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%