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Teaching Stoichiometry Made Simple

“Stoichiometry” – that big, scary word in chemistry that makes students’ eyes pop open. When teachers throw it out there, you can practically see the “uh-oh” expression on students’ faces. But although the word sounds complex and intimidating, it’s simply the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction.

Use the analogy of cooking. Ask your students to imagine making a dish, and instead of ingredients, we call them reactants. The finished dish? That’s the product of the reaction. Let’s take baking cookies, for instance. You toss in some flour, butter, and sugar, and boom! – You get a batch of cookies. Stoichiometry is just about figuring out how much of each ingredient (or reactant) you need to whip up your chemical creation.

The word “stoichiometry” is derived from the Greek words “stoicheion,” meaning element, and “metron”, meaning measure. It’s essentially measuring the amount of elements consumed and produced in a chemical reaction based on a balanced equation.

Let’s run through the key concepts your students should master in stoichiometry first and then we’ll discuss why stoichiometry matters and dive into a specific recipe example to solidify this concept.

Key Concepts of Stoichiometry

1. Balanced Chemical Equations:

Before diving into stoichiometric calculations, it’s crucial to have a balanced chemical equation. This means that the number of atoms of each element on the reactant side must equal the number of atoms of the same element on the product side.

Since matter cannot be created nor destroyed, the atoms you start with should be the same atoms you end up with. Think of it like a set of LEGO. You can use the same pieces to make different products, but in the end, the pieces have not changed in themselves.

2. Moles:

In stoichiometry, chemists use moles as a unit of measurement. A mole is a quantity of substance that contains Avogadro’s number of entities, where Avogadro’s number is approximately 6.022 x 10^23.

If your students are having a hard time grasping this concept of “moles”, ask them how many is a pair (2). A dozen (12). A grand (1000). A mole is simply another quantity, representing 6.022 x 10^23. If I have a mole of flowers, I have 6.022 x 10^23 flowers (that’s more than enough to fill the entire surface area of the earth!)

The reason why we work with such a high number in chemistry is because atoms and molecules are microscopic. Imagine, a mole of carbon-12 atoms (meaning 6.022 x 10^23 carbon atoms) has a mass of only 12 grams!

3. Molar Ratios:

Molar ratios express the proportional relationship between the amounts of reactants and products in a chemical equation. These ratios are determined by the coefficients of the balanced equation.

For example, in the equation for the formation of water: 2H2 + O2 → 2H2O, there is a molar ratio of 2:1 between hydrogen (H2) and oxygen (O2). This means that two moles of hydrogen will react with one mole of oxygen to produce two moles of water.

Once you know the amount in moles of one substance in the balanced chemical equation, you can calculate the amount of moles of any other substance in the balanced chemical equation. It’s that easy!

Why Do We Use Stoichiometry?

Students are always asking why they have to learn certain topics. What’s the use? Well, stoichiometry allows chemists to plan and optimize chemical reactions. Using stoichiometry, you can calculate the exact quantities of reactants needed to produce a desired amount of product. This minimizes waste and ensures efficiency in the lab.

You can also calculate how much product is expected from any chemical reaction so you know if the reaction is complete. To do so, you’ll first have to find the limiting reactant, which is the reactant that gets used up first. Even if the other reactants are still available, as soon as one reactant is used up, the reaction will no longer proceed.

Stoichiometry also helps us gauge the efficiency of a reaction process. To do this, we calculate the “theoretical yield”, which is the expected product amount based on the balanced chemical equation. We then compare this to the “actual yield,” the amount of product collected during the experiment. By examining how closely these numbers align, we can determine if the experiment is efficient or if improvements are needed to boost yield and minimize losses.

Using a Sandwich Recipe Analogy to Teach Stoichiometry

As mentioned in the introduction, you can ask your students to imagine a chemical reaction as a cooking recipe. In a recipe, you have a list of ingredients with specific quantities, which will yield a particular amount of “product”.

Consider a simple example of making a sandwich. Let’s say to make a sandwich, you need two slices of bread, one leaf of lettuce and three strips of bacon. Written in the form a balanced equation, it may look something like this:

2 slices of bread + 1 leaf of lettuce + 3 strips of bacon → 1 sandwich

Students can easily see that by adjusting the quantities of the ingredients, you can produce different quantities of sandwiches. If we want to make two sandwiches, all we have to do is double all of the ingredients. Just as you can adjust a recipe for the number of sandwiches, you can also adjust the quantities in stoichiometric calculations.

The sandwich analogy can be used to introduce the concept of “limiting reactants” as well. Give your students varying quantities of bread, lettuce and bacon and ask them how many sandwiches can be made. For example, let’s suppose we have 6 slices of bread, 10 leaves of lettuce, and 18 strips of bacon. Which one of these ingredients is going to limit how many sandwiches can be made?

Well, with 6 slices of bread, you can make 3 sandwiches. With 10 leaves of lettuce, you can make 10 sandwiches. With 18 strips of bacon, you can make 6 sandwiches. However, because we only have 6 slices of bread, the maximum amount of sandwiches we can make is only three. This makes bread our “limiting reactant” and the rest of the lettuce and bacon will remain in excess.

Just like in a chemical reaction, you’ll have different amounts of reactants and in most cases, one will be used up first. This one will be considered the “limiting reactant” as it limits how much product can be produced. The remaining reactants are called “excess reactants”. By identifying the limiting reactant, you can calculate the maximum amount of product that can be produced by this particular reaction.

We hope this article will help you chemistry teachers get your students familiarized with the topic of stoichiometry. Do you use any other analogies to teach this concept? Share them in the comments below!

Ellier Leng
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