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The Concept Of Limiting Reagent

So far we have been considering reactions in which all the reactants exist in adequate quantities. In this section, we will consider what happens when the amount of one of the reactants available is less than the amount required to complete the reaction. When such a condition exists, we call that reactant or reagent the limiting reagent.

We will further explore this scenario through the following examples.

Example 2-6
A reaction mixture contains 60.75 g magnesium and 146 g hydrogen chloride. Predict the limiting reagent if the reaction occurs as  shown below.

Solution

First, we have to convert the grams of the substances to moles. Then make the comparison to see which one is the limiting reagent. By now, you should be comfortable with the conversion of moles to grams and vice versa. The number of moles of magnesium present is 2.5 moles. According to the equation, 1 mole of magnesium reacts with 2 moles of hydrogen chloride. For magnesium to completely react, there should be at least 5 moles of hydrogen chloride present. If you calculate the number of moles of hydrogen chloride present, you will get 4 moles. This amount of hydrogen chloride is not enough to completely react with the amount of magnesium present. So the limiting reagent is hydrogen chloride.