Chapter 12: Atom Economy and Percentage Yield

12.1-Atom Economy

Atom economy % of reactants forming useful products

• A lot of reactions make more than one product
• The atom economy 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 product
• Atom economy = relative formula mass of desired products / relative formula mass of all reactants x 100
• 100% atom economy means that all the atoms in the reactants have been turned into useful products, the higher the atom economy, the greener the process
• Calculating the atom economy
• Identify the desired product
• Work out the Mr of all reactants
• Work out the total M of the desired product
• Use the formula to calculate the atom economy

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
• This makes them unsustainable
• They usually aren’t very profitable
  • Raw materials are expensive to remove and dispose of responsibly
• Reactions with highest atom economy are the ones that only have no product
• The reactions have an atom economy of 100%
  • The more products, the lower the atom economy is likely to be

12.2-Percentage Yield

Percentage yield compares actual and theoretical yield

• The amount of product you get is known as the yield
• The more reactants you start with, the higher the actual yield will be
  • But the percentage yield doesn’t depend on the amount of reactants you started with, it’s a percentage
  • Percentage yield = mass of product actually made(g) / maximum theoretical mass of product x 100
  • Maximum theoretical mass can be calculated using balanced reaction equation
• Percentage yield is always somewhere between 0 and 100%
• 100% yield means that you get all the product you expected to get
• 0% yield means that no reactants were converted into products
• Industrial processes should have as high a percentage yield as possible to reduce waste and reduce costs

Yield are always less than 100%

• In real life, you never get a 100% yield
• Some products or reactant always gets lost along the way, and that goes fr big industrial process as well as school lab experiments
• Depends on what sort of reaction it is and what apparatus is being used

Not all reactants react to make a product

• In reversible reactions the products can turn back into reactant so the yield will never be 100%
  • For example, in the Haber process at the same time as the reaction N2 + 3H2 - 2NH3 is taking place, the reverse reaction is also happening
  • This means it never goes to completion

Practice Questions

• In an industrial process, a factory produces 158 million grams ammonia from a theoretically possible yield of 200 million grams . Calculate the percentage yield of this process:

• 158,000,000/200,000,000 x 100

• 79%

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