atom economy heading

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Atom economy

Chemist working on a new product in the lab Chemical companies are always looking to develop new and valuable products to bring to market and to improve the way they make existing products to increase their profitability, reduce environmental pollution and also to try and ensure that any new products are produced in a sustainable way; that is one which meets the needs of current generation without compromising the ability of future generations to meet their own needs. Chemists and chemical engineers will study a number of factors involved in manufacturing a new product before any large scale production takes place. Some of the most important factors are:

Atom economy- why is it important?

Let's suppose that a chemical company intends to start manufacturing a new product which they hope will to bring to market and increase their profits, let's call this new product; product C. Now the chemists working for this particular chemical company have come up with two separate methods to make this new product C; these two methods are shown below:

Method 1

A + B → C

Method 2

D + E ⇌ C + F

So which method would you choose to make the new product C? Which method do you think will be the most profitable, sustainable and environmentally friendly? Let's look at some of the advantages and disadvantages for each of the proposed methods.

Advantages and disadvantages of each method

A chemist working on a new product in the lab Method 1 has one major advantage over method 2, method 1 only produces the desired product C. This means that all of the reactants are going into making the desired product and no other unwanted products are formed. This method also goes to completion, the arrow (→) used in the equation indicated that ALL of the reactants are turned into products. This is excellent since if there is only one product formed then there will be no expense incurred as a result of separation issues which would be the case if the reaction produced a mixture of two or more products. There is also no waste to be disposed of, which is likely to be both expensive and also environmentally unfriendly.

Method 2 produces an unwanted product; product F. This product may be able to be sold for profit but is could also be a waste product with little or no value. The other problem with method 2 is that the desired product C will have to be separated from the unwanted or waste product F and since this reaction is a reversible one not only will the unwanted product F be present but also unreacted reactants D and E will also be present, that is the reaction mixture will consist of an equilibrium mixture of reactants and products. This means a separation plant will have to be built to separate out the desired product C from this equilibrium mixture and this will incur a cost both financially and environmentally.

Other factors to consider

The goal of any good chemist is to design a process where the maximum amount of the desired product is obtained with as little waste and undesired products as possible being produced. That is the reaction should have a high percentage yield, it ideally should have a reasonable rate of reaction and not involve waiting for days or even longer before any reasonable amount of the desired product is produced.

Reactions which are reversible will obviously result an equilibrium mixture being produced which could mean separation issues and additional expense before any of the desired product is obtained. Any waste products produced will have to be disposed of and this will incur costs both financial and environmental. Producing lots of unwanted products or waste materials also uses up valuable natural resources and will cause additional environmental pollution both in their extraction and their disposal; these processes are likely to be unsustainable.

Calculating atom economy

One of the most important considerations in deciding which of the two methods above to use in producing the new product C is the atom economy or atom utilisation for each method. The atom economy or atom utilisation is used as a measure of how much of the reactants end up in the desired product; it can be calculated using the formula shown in the box below:
Formula to calculate atom economy
The higher the value of the atom economy the more of the reactants end up in the desired product, while any chemical reaction with a low atom economy is one where a large amount of the reactants end up in unwanted or waste products, this means that large amounts of reactants are likely to be need to manufacture reasonable amounts of the desired product and lots of waste products will be formed; this is unlikely to be profitable, environmentally friendly and certainly not sustainable.

Example 1

Display of alcholic drinks, beer and wine which all contain the alcohol ethanol

As an example consider the fermentation of glucose to form the alcohol ethanol and carbon dioxide gas, word and symbolic equations for this reaction are shown below:

glucose →   ethanol   +   carbon dioxide
C6H12O6 →  2C2H5OH   +   CO2

Now the relative formula masses of all the reactants and products are:

The symbolic equation shows that fermentation produces 2 moles of ethanol for every one mole of glucose, so the mass of 2 moles of ethanol is 46 x 2= 92g. While glucose has a Mr of 180. So to work out the atom economy of this fermentation reaction simply put the Mr values into the above formula for atom economy and we have:

So the atom economy = (Mr of ethanol÷ Mr of glucose) x 100%
= (92 ÷ 180 ) x 100% = 51.1%

The fermentation process has a relatively low atom economy of just over 51%, meaning that just over half of the mass of the reactants ends up in the desired product; the ethanol. Improving the atom economy of this reaction is very desirable in order to reduce the amount of waste or undesired product and make the process more sustainable and cost-effective.

Example 2 - Making ammonia

Ammonia (NH3) is a very valuable substance; its main uses are in the manufacture of fertilisers and explosive. Ammonia is made industrially in a process called the Haber process. Word and symbolic equations for the synthesis of ammonia in the Haber process are shown below:
nitrogen + hydrogen ⇌   ammonia
N2 + 3H2 ⇌   2NH3

The relative formula masses for all the reactants and products are:

So to the atom economy = (34 ÷ 28 + 6) x 100% = 100%

Now since there is only one product; the ammonia; then all of the reactants must end up as products and so the atom economy will be 100%. So although this sounds ideal as a possible route to making ammonia on an industrial scale it does not tell the whole story. As was mentioned above atom economy is important in deciding if a chemical process will likely be profitable and sustainable, however it is not the only factor that needs to be considered. The reaction shown to make ammonia is a reversible reaction and so will involve an equilibrium mixture of products and reactants; it just so happens in this particular reaction that the position of equilibrium lies very much to the left, that is in favour of the reactants. Finally the rate of the forward reaction is also very slow due to the fact that it has a large activation energy. So while atom economy is very important it is not the only factor that should be considered in deciding whether a reaction will be profitable, sustainable and environmentally friendly.

Key points

The image below shows two different chemical factories. One factory utilises reactions with high atom economies which produce little waste and which cause low levels of pollution; this factory also utilises all of the raw materials it consumes and so is sustainable. The other chemical factory does none of these things!!

An image which compares chemical factories with high and low atom economies and which are sustainable and environmentally friendly.

Practice questions

Check your understanding - questions on atom economy

Check your understanding - additional questions on atom economy