5.10 - Yields and Atom Economies

• limiting reagent

  a substance present in an amount that limits the theoretical yield

  converting all of it to the desired product gives 100% yield
• use of yield calculations

  assess the efficiency of the chemical process
• theoretical yield

  mass of product that should be made in a reaction if no chemicals are lost in the process
• actual yield

  mass of product obtained from the reaction

  always less than the theoretical yield
• percentage yield

  actually amount of products you collect as a percentage of the theoretical yield

  tells you how efficient the process is
• percentage yield equation

  (actual yield ÷ theoretical yield) × 100
• reasons why yields aren't 100%

  reactants aren't totally pure

  some of the products may be lost during transfer of chemicals

  there may be side reactions where reactants form different products

  some of the reactants may not react because reaction is too slow or has reached equilibrium
• atom economy

  a measure of the proportion of reactant atoms that become part of the desired product

  measures how efficient the reaction is
• atom economy equation

  (molar mass of desired product ÷ sum of molar masses of all products) × 100
• what lowers atom economy

  unwanted by-products
• atom economy of addition reactions

  always 100% because all reactants combine to form a single product
• importance of atom economy

  companies will try to use reactions that tend towards 100% atom economy

  high atom economies means raw materials are used more sustainably as reaction is more efficient

  high atom economy produces less waste so benefits the environment

  high atom economy means less by-products so less time and money spent separating those from desirable products