Percentage yield and atom economy

Percentage yield and atom economy

Lessons

In this lesson, we will learn:
  • To understand the definitions of atom economy and percentage yield and the difference between them.
  • To calculate atom economy and percentage yield from example chemical reactions.
  • To explain the importance of atom economy as a chemist when planning chemical reactions and processes.

Notes:

  • The atom economy of a reaction is the percentage of atomic mass of useful products in a reaction. It is calculated by: AtomEconomyAtom\;Economy \; (%)=AtomicmassofusefulproductsTotalatomicmassofproducts100\;= \frac{Atomic\;mass\;of\;useful\;products}{Total\;atomic\;mass\;of\;products} * 100

  • A high atom economy means that most of what the chemical process makes is useful! In the same way, a low atom economy tells you that a reaction is mostly producing unwanted waste products.

  • The atom economy is used to show an efficient reaction that makes lots of valuable chemicals and not much waste product that will require disposal.

  • A quick tip – reactions with a single product have a 100% atom economy because the only chemical being produced is the desired product.

  • The percentage yield of a reaction is the mass of products formed as a percentage of how much could have been formed given the mass of reactants used.

  • A low percentage yield means that not much of the reactants you used has become products. A high percentage yield therefore means that a lot of the reactant chemicals you used successfully reacted to make the products.

  • Know the difference between the two! Atom economy is about how wasteful the reaction is, and yield is about how much reactant was successfully converted to products!
  • Introduction
    Why do chemists do chemical reactions?
    a)
    Things that happen during a chemical reaction.

    b)
    The definitions of atom economy and percentage yield.

    c)
    The difference between them.

    d)
    Environmental and economic chemistry.


  • 1.
    Find the atom economy and percentage yield of chemical reactions.
    Water can be produced by reaction of hydrogen and oxygen gas according to this equation:
    2 H2+_2 + O2_2 →2 H2_2O
    a)
    What is the atom economy of this reaction?

    b)
    If the reaction was performed and 150 g was the theoretical yield of water and the actual yield was only 92 g, what is the percentage yield?


  • 2.
    Find the atom economy and percentage yield of chemical reactions.
    The reaction to make iron metal by reduction uses carbon. CO2_2 is an unwanted side-product. The reaction is as follows:
    2 Fe2_2 O3+_3 + 3 C→3 CO2+_2 + 4 Fe
    a)
    What is the atom economy of this reaction?

    b)
    280 g of Fe2_2O3_3 was used in this reaction and 104 g of Fe metal was collected. Calculate the theoretical yield and then the percentage yield of this reaction.

    c)
    This reaction was done again and a percentage yield of 90% was achieved this time. If 250 g of Fe was collected this time, how much Fe2_2O3_3 was used up?


  • 3.
    Compare the viability of two reactions by finding the atom economy and percentage yield.
    Magnesium reacts with hydrochloric acid as shown in the following equation:
    Mg+ + 2 HCl→MgCl2+_2 +H2_2
    a)
    If this reaction was performed to make hydrogen gas, what would be the atom economy?

    b)
    55 g of Mg metal was used in this reaction with HCl in excess. Only 4.21 g of H2_2 gas was produced from this reaction. What is the percentage yield of this reaction?

    c)
    Hydrogen gas could also be made by electrolysis of water, in the following reaction:
    2 H2_2 O→2 H2+_2 + O2_2

    Find the atom economy of this reaction. Is it a better or worse way to make hydrogen than reacting magnesium with hydrochloric acid?