NaOH, NaCl and HCl

[sunspot]

Here is an interesting observation, is we mix NaOH or HCl into water it is very exothermic and the solution gets warm or hot. Yet if we mix the same amount of NaCl into water it is endothermic and the solution gets cool. Does this imply that the Na+ aspect of NaOH and the Cl- aspect of HCl are also both endothermic, and that extra exothermic comes from the OH- and H+ (H30+), respectively, even though charges are the same?

[aj47]

I think the reason NaCl is endothermic is that when looking at bond enthalpies, enthalpy of atomisation would be greater than the enthalpy of hydration, therefore the overall enthalpy of solution would be a positive value e.g. endothermic.

 

On the other hand for NaOH and HCl, the enthalpy of hydration would be greater than enthalpy of atomisation so the value would be negative e.g. exothermic.

[sunspot]

That is correct. What this example shows is that all equal charge is not created equal. Both H+ and Na+ have one extra positive charge yet the H+ contains far more potential as reflected by its exothermic versus the endothermic output of Na+ going in the water. This compounded by HCl having to break a covalent bond while NaCl only ionic bonds. The same is true of Cl- and OH-, both have one extra electron and therefore one extra negative charge, yet the -OH has far more impact on the water as reflected by its exothermic relative to Cl-'s endothermic.

 

In other words, Na+ and Cl- both disrupt the hydrogen bonding within the water causing an endothermic affect. The Na+ takes the place of hydrogen within the water, by binding with the oxygen of water, while the Cl- takes the place of oxygen within the water, by binding with the hydrogen. But neither does as good a job as only the slightly polarized H2O, which does not even have a full charge value within the polarizaiton. Here is a case where less opposite charge is more exothermic than more opposite charge value. Charge alone cannot explain this anomaly.

 

The only other thing that is at work, which can explain this anomaly is magnetic force. Or charge in motion, like electrons in orbitals, will set up magnetic fields. This allows water to bind to itself tighter than if the extra charge of Na+ and Cl- are present. The charge helps neutralize the dipole but disrupts magnetic addtion in the process for a net lowering of stability.

 

One may argue too much charge may be the problem. This should cause the NaCl to stay attracted together lowering its solubility, which is not observed. The Na+ and Cl- ions will form hydration spheres to spread out the charge. This extended and structured hydrogen bonding should be exothermic since stable hydrogen bonds will formed within the hydration spheres. Yet Na+ and Cl-, with hydration spheres and all, are endothermic relative to pure water. Charge is not the limited step but disruption of magnetic stabilty.