reacting quantities

• The maximum possible amount of product is the theoretical yield which is the value you would calculate. Experimentally, it is difficult to achieve the theoretical yield because:

  • the reaction may not have gone to completion
  • other reactions (side reactions) may have taken place alongside the main reaction
  • 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)x100
• If you don't know which reagent is in excess, you need to find out by working out the amount in moles of each reactant and comparing with the equation. For example:

  $2H_2(g) +$$O_2(g) -> 2H_2O(l)$

  2 mol of hydrogen are required for every 1 mol of oxygen. If equal amount of hydrogen and oxygen were to react, hydrogen would be used up first, and half the oxygen will be unreacted. The limiting reactant is hydrogen.
• Calculations must be based on the limiting reagent
• The atom economy of a chemical reaction is a measure of how well atoms have be utilised
• Reactions with high atom economies:

  • produce a large proportion of desired products and few unwanted waste products
  • 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 / sum of molar masses of all products) x 100
• 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%
• The efficiency of a reaction depends on both percentage yield and atom economy