Percentage Yield & Atom Economy (OCR A Level Chemistry)

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

• 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 M_r of the desired product

• In addition reactions, the atom economy will always be 100% because all of the atoms are used to make the desired product
  • Whenever there is only one product, the atom economy will always be 100%
  • For example, in the reaction between ethene and bromine:

• The atom economy could also be calculated using mass, instead or M_r
  • 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 asked 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:

• Chemists use percentage yield as one possible measure of how efficient a reaction is
  • A high percentage yield suggests that a process is effective at converting reactants into products
• The estimated percentage yield for a single run of the Haber Process is around 15%
  • This is a compromise due to the cost and safety of the required conditions against the overall rate of ammonia production
  • Any unreacted materials are also recycled so it is estimated that the percentage conversion of all reactants to products is around 97%
• Whilst a high percentage yield can be good for profits, it does not account for any waste products
  • A reaction can have a high percentage yield but low atom economy which essentially means that more waste products are produced
• Atom economy is a measure of the percentage of reactants that become useful products and is calculated by:

Atom Economy and Green Chemistry

• Chemists will often have several choices of reaching a target molecule and those choices need to take into the principles of Green Chemistry

• By choosing a reaction pathway that has fewer steps, you can prevent waste and reduce energy demands which is better for the environment
  • This also reduces production costs

• One of the key ideas behind Green Chemistry is to find reaction pathways with high percentage yield and high atom economy
• By analysing the atom economy of each step, you can select reactions that give a higher atom economy and / or select other reactions to reduce the number of steps involved in a reaction pathway
  • For example, the synthesis of ibuprofen that was patented by Boots in the 1960's was a six-step synthesis
  • Even if each step has an atom economy of 90%, a six-step synthesis would have an overall atom economy of 53%
  • The modern production of ibuprofen is a three-step synthesis, which with the same assumptions as before, gives an overall atom economy of 73%  
• Higher atom economy means that there is less waste produced
  • This can be considered environmentally friendly even though it may not influence the reaction conditions
  • It also means that reactions are more sustainable and often use less natural / finite resource