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Mass loss in common chemical reactions?

  1. Jan 14, 2013 #1
    In any common chemical reaction that releases energy, say the reaction 2H2 + O2 = 2H2O, what mass is converted to energy via the E = MC2 equation? What sub-atomic particles are converted to energy during ordinary chemical reactions? I was taught us in HS and under-grad chemistry classes that mass is conserved in chemical reactions but I've read lately in Relativity Theory that it is mass-energy that is conserved. So, chemical reactions that release energy are converting a very small amount of mass to energy in the same, but opposite, manner that a substance warmed by sunlight is actually gaining mass.
  2. jcsd
  3. Jan 14, 2013 #2

    D H

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    Your second question first: None. Ordinary chemical reactions don't do that. Ordinary chemical reactions, even a highly reactive one, barely change the mass at all. As far as chemists are concerned, mass is conserved. Unless one is extremely careful and precise in measuring mass, the change in mass is immeasurably small in chemical reactions. (There is a change; it's just very small.)

    Since subatomic particles are *NOT* destroyed, what does change?

    The answer lies in binding energy. It takes a good deal of energy to strip all of the electrons from an atom. The amount of energy needed is the binding energy, and it is by this quantity (divided by the speed of light squared) that that the combined mass of a bare nucleus and the freed electrons exceeds the mass of the neutral atom. For example, the mass of a neutral hydrogen atom is slightly less than the sum of the masses of a proton and an electron.

    The same concept applies to chemical compounds. That binding energy released when hydrogen and oxygen ignite to form water means that water is slightly (very slightly) less massive than the constituent oxygen and hydrogen molecules.
    Last edited: Jan 14, 2013
  4. Jan 14, 2013 #3
    Thank you for the great answer! Much appreciated.
  5. Jan 14, 2013 #4
    Have they changed the definition of binding energy?

    I learned, many years ago that

    Binding energy is the difference between the sum of the masses of neutrons and protons in the free state and the masses of the same number of neutrons and protons in a nucleus.

    (Semat page 85)

    Also the masses of the electrons are the same in an atom and free so cancel out.

    Semat gives the folowing example: in atomic mass units

    Lithium = 7.01822

    4 Neutrons = 4 x 1.008987 = 4.03595
    3 Protons = 3 x 1.008145 = 3.02444
    total = 7.06039

    The difference is the binding energy of 0.04217amu which can be equated to the binding energy by Einstein's equation.
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