Unraveling the Mystery of NaCl's Low Solubility Curve

[enthropy]

Why is Δsolubility / Δtemperature so much lower for NaCl than for other ionic compounds like KCl and NaNO₃? (It's solubility curve is almost flat).

 

[Gokul43201]

Do you know how the slope of the solubility curve is related to the enthalpy of dissolution? This is generally covered in any standard Physical Chemistry text in the chapter on Solutions. Or it may appear in the discussion about Le Chatelier's principle.

 

[enthropy]

It doesn't, but I found the Van't Hoff equation and from there it is fairly straightforward.

 

[Gokul43201]

It doesn't but I found the Van't Hoff equation and from there it is fairly straightforward.

Could you explain further?

 

[enthropy]

Sure. Integrating and solving for K, we get $$K \alpha e^{\Delta H /RT}$$. Smaller ΔH means smaller rate of change, and NaCl's is much smaller than KCl

 

[Gokul43201]

Close enough (your signs may be off or you use a non-standard sign convention).

I hope you realize that the ΔH here is the enthalpy of dissolution, which is what I was referring to above. And that's why your response "It doesn't" had me a little confused.

 

[enthropy]

I dropped the negative sign when typing it here, it becomes $$K\proptoe^{\frac{-\Delta H}{RT}}$$
"it doesn't" referred to it's appearance in the text, not my awareness of the relation.

 

[alxm]

Dissolving NaCl?

"Such unjustified aspersion of the molecular character of our most necessary condiment must not be allowed any longer to pass unchallenged."

-- H. E.. Armstrong, Nature, v120, p478 (1927)

He was the last hold-out against ionic theory.