Atom Economy

Specification Point 5.13C:
  • Recall the atom economy of a reaction forming a desired product.
  • Along with the percentage yield, atom economy is used to analyse the efficiency of reactions.
  • Atom economy studies the amount of reactants that get turned into useful products.
  • It illustrates what percentage of the mass of reactants become useful products.
  • It is used extensively in the analysis of systems and procedures in industries, in an effort to obtain sustainable development.
  • It is also a very important analysis for economical reasons as companies prefer to use processes with higher atom economies.
  • The higher the atom economy of a process then the more sustainable that process is.

Calculating Atom Economy

Specification Point 5.14C:
  • Calculate the atom economy of a reaction forming a desired product.

Equation:

Atom economy   =   (total Mr of desired product  ÷ total Mr of all products) x 100

Example:

Hydrogen gas is obtained from methane in a process called steam-methane reforming. Calculate the atom economy of this reaction.

Equation Of Reaction:    

CH4 (g)    +    H2O (g)     →     CO (g)    + 3H2 (g)

Step 1  Calculate the total Mr of all the products:

Total Mr = 28 (CO:12 + 16) + (3H2: 6 x 1) = 34

Step 2  Calculate the Mr of the desired products:

Mr of 3H2 (6 x 1) = 6

Step 3  Calculate the atom economy:

Atom economy = (6 ÷ 34) x 100 = 17.6%

Therefore in this process 82% by mass of reactants is wasted.

Higher Tier Only

Choosing a Reaction Pathway

Specification Point 5.15C:
  • Explain why a particular reaction pathway is chosen to produce a specified product, given appropriate data such as atom economy, yield, rate, equilibrium position and usefulness of by-products.
  • Reactions that have low atom economies use up a lot of resources and produce a lot of waste material which then needs to be disposed of.
  • These reactions are thus unsustainable as they use up too much raw material to manufacture only a small amount of product.
  • They are not economically attractive as raw materials tend to be expensive, as does waste disposal which requires specialist chemicals, equipment, space and transport.
  • Companies continually analyse reactions and processes and evaluate several factors in an effort to improve efficiency.
  • Atom economy, percentage yield, rates of reaction and equilibrium position are important factors which need to be considered when choosing a reaction pathway.
  • High percentage yields and fast reaction rates are desirable attributes in industrial chemical processes.
  • In reversible reactions, the position of the equilibrium may need to be changed in favor of the products by altering reaction conditions.
  • If the waste products can be sold or reused in some way that would improve atom economy.
  • Alternative methods of production could also be considered that may produce a more useful by-product.

Exam Tip

  • Look for information on percentage yield, atom economy, rate and equilibria in questions based on choosing reaction pathways.
  • Your answer should then be based on evaluating this information.

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