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Why is the salt solubility curve flat?

by MadViolinist
Tags: curve, flat, salt, solubility
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MadViolinist
#1
Dec20-12, 03:25 PM
P: 18
I know most salts' have increased solubility in 100g of water with an increase in temperature, a few have an inverse relationship, but why does NaCl flatline regardless of temperature? Like is there a mechanism that explains this phenomenon? Thanks in advance.
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pa5tabear
#2
Dec20-12, 05:04 PM
P: 176
Is it completely flat? Not just mostly flat?

My guess is that the enthalpy change of dissociation is sufficient that temperature has a negligible effect.

I know that Sodium and Chlorine are strong acids/bases respectively, and we were taught that strong acids/bases dissociate completely.
Borek
#3
Dec21-12, 05:08 AM
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Borek's Avatar
P: 23,535
It is not perfectly flat.

Note that if some salts have solubility that increases with temperature and some have solubility that decreases with temperature, there is nothing strange with some salts being in between - and having temperature coefficient of solubility close to zero. NaCl happens to be one of these salts.

morrobay
#4
Dec22-12, 01:15 AM
P: 380
Why is the salt solubility curve flat?

The solubility curve is related to the molar enthalpy of ion formation in solution,
NaCl(s) --> Na+ + Cl- (aq)
ΔH (298K) = 3.9 kJ/mole
Which is a two part process : Lattice enthalpy, breakdown of crystal lattice and ionic enthalpy of hydration.
The first is endothermic the second exothermic. So with Le Chatelier principle a temperature
increase favors the first process and is against the second process.
In the case of NaCl these two parts of the process cancel each other out with increasing temperature.
Entropy is more important than temperature in free energy.
Δ G = ΔH - TΔS , - 8.94 kJ/mole = 3.9 kJ/mole - 12.84 kJ/mole


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