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My book says,

When NaCl forms, what really happens is that $\ce{Na+}$ and $\ce{Cl-}$ come together and form an ionic bond and thus NaCl is formed and it is a quite exothermic reaction.

Why and from where is this energy coming from? Ok Ok, I read that every system wants to reduce energy, but where literally is this energy coming from, is it because of some electrons or from the nucleus? Or from where?

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  • $\begingroup$ To be honest I think this is bordering on a physics.SE question. You are essentially asking "why is it more favourable for positive and negative charges to be next to each other". Answer: electrostatic forces. $\endgroup$ – orthocresol Sep 15 '16 at 16:25
  • $\begingroup$ @orthocresol Who says that? I never asked for that! I only asked about the exact reason that why, when an electron goes from Na to Cl, there is a HUGE release of energy? $\endgroup$ – Aaryan Dewan Sep 15 '16 at 16:27
  • $\begingroup$ Because sodium becomes positively charged, chlorine becomes negatively charged, and positive and negative like being next to each other. No need to shout at me. $\endgroup$ – orthocresol Sep 15 '16 at 16:28
  • $\begingroup$ Haha, It's who whose shouting now. What do you mean by they "like" to be next to each other? This ain't philosophy my friend! $\endgroup$ – Aaryan Dewan Sep 15 '16 at 16:31
  • $\begingroup$ @orthocresol True, but OP’s doing his best to ask it in a chemical way … $\endgroup$ – Jan Sep 15 '16 at 18:40
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Why and from where is this energy coming from?..

If we go much deeper it can be explained physically by the famous equation (energy mass equivalence) $ E = M C^2 $. So the energy comes from the mass defect which gets converted to energy.( I hope you know what mass defect is). As we know that energy released in a chemical reaction is very small in comparison to a nuclear reactions, so the mass defect in chemical reactions is also very small in comparison to nuclear reaction. So in general we do not take into consideration the mass defect in a chemical reaction(since it can be neglected) So we generally explain evolution of energy on the basis of other energies but all of these point again to energy mass equivalence.

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The negative standard enthalpy of formation of sodium chloride (i.e. ‘the formation is exothermic’) is primarily due to the formation of a crystal lattice. This crystal lattice is a macroscopic arrangement of ions in a cubic fashion with each sodium ion surrounded by six chlorides and each chloride ion surrounded by six sodium ions.

If it were not for the generation of this lattice and its inherent stabilising properties (a very good charge distribution, charges neatly alternate, contact between species of identical charge is strongly inhibited) the formation of sodium chloride would be much less exothermic.

Note most importantly, that it is not merely the generation of sodium cations (that process is endothermic) or the generation of chloride anions (that process is only weakly exothermic) that causes the overall exothermicity of the entire reaction.

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  • $\begingroup$ "Note most importantly, that it is not merely the generation of sodium cations (that process is endothermic) or the generation of chloride anions (that process is only weakly exothermic) that causes the overall exothermicity of the entire reaction" . This is exactly , what my question is. I want to know that what causes the sodium conversion to sodium anion to loose energy? I know it's getting stable, but what really is causing it to loose energy? $\endgroup$ – Aaryan Dewan Sep 15 '16 at 14:55
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    $\begingroup$ @AaryanDewan That’s not exactly the question you asked, but here is the answer: Electron and nucleus attract each other. To strip away the electron, you need to overcome the attraction. That costs energy. Note that calling metal cations ‘stable states’ is typically wrong: If something lost an electron by ionisation (as metals do if they become cations) that will always cost energy and thus metal cations are never more stable than the pure metal. $\endgroup$ – Jan Sep 15 '16 at 18:35
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The answer is lattice energy.

The enthalpies involved in the formation of ionic compounds can be understood by Born-Haber cycle.

The following steps are involved in the formation of $\ce{NaCl}$ for example.

$$\ce {Na(s) -> Na(g)}\tag{1}$$

$$\ce{Na(g) -> Na+ + e-}\tag{2}$$

$$\ce{1/2 Cl2(g) -> Cl(g)}\tag{3}$$

$$\ce{Cl(g) + e- -> Cl^{-}(g)}\tag{4}$$

$$\ce{Na+(g) +Cl^{-}(g) -> NaCl(s)}\tag{5}$$

You can see that energy is required in the steps 1-3 whereas energy is evolved in 4th and 5th steps.

The energy evolved in the 5th step is called the lattice energy of the ionic compound and this only is responsible for the overall process being exothermic.

Now why lattice formation is exothermic? This is because when a lattice is formed, new attractive forces are set up that are dominant over the new repulsive forces set up. As it is known that attractive forces decrease the potential energy of a system(or increase the stability), so energy is released as heat.

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  • $\begingroup$ Thanks for the answer! But, Please tell me the exact reason for the reduce of energy. Born-Haber cycle and Lattice energy are words, not explanations! $\endgroup$ – Aaryan Dewan Sep 15 '16 at 14:56
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At the first step in the formation of NaCl, the solid Na should be converted to gaseous Na by the process sublimation so there comes sublimation entalpy, secondly the gaseous Na should lose one electron to become a Na+ ,so there comes the role of ionisation entalpy and if we see the case of chlorine molecule at first they need to dissociate to 2 cl and of electron affinity they should get an electron becoming stable. At last we need to bring both of them to an equilibrium position such that the released energy is sufficient for them to make a bond (lattice energy) then in the same Na+and Cl-ions the electron and the protons in the nucleus attract so some energy is again spend against that. Thus while looking overall we can under that the process of formation of NaCl is an exothermic process

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