Atom Economy

• It's important in industrial reactions thta as much of the reactants as possible get turned into useful products. This depends on the atom economy and the % yield of the reaction
• Atom economy - % of reactants forming useful products
• A lot of reactions make more than one product. Some of them will be useful, but others will be waste
• The atom economy (or atom utilisation) of a reaction tells you how much of the mass of the reactants is wasted when manufacturing a chemical and how much ends up as useful products
• (Relative formula mass (Mr) of desired products / relative formula mass (Mr) of all reactants) x 100 = atom economy
• 100% atom economy means that all the atoms in the reactants have been turned into useful (desired) products. The higher the atom economy, the 'greener' the process
• Example

  Calculate the atom economy of the following reaction to produce hydrogen gas: CH4(g) + H2O(g) -> CO(g) + 3H2(g)

  1)Identify the desired product - that's the hydrogen gas

  2) Work out the Mr of all the reactants

  > Mr(CH4) = 12 + (4 x 1) = 16 Mr(H2O) = (2 X 1) + 16 = 18 16 + 18 = 34

  3)Work out the total Mr of the desired product

  > 3 x Mr(H2) = 3 x (2 x 1) = 6

  4) Use the formula

  > 6/34 x 100 = 17.6%
• High atom economy is better for profits and the environment
• Reactions with low atom economy use up resources very quickly. At the same time, they make lots of waste materials that have to be disposed of somehow. That tends to make these reactions unsustainable - the raw materials will run out and the waste has to go somewhere
• For the same reasons, low atom economy reactions aren't usually profitable. Raw materials are often expensive to buy, and waste products can be expensive to remove and dispose of responsibly
• There's often more than one way to make the product you want, so the trick is to come up with a reaction that gives useful ''by-products'' rather than useless ones
• The reactions with the largest atom economy are the ones that only have one product. Those reactions have an atom economy of 100%
• The more products there are, the lower the atom economy is likely to be
• Atom economy isn't the only factor to be considered when choosing which reaction to use to make a certain product. Things such as the yield, the rate of reaction and the position of equilibrium for reverse reactions also need to be thought about
• A reaction with a low atom economy, that produces useful by-products, might also be used