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Why is the formation of ionic compounds exothermic?

  1. Jan 16, 2012 #1
    Hello everyone,

    I've been trying to figure out why when ionic compounds are formed that they are exothermic (meaning they give off heat). The only theory I have is that when the ions are coming together, the motion from them moving and possibly the friction of their surfaces making contact generates heat.

    Does this seem feasible?

    Thanks, - Brim
  2. jcsd
  3. Jan 17, 2012 #2


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    Staff: Mentor

    Friction has nothing to do with the process. It is all about electron placement (in a way that's what all chemistry is about).
  4. Jan 18, 2012 #3
    Borek, could you perhaps elaborate more on this? I do not understand how the heat is generated during the ionic compounding.
  5. Jan 18, 2012 #4
    Hey Brindley,

    Ionic compounds are formed when an atom(s) with normally 1,2 or 3 electrons in its outer valence bonds with another atom(s) with 5,6 or 7 electrons in its outer valence. The electrons in the first atom can attain a lower energy state by making up a complete valence shell in the second atom. So they do. The energy released is the difference between the electrons initial state and its resultant state. This electron exchange leaves one atom as a cation (losing electrons) and the other as an anion (gaining electrons) which contributes to the strength of bond - ionic compounds usually have a much higher melting point. Hope this helps.

  6. Jan 18, 2012 #5
    Hello ch@rlatan and thank you for your response!

    Could you give more detail as to what you mean by lower/high energy state? What do you mean?

    I don't understand how the energy released relates to heat, as you said the difference between the electrons initial state and its resultant state was the energy. Does this mean the physical motion of the electrons in transit causes heat? friction? I understand the higher melting point note, but I'm still not understanding the big picture as to why the ionic compounding process is exothermic.

    Any and all help is appreciated!
  7. Jan 19, 2012 #6
    Hey Brimley,

    Ahh. I think this is more to do with the nature of energy itself. Heat or temperature is a measure of energy and there are many - pressure, force, kinesis (motion), luminosity to name but a few. But the true nature of energy is not very well understood at all. So heat is simply an indication of the amount of energy released.

    You might like to think of the electrons in the first atom as being kinetically energetic and by moving to (being captured by) the second atom that kinesis is converted to energy. You might like to think that by completing a valence shell (forming a noble gas structure) the electrons are bonded in such a way that they can pull in tighter around the atom reducing their mass and emitting energy according to e=mc2. But the truth is that the nature of the atom and the processes within it are quantum mechanical and can only really be interpreted through the mathematics of quantum mechanics. You can imagine a fistful of scenarios that are consistent with the release of energy but it's not yet possible to experimentally confirm with absolute certainty any of them.

    One thing I can tell you is that friction is a more complicated (macroscopic) concept and will involve one or more of the already established forms of energy.

  8. Jan 19, 2012 #7
    Hello ch@rlatan,

    so in summary, would you say that ionic compounding is exothermic due to energy being emitted via the bonding process which entails kinesis and thus the conservation of energy states that this kinesis will change form to heat once the electrons have finished the kinetic process?

    Does that sound right? Also, what did you mean by emitting energy according to e=mc2 ?
  9. Jan 19, 2012 #8


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    That's a question of chemical kinetics. In the simplest case, that of reaction in the gas phase of two atoms, the two atoms exchange an electron when they come close to each other and after that accelerate to each other due to the Coulomb attraction. To form a bond and not only get scattered from each other they have to collide with a third atom or molecule which carries with it part of the energy leaving behind a bound pair of cation and anion.
  10. Jan 19, 2012 #9

    No. The scenarios I proposed are as valid as saying 'There's a little leprechaun in the atom that grabs the electron to fill his magic box of eight at which time it sings and we see that as energy'.

    It's like this. The textbook response to your question is that electrons are in a higher energy state in the first atom and the bonding process occurs because by bonding with the second atom they can achieve a lower energy state. The transition of states from higher to lower allows for the release of energy making the reaction exothermic.
    If you don't really know what 'state' means or understand the mechanism for the reaction then join the queue. Science is about what can be shown mathematically.
    So I re-iterate ;- 'But the truth is that the nature of the atom and the processes within it are quantum mechanical and can only really be interpreted through the mathematics of quantum mechanics.'

    If you want to 'chew the cud' about what is possibly going on in the atom then this forum is not the place. I know that from experience.

    When you say 'that this kinesis will change form to heat' you are still confusing heat and energy. In this scenario it would be correct to say 'that this change in kinesis will be converted to energy which is then measured as heat.'


    'Also, what did you mean by emitting energy according to e=mc2?'

    Energy has three forms. Potential - by virtue of a bodies position with respect to a field of force. Kinetic - by virtue of a bodies movement (potential is converted to kinetic when it is free to move in the field of force - like something falling off a shelf). And rest-mass - the intrinsic energy contained within the matter of the body. The equation e=mc2 states that matter can be converted to energy by a factor c2. For more info on that search 'mass-energy equivalence'.

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