Question about exothermic/endothermic reactions

  1. I couldn't find the answer to this anywhere so I figured I'd make a thread.

    Can exothermic/endothermic reactions occur in a vacuum? Or if there are no surroundings, then what would the transfer of heat be? Thanks.
     
  2. jcsd
  3. mathman

    mathman 6,656
    Science Advisor
    Gold Member

    My guess: exothermic - emit photons. endothermic - needs incoming photons.
     
  4. Nugatory

    Staff: Mentor

    Or end up with more or less thermal energy in the reaction products than in the initial reagents.... That's certainly how I'd expect the (remarkably exothermic) decomposition of TNT to behave.
     
  5. ^ Exactly. The energy does not have to come from or go to the surroundings.

    Lets use the dissolution of ammonium nitrate as an example of an endothermic reaction. If we put NH4NO3 and water in a thermos bottle or other heat-insulated container, what happens? The ammonium nitrate dissolves and the resulting solution is colder than the water originally was. The energy needed for the reaction has been taken from the vibrational/rotational excitations of the water molecules.

    One example of an exothermic reaction is the explosive decomposition of nitrogen trichloride to form nitrogen and chlorine gases: ##2NCl_{3}\longrightarrow N_{2}+3Cl_{2}##. The compound can explode in vacuum just as well as in normal atmosphere, the energy change of 232 kJ/mol in the reaction just means that the reaction products are very hot when they are formed.
     
  6. russ_watters

    Staff: Mentor

    How can a vacuum contain reactants?

    Regardless, the others are right; reactions do not require heat transfer to surroundings.
     
    Last edited: Oct 30, 2013
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