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If I mix sulfuric acid with sodium hydroxide, the solution really heats up. Could someone explain why acid-base reactions are exothermic from a molecular perspective?

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  • $\begingroup$ Sodium hydroxide and Sulphuric acid have $-\Delta_fH^\circ$ around $\pu{1300 kJ/mol}$ and sodium sulfate and water have around $\pu{1500 kJ/mol}$. So we have an exothermic reaction. $\endgroup$ – A---B Mar 31 '17 at 1:59
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    $\begingroup$ When an acid meets a base in a beaker... How easy is it to find a strong acid or base in nature? It isn't - it is hard work to make them. Well, all that work comes out when you mix them back together again. Write out the reactions and see what happens. $\endgroup$ – Jon Custer Mar 31 '17 at 2:02
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    $\begingroup$ Sorry if this is too basic, but imagine you ripped a bunch of water molecules in two, throwing the H+ pieces in 1 jar and the OH- pieces in another. All the energy put into that comes crashing back at you when you mix the solutions together, allowing the oppositely charged pieces of water to come back together again. No matter the path actually taken to get to this state of separated water molecules (H+ in the strong acid, OH- in the strong base), that's the energy that you get back in the form of heat when you mix the two. Is that what you're shooting for when you say "molecular" perspective? $\endgroup$ – airhuff Mar 31 '17 at 3:06
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One can think of it as simply because of electrostatic attraction. Basically, an acid produces $\ce{H+}$ ions in water while a base produces $\ce{OH-}$ ions in water. So when you mix an acid and a base (e.g. sulfuric acid and sodium hydroxide) in a beaker, you get a solution containing $\ce{H+}$ and $\ce{OH-}$ ions. So, when these ions are far apart, there exists potential energy between them (because opposite charges attract each and want to come closer together). So when these ions come closer together, the attraction between them becomes strong enough that a bond forms between the ions to form $\ce{H2O}$. During this process, the initial potential energy between the ions decreases and is converted to heat energy. Hence this is why it is exothermic.

So generally speaking, energy is released when a bond is formed, while energy is required (energy is absorbed) to break a bond. So in the case of acid and base reactions, no bond is being broken, bonds are only being formed. So it must be an exothermic reaction then.

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For a simple answer, an acid, likes do donate its $\ce{H+}$ ions, and a base likes to give off its $\ce{OH-}$ ions. The bond enthalpy of the A-H and B-OH bonds are low. Now the $\ce{H+ and OH-}$ fuse to form a water molecule, giving rise to new bonds, hence is an exothermic process.

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From a purely enthalpic point of view , $H^+$ when combines with $OH^-$ it gives a water molecule, that is a well known stable molecule. Since the energy must be conserved, when you "becker system" gains energy via the condensation reaction just named, the surrounding must loose energy: this happens since it is heated up by this exothermic reaction. Therefore you can think of the exothermicity as a consequence of the conservation of energy.

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  • $\begingroup$ This doesn't explain why acid-base reactions are exothermic, except a small hint that water is stable. Therefore a comment and an NAA $\endgroup$ – M.A.R. ಠ_ಠ Mar 31 '17 at 13:46

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