# Why is dilution exothermic?

The breaking of bonds usually absorbs energy and forming bonds usually releases energy.

Then, why is the dilution of acids and bases exothermic?

• Except when it is endothermic; I have water soluble fertilizer that is strongly endothermic. – blacksmith37 Dec 10 '20 at 16:49

Dissolution can be both exothermic like for $$\ce{NaOH}$$ or $$\ce{H2SO4}$$, or endothermic like for $$\ce{CaCl2 . 6 H2O}$$, $$\ce{KNO3}$$ or $$\ce{KClO3}$$.

The net thermal effect sign is determined by combination of endothermic breaking of a ionic lattice or chemical bonds and of exothermic ion hydration.

$$\Delta H_\mathrm{dissol} = \Delta H_\mathrm{ioniz} + \Delta H_\mathrm{hydr}$$

Compounds with the greater absolute value of lattice or bond energy ( the former term ) than hydration energy ( the latter term ) have endothermic dissolution. Compounds with the opposite relation have exothermic dissolution.

Dilution of concentrated solutions is exothermic, as the exothermic ion hydration is gradual, depending on H2O : ion molar ratio.

For liquid concentrated acids like $$\ce{H2SO4}$$ or $$\ce{HNO3}$$, the process of dissolution gradually switch to dilution, with acid molecules being gradually ionized and hydrated.

I suppose a complicated process with a complicated PE graph, as it is not a single reaction like e.g. SN2 nucleophilic substitution. E.g. hydration of protons comes with multiple steps and several layers.

$$\ce{H+ -> H3O+ -> H3O+ . H2O -> H3O+.(H2O)2 -> H3O+.(H2O)3 -> \\ .. -> H3O+.(H2O)6 -> .. -> H3O+.(H2O)20 }$$

See Wikipedia - hydronium - solvation. Additionally, dissociation and hydration are not subsequent processes, but are ongoing simultaneously.

Similar would happen with anions.

• The question was about dilution, pretty sure that's no typo. – Karl Dec 10 '20 at 7:46
• @Karl Ah, I see. I know difference, of course, I have just read something else than what was written. :-) It was before my morning coffee. – Poutnik Dec 10 '20 at 7:50
• Probably all the steps described by Poutnik 2 days ago are exothermic. I repeat them H+⟶H3O+⟶H3O+⋅H2O⟶H3O+⋅(H2O)2⟶H3O+⋅(H2O)3⟶⋅⋅⟶H3O+⋅(H2O)6⟶⋅⋅⟶H3O+⋅(H2O)20 – Maurice Dec 12 '20 at 12:10
• @Poutnik In the case of bases, I think, there is no hydration going on during dilution. So, how is that exothermic? – Shub Dec 28 '20 at 5:22
• @Shub Because you think wrong here. – Poutnik Dec 28 '20 at 6:26