Percentage Yield Calculations.

However, not all of the reactants may actually be converted into products.

As we have seen, some reactions have 100% atom economy i.e. all the atoms of the reacting molecules are converted into useful products.

C2H4 H2O C2H6O+e.g.

There are several reasons for this :a. The reaction may be reversible.

Such reactions never go to completion and some of the reactants always remain.

N2 + 3H2 2NH3

e.g.

The Haber Process

What happens to the un reacted nitrogen and hydrogen gases ?

Unreacted gases are recycled

b. An un wanted ‘side reaction’ may occur reducing the yield of the desired product.

This often occurs in Drug manufacture.

Unwanted Impurity

Unwanted Impurity

Wanted Product

c. The product may be impure and require purification before use.

This purification process always causes mass to be ‘lost’.

e.g. pharmaceutical products

We can assess how efficient the production process will be by calculating the percentage yield.

Percentage Yield = Actual Yield of Product

Maximum possible yieldx 100

For example:

A pharmaceutical process is being used to produce aspirin.The process could produce a maximum possible yield of 350g.The actual yield of aspirin after purification is only 270g.

Calculate the Percentage Yield.Answer

Percentage Yield = Actual Yield of Product Maximum possible

yield

x 100

= 270 x 100350

= 77.1 %

e.g. 1

The Haber Process.Nitrogen reacts with Hydrogen to form Ammonia

according to the following equation:

N2 + 3H2

2NH3

200 kg of nitrogen are used to produce the ammonia.48.6 kg of ammonia are produced.

(i.) Calculate the maximum possible yield of ammonia.(ii.) Calculate the actual percentage yield.

Answer

(i.)

Step 1. Write the information under the equation.

200 kg Yield 48.6 kg

Step 2. Calculate the Mr values for each molecule.

Mr (N2) = 2 x 14 = 28

Mr (NH3) = 14 + 3 x 1 = 17

Mr (N2) = 28

Mr (NH3) = 17

Atom Economy100 %?????

Step 3

Use the equation

1 N2 produces 2 NH3Step

4Convert to masses.1 x 28 kg of N2 produces 2 x 17 kg

of NH3 28 kg of N2 produces 34 kg of NH31 kg of N2 produces 34 / 28 kg

of NH3200 kg of N2 produces 34 / 28 x 200 kg of NH3 = 243 kg of

NH3

Maximum Possible Yield

N2 + 3H2

2NH3

Mr (N2) = 28Mr (NH3) = 17

Percentage Yield = Actual Yield of Product Maximum possible

yield

x 100Answer

(ii.)

= 48.6 / 243 x 100= 20 %

Atom Economy

100%

Although the reaction has a 100% atom economy, only 20% of the reactants are actually converted into products.