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Percentage yield and atom economy

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# Atom economy

Atom economy is a way to measure the atoms wasted when making a chemical. The higher the atom economy, the 'greener' the process. 100 per cent atom economy means that all the atoms in the reactants have been converted to the desired product.

## Calculating atom economy

Atom economy = Mr of desired productssum of Mr of all products × 100

For example, calculate the atom economy for making hydrogen by reacting zinc with hydrochloric acid: Zn + 2HCl → ZnCl2 + H2

Mr of H2 = 1 + 1 = 2

Mr of ZnCl2 = 65 + 35.5 + 35.5 = 136

Atom economy = 2136 + 2 × 100 = 2138 × 100 = 1.45%

## Industrial processes - Higher tier

Industrial processes need as high an atom economy as possible, because this:

• Reduces the production of unwanted products
• Makes the process more sustainable [sustainable: Doing something in a way that minimises damage to the environment and avoids using up natural resources, eg by using renewable resources. ]

For example, hydrogen can also be made from methane and steam: CH4 + 2H2O → CO2 + 4H2

• Mr of H2 = 1 + 1 = 2
• Mr of CO2 = 12 + 16 + 16 = 44

Atom economy = 4 × 244 + (4 × 2) × 100 = 852 × 100 = 15.4%

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