Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

When reactions occur, how can you differentiate between A2B2 and 2AB?

  1. Nov 30, 2004 #1
    Take this double replacement reaction for example:

    [tex]Na_2SO_4 + CaCl_2[/tex]

    How am I supposed to tell whether the product will be [tex]CaSO_4 + Na_2Cl_2[/tex] or [tex]CaSO_4 + 2NaCl[/tex]?

    Obviously, there's a difference between Calcium Sulfate + 2 Sodium Chloride molecules and Calcium Sulfate + Sodium DiChloride, but how can I figure out which it will be?

    Thanks for any help,
    Last edited: Nov 30, 2004
  2. jcsd
  3. Nov 30, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    A2B2 (almost) never exists. Whenever the numbers of different atoms in a molecule have a common divisor, you divide by that number to reduce it to the simplest ratio. Why ? Because in most cases, it can be shown that the simplified structure (AB rather than A2B2) is the energy minimizing structure.

    So it's extremely rare to find (inorganic) molecules like A2B2, A2B4, A3B6, A2B2C4, etc. Hydrogen peroxide is a notable exception, but then again, peroxide linkages are usually quite unstable.

    So, in your specific problem, the product formed is 2NaCl (not Na2Cl2).
  4. Nov 30, 2004 #3
    The products will be [tex]CaSO_4 + 2NaCl[/tex] Why would you get [tex]Na_2Cl_2[/tex]? Sodium ions have a charge of +1 and Chloride ions have a charge of -1. It only takes 1 sodium ion to go with 1 chloride ion.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook