You might spend a lot of time working out a method to make a product that you hope to sell for
vast profit, let's call it substance C. Your method is to use the reactants A and B to make your
desired product C.
A + B → C
Looking at this equation it would appear at first glance to be a good method to make substance C,
there are no other products so 100% of your reactants should turn into the desired product. The
theoretical yield of a reaction is the mass of the product you expect
to make based on your calculations
and the actually yield is the mass of the product that you actually manage to make,
and that you can bottle up and
sell. In an ideal world the theoretical yield, the calculated yield and actual yield would be the
same, and be 100%. Unfortunately in the real world actual yields are rarely if ever 100%.
There are many reason why yields are never 100%, these include:
- One of more of your reactants is not 100% pure. This would means that if you weighed
out 20g of it, then there might only be 19g of the actual reactant you need, the other 1g would
be a contaminant. This would mean you make less product than you expect.
- The method chosen by you to make the desired product could be an equilibrium reaction
and not one which goes to completion. If it was an equilibrium reaction then a mixture of
reactants and products would be obtained at equilibrium and the amount of product you expect
to get would be much less than you hoped for.
- There may be side reactions that you did not expect. This would mean some of your reactants are
turned into an unwanted product. This would reduce your actual yield of desired product.
- When products are transferred from one container to another chemicals are always left behind.
The more steps and the more transfers that take place the more chemicals will be lost and the actual
yield will go down. So it is crucial to design a method which involves as few steps as possible. It maybe that
it is difficult to separate the desired product from the reaction mixture.
- Some gases, liquids or solids could escape into the air, this would mean you lose some reactant or
product and again your actual yield will go down.
To calculate the % yield of a reaction use the formula below:
In an experiment Joe made 7g of substance A. He calculated that the maximum mass of product should
be 10g. What is the % yield of the reaction?
Simply use the equation given above to calculate the % yield.
Simply substitute these numbers into the equation to get the % yield.
- Mass of desired product = 7g
- theoretical (calculated) mass was 10g.
% yield = (7g/10g) x 100% = 70% yield.
2.5g of new drug is made but the scientists were expecting a maximum yield of 15g. What is the % yield for this reaction?
So simply substitute these numbers into the formula to give:
- Mass obtained of desired product= 2.5g
- Maximum mass that could be obtained is 15g.
% yield = (2.5g/15g) x 100% = 17% yield.