 kg of carbon monoxide is reacted with 8.60 kg of hydrogen gas. Calculate the theoretical yield of methanol. If 3.57 x 104g of methanol is actually produced, what is the percent yield of methanol?