Reacting Quantities

• The maximum possible amount of product is called the theoretical yield
• Why is it difficult to achieve the theoretical yield?

  1. the reaction may not have gone to completion
  2. other reactions (side reactions) may have taken place alongside the main reaction
  3. purification of the product may result in loss of some product
• The actual yield obtained from a reaction is usually lower than the theoretical yield.
• The conversion of starting materials into a desired product is expressed by the percentage yield
• percentage yield = (actual yield / theoretical yield) X 100
• The reactant that is not in excess will be completely used up first and stop the reaction - the limiting reagent
• For example, when hydrogen and oxygen gases react to form water, 2 mol of H2 are required for every 1 mol of O2:

  2H2 + O2 = 2H2O

  If equal amounts of hydrogen and oxygen are allowed to react, the hydrogen will be used up first, and half the oxygen will be unreacted. The limiting reagent is hydrogen, so calculations must be based on hydrogen
• The atom economy of a chemical reaction is a measure of how well atoms have been utilised
• Reactions with high atom economies:

  1. produce a large proportion of desired products and few unwanted waste products
  2. are important for sustainability as they make the best use of natural resources
• Atom economy is based solely on the balanced chemical equation for a reaction and assumes a 100% yield
• Atom economy = sum of molar masses of desired products X 100

  sum of molar masses of all products
• The idea of atom economy has been developed alongside awareness of dwindling finite resources and environmental concerns about processing or disposing of harmful waste. Improving atom economy makes industrial processes more efficient, preserves raw materials and reduces waste. In an ideal chemical process, a use would be found for all products and thus the atom economy would be 100%
• Atom economy only provides part of the answer:

  1. the process uses reactants that are readily available, carbon from coal, and steam from water. Energy will be needed to produce the steam, but costs for obtaining such materials are low
  2. other reactions may have a much larger atom economy but poor percentage yields. Efficiency will depend on both factors