Percentage Yield & Atom Economy (OCR A Level Chemistry A): Revision Note

Percentage Yield Calculations

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

Atom Economy Calculations

• 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:

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

• 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

Benefits of High Atom Economy

• 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

The twelve 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