Limiting Reactant: Definition, Formula & Examples Video

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  • 0:03 What Is a Limiting Reactant?
  • 0:58 Calculating the…
  • 4:21 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we'll use pizza, along with common chemical compounds and elements, to learn about limiting reactants in chemistry. Using easy-to-follow instructions, we'll also find out how to identify the limiting reactant in a chemical reaction.

What Is a Limiting Reactant?

Let's say that you and your friend have decided to have a pizza party and evenly split the costs. Each pizza costs $10, but while you have $10, your friend only has $5. Instead of each one of you having your own pizza, you'll have to split one. And you were counting on eating a pepperoni and olive pie all by yourself!

In this example, your friend's $5 served as the limiting reactant on how much pizza you could have. The same thing happens in chemical reactions: there is always a limiting reactant, which is a chemical element or substance that limits the amount of product made during a chemical reaction. Typically, there is also an excess reactant, or the amount of an element or substance left over after the reaction stops. In our example, the excess reactant would be the amount of money you'd have left over after you paid for your share of the pizza pie: $5.

Calculating the Limiting Reactant

In order to find the limiting reactant in chemistry, you need a balanced equation, or one where the number of atoms found on the reactant side is equal to the number of atoms found on the product side. You also need to know how much of each reactant you have (typically in grams) and the molecular weight of the reactants. If you were in a chemistry lab, you could determine the mass of each reactant in grams by weighing it; sometimes, that information will be provided. You can also use the periodic table of elements to find the molecular weight of a reactant, such as iron.

Iron on periodic table

This is a close-up of the chemical element iron as shown on the periodic table. Fe is the abbreviation for iron, while the 26 refers to the number of the element. According to the table, the molecular weight of iron is 55.85, which means that 1 mole, or unit, of iron is equal to 55.85 grams of iron. This tells you how many grams of product will be produced from each reactant; the reactant producing the least amount of product is the limiting reactant.

Example of a Limiting Reactant

Let's look at an example of a limiting reactant at work. In this balanced equation, we can see how much propane and oxygen we'd need to obtain water and carbon dioxide during a chemical reaction.

Propane balanced equation

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