Edexcel A Level Chemistry:复习笔记1.7.4 Yield & Atom Economy

Percentage yield

• In a lot of reactions, not all reactants react to form products which can be due to several factors:
  • Other reactions take place simultaneously
  • The reaction does not go to completion
  • Products are lost during separation and purification

   

• The percentage yield shows how much of a particular product you get from the reactants compared to the maximum theoretical amount that you can get:
• 
• The actual yield is the number of moles or mass of product obtained experimentally

• • The theoretical yield is the number of moles or mass obtained by a reacting mass calculation

   Answer:

   Step 1: The balanced symbol equation is:

   Step 2: Calculate the amount of zinc reacted in moles

   Step 3: Calculate the maximum amount of copper that could be formed from the molar ratio:
   Since the ratio of  Zn(s) to Cu(s) is 1:1 a maximum of 0.10 moles can be produced
   Step 4: Calculate the maximum mass of copper that could be formed (theoretical yield)

   Step 5: Calculate the percentage yield of copper

Atom economy

• The atom economy of a reaction shows how many of the atoms used in the reaction become the desired product
  • The rest of the atoms or mass is wasted

   

• It is found directly from the balanced equation by calculating the Mr of the desired product

• The atom economy could also be calculated using mass, instead or Mr
  • In this case, you would divide the mass of the desired product formed by the total mass of all reactants, and then multiply by 100
• Questions about atom economy often ask in qualitative or quantitative terms

Answer

Hydration of ethene has a higher atom economy (of 100%) because all of the reactants are converted into products, whereas the substitution of bromoethane produces NaBr as a waste product

   Answer

   Step 1: Write the equation:

   Step 2: Substitute values and evaluate: