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Percentage Yield

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Chemistry

We can get an idea of how much product (or yield) we will get from a reaction by using the molecular mass of the samples involved.

Let us use the reaction between ethene and water to produce ethanol as an example. Have a look at the molecular masses of ethene, water and ethanol shown below.


Percentage Yield comparing ethene, water and ethanol StudySmarterPercentage Yield. Olive [Odagbu], StudySmarter Originals

What is percentage yield?

You can see from the balanced equation in the image above, that 1 mole of ethene reacts with water to make 1 mole of ethanol. We can guess that if we react 28g of ethene with water, we will make 46g of ethanol. But this mass is only theoretical. In practice, the actual amount of product we get is lower than the amount we predict due to the inefficiency of the reaction process.

If you were to carry out an experiment with exactly 1 mole of ethene and excess water, the amount of the product, ethanol, would be less than 1 mole. We can work out how effective a reaction is by comparing the amount of product we get in an experiment to the theoretical amount from the balanced equation. We call this percentage yield.

Percentage yield measures the effectiveness of a chemical reaction. It tells us how much of our reactants (in percent) successfully turned into a product.

  • We will learn why the reaction process is inefficient, and how to calculate percentage yield.
  • We will consider limiting reactants and how to find the limiting reactant in a chemical reaction.
  • We will also learn about the atom economy of a reaction, and what makes a reaction efficient.

Factors that affect percentage yield

The reaction process is inefficient for some of the reasons listed below.

  • Some of the reactants do not convert to a product.

  • Some of the reactants get lost in the air (if it's a gas).

  • Unwanted products get produced in side-reactions.

  • The reaction reaches equilibrium.

  • Impurities stop the reaction.

Calculating percentage yield

We work out percentage yield using the formula:

Actual yield is the amount of product you practically get from an experiment. It is rare to get 100 percent yield in a reaction due to the inefficiency of the reaction process.

Theoretical yield (or predicted yield) is the maximum amount of product you can get from a reaction. It is the yield you would get if all the reactants in your experiment turned into a product.

Let's illustrate this with an example.

In the following reaction 34g of methane reacts with excess oxygen to make 73g of carbon dioxide. Find the percentage yield.

1 mole of methane () makes 1 mole of carbon dioxide ()

= 16g/mol

34g of methane = 34 ÷ 16 = 2.125 mol since

According to the equation, for each mole of we get one mole of , so theoretically we should also produce 2.125 mol of carbon dioxide.

The molecular mass of is 44 g/mol:

M(C) = 12

M(O) = 16

so M() = 12 + 2 x 16 = 44 g/mol

Remember

By multiplying the molecular mass of with the amount of substance we can obtain the theoretical yield.

44g x 2.125 = 93.5g

The theoretical (maximal) yield is therefore 93.5g of carbon dioxide.

Actual yield = 73g

Theoretical yield = 93.5g

Percentage yield = (73 ÷ 93.5) x 100 = 78.075%

This means that the percentage yield is 78.075%

What are limiting reactants?

Sometimes we do not have enough of a reactant to form the amount of product we need.

Imagine you make nine cupcakes for a party but eleven guests show up. You should have made more cupcakes! Now the cupcakes are a limiting factor.


Percentage yield | Limiting Reactant | StudySmarterLimiting reactant. Olive [Odagbu], StudySmarter Originals

In the same way, if you do not have enough of a certain reactant in a chemical reaction, the reaction will stop when it is all used up. We call the reactant a limiting reactant.

A limiting reactant is a reactant that is all used up in a chemical reaction. Once the limiting reactant is all used up, the reaction stops.

One or more of the reactants may be in excess. They are not all used up in a chemical reaction. We call them excess reactants.

How to find the limiting reactant

To figure out which of the reactants in a chemical reaction is the limiting reactant, you must start with the balanced equation for the reaction, then work out the relationship of the reactants in moles or by their mass.

Let's use an example to find the limiting reactant in a chemical reaction.

The balanced equation shows 1 mole of ethene reacts with 1 mole of chlorine to produce 1 mole of dichloroethane. Ethene and chlorine are all used up when the reaction stops.

What if we use 1.5 moles of chlorine? How much of the reactants are left over?

1 mole of ethene and one mole of chlorine react to make 1 mole of dichloroethane. 0.5 moles of chlorine is left over. Ethene is the limiting reactant in this case as it is all used up at the end of the reaction.

You can also use the trick of dividing the number of moles of each reactant by its stoichiometric coefficient to determine which reactant is limiting. The reactant with the smallest mole ratio is limiting.

For the above example:

Stoichiometric coefficient of = 1

Number of moles = 1

1 ÷ 1 = 1

Stoichiometric coefficient of = 1

Number of moles = 1.5

1.5 ÷ 1 = 1.5

1 < 1.5, therefore, is the limiting reactant.

Percentage Yield - Key takeaways

  • Factors that affect percentage yield: the reactants do not convert to a product, some reactants get lost in the air, unwanted products get produced in side-reactions, the reaction reaches equilibrium, and impurities stop the reaction.
  • Percentage yield measures the effectiveness of a chemical reaction. It tells us how much of our reactants (in percentage terms) successfully turned into a product.
  • The formula for percentage yield (actual yield/theoretical yield) is 100.
  • Theoretical yield (or predicted yield) is the maximum amount of product that you can get from a reaction.
  • Actual yield is the amount of product you practically get from an experiment. It is rare to get the 100 percent yield in a reaction.
  • A limiting reactant is a reactant that is all used up at the end of a chemical reaction. Once the limiting reactant is all used up, the reaction stops.
  • One or more of the reactants may be in excess. They are not all used up in a chemical reaction. We call them excess reactants.

Percentage Yield

We work out percentage yield using the formula below:

actual yield/ theoretical yield x 100

Percentage yield measures the effectiveness of a chemical reaction. It tells us how much of our reactants (in percent) successfully turned into a product.

A high percentage yield lets us know how effective our reaction was. We usually only care about one of the products in a chemical reaction. Percentage yield lets us know how much of our reactants turned into a desired product.

Final Percentage Yield Quiz

Question

Select factors that affect percentage yield?

Show answer

Answer

Some reactants get lost in the air

Show question

Question

What is the percentage yield?

Show answer

Answer

Percentage yield measures the effectiveness of a chemical reaction. It tells us how much of our reactants (in percent) successfully turned into a product.

Show question

Question

What is theoretical yield?

Show answer

Answer

Theoretical yield (A.K.A. predicted yield) is the maximum amount of product that you can get from a reaction.

Show question

Question

What is actual yield?

Show answer

Answer

Actual yield is the amount of product you practically get from an experiment. It is rare to get 100 percent yield in a reaction due to the inefficiency of the reaction process.

Show question

Question

What is a limiting reactant?

Show answer

Answer

A limiting reactant is a reactant that is all used up at the end of a chemical reaction. Once the limiting reactant is all used up, the reaction stops.

Show question

Question

What is an excess reactant?

Show answer

Answer

One or more of the reactants may be in excess. They are not all used up in a chemical reaction.

Show question

Question

Find the limiting reactant in the reaction below?

5 g of iron and 5 g of sulfur were used to make iron sulphide

Fe(s) + S(s) → FeS(s) 

Show answer

Answer

Iron (Fe) is the limiting reactant.


(1 mole of iron reacts with 1 mole of sulfur to make 1 mole of iron sulphide. 

Fe = 56/5 = 0.089 mol. Fe

S = 32/5 = 0.156 mol. S 

There is not enough iron to react with the sulfur.)



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