Atom Economy (Edexcel GCSE Chemistry) : Revision Note

Hydrogen gas is obtained from methane in a process called steam-methane reforming. The reaction is as follows:

CH₄ (g) + H₂O (g) → CO (g) + 3H₂ (g)

Calculate the atom economy of this reaction.

Answer:

• Step 1: Calculate the total M_r of all products:

  • Total M_r = CO + 3H₂ 

  • Total M_r = (12 + 16) + (3 x 2 x 1) = 34

• Step 2: Calculate the M_r of the desired product:

  • M_r of 3H₂ = (3 x 2 x 1) = 6

• Step 3: Substitute the values into the percentage atom economy equation:
  

  • Atom economy = 

  • Atom economy =  = 17.6%

• Therefore, in this process 82.4 % of reactant material is wasted

  • 100 - 17.6 = 82.4 %

Unwanted byproducts can sometimes be put to use so although a low atom economy is a sign that a process is not green (sustainable) it doesn’t necessarily imply that the process is not economically viable.