Limiting reactants and percent yield

Video summary

A limiting reactant in a chemical reaction is the reactant that's used up first, so, it's the one that limits the amount of products that are created in a chemical reaction. The other reactants are then said to be in excess. To get the most out of a chemical reaction, it's important to use the correct amounts of all the reactants. If you have too much of one reactant and not enough of another, you won't get the maximum yield.

The percent yield is a measure of how efficient a chemical reaction is. It's calculated by comparing the amount of product that's actually produced to the maximum amount that could be produced under perfect conditions. A percent yield below 100% means there was some loss of product during the reaction. This can happen due to incomplete mixing or because some reactants are used up before others.

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Revisores de contenido

Rishi Desai, MD, MPH

Limiting reactants limit the amount of products a chemical reaction can produce.

Let's use a simple example.

Let’s say we have 10 people and 5 chairs.

If only one person can sit in a chair, then only five people can sit down at a time while five others stand.

We can write this as a chemical equation:

1 Person + 1 Chair —> 1 Sitting Person

So because it takes one person, and one chair, to produce one sitting person, then we know that with 5 chairs and 10 people we are limited to producing at most 5 sitting people.

So the number of chairs is the limiting reactant, or limiting reagent, in our example.

But sometimes figuring out the limiting reactant is not very intuitive.

And so we will use the chair example to learn how to find the limiting reactant more systematically, by using mole ratios.

If we want to analyze this problem using mole ratios, we start by looking at the coefficients in the balanced chemical equation.

In the equation, the coefficients of “Person” and “Chair” are both one---so the mole ratio between the reactants is one-to-one:

Person/Chair = 1/1

Now that we have the mole ratio between the reactants, we pick either one of the numbers that we are given in our problem---we can pick either 5 chairs, or we can pick 10 people, it does not matter.

Here, suppose we pick 10 people.

We write this number into our mole ratio equation, and then put an “x” in place of the number of chairs

Person/Chair = 1/1 = 10/x

If we solved the equation, we would find that “x” is equal to “10”

This represents how many chairs would be needed to have a perfectly balanced reaction with 10 people.

So, since we know from our problem that we only have 5 chairs, we now know that chairs must be our limiting reactant---we have fewer chairs than we need to react with every person.

Let’s say that instead of picking 10 people as our first number, we instead picked 5 chairs.

In this case, we would write out the mole ratio with an unknown “y” in place of the number of people:

Person/Chair = 1/1 = y/5

We can solve this equation in order to get y equals 5, meaning that we would need exactly 5 people to react with 5 chairs.

However, we know that we actually have 10 people, meaning that the number of people is in excess---we have more people than chairs.

Once again, this would make chairs the limiting reactant in this problem.

We can also set this problem up by drawing a little table.

Once we know our limiting reactant, drawing a table helps us figure out how much product we will make.

The columns of our table correspond to each part of our chemical reaction: here, these are people, chairs, and sitting people.

We label the rows with “I” for “Initial”, “C” for “Change”, and “F” for “Final.”

In the “initial” row, we put down all the values we start with, before the reaction takes place.

So we write down numbers corresponding to 10 people, 5 chairs, and zero Sitting people.

We just determined that chairs were our limiting reactant, and so all of the chairs get used up in the reaction.