Atom economy is a really neat way of working out how much waste there will be in a chemical reaction. It is covered in section 1.1 of the course and if you have a copy of the Chemistry subject guide you will find it on the right hand side under the sub heading ‘Utilization’.
It is often confused with percentage yield but is in fact a very different concept. Percentage yield will tell you have good (or bad) a chemical reaction has been. It lets you know how effective a reaction has been in turning products into reactants.
Atom economy gives an idea of waste. It does not give you any clues as to how efficient a reaction is. So from an industrial point of view it is another tool in deciding if a reaction is worth carrying out (alongside percentage yield).
To calculate atom economy is very simple. It is given by:
[Total mass of desired product / Total mass of all products] x 100
For example, if we were to compare the atom economy of two reactions – the production of Iron by the thermite reaction (1) and the blast furnace (2)
In the thermite reaction, aluminium reacts with iron (III) oxide to produce molten iron and aluminium oxide:
Fe2O3 + 2Al –> 2Fe + Al2O3
The atom economy would be given by:
[Total mass of iron / Total mass of iron plus total mass of aluminium oxide] x 100
[2 x 56] / [(2 x 56) + ((27×2)+(16 x 3))] x 100 =
(102 / 204) x 100 = 50%
In the blast furnace the reaction is:
Fe2O3 + 3CO –> 2Fe + 3CO2
This means that the atom economy would be give as:
[Total mass of iron / total mass of iron + total mass of carbon dioxide] x 100
[2 x 56] / [(2 x 56) + (3 x (12 + (16×2)))] x 100
(102 / 234) x 100 = 44%
So, what does this tell us? Well, purely from the perspective of waste, the thermite reaction is better. More of the desired product is produced by mass compared to the waste.
So, why isn’t iron manufacture this way? Well, as you are probably guessing, there are many more factors in play.
The percentage yield is one – how much iron is actually produced by each reaction?
What about operating conditions? Is it cheaper and easier to build a plant to extract iron in the blast furnace than by using the unpredictable and highly exothermic thermite reaction? The blast furnace is a continuous reaction but the blast furnace operates by a batch process – which means extra costs are involved stopping and starting the reaction.
Finally, what about the raw materials? Making iron through the thermite reaction is actually really expensive as aluminium has to be extracted first.