Chemical Forums

Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: BHAVESH on January 29, 2020, 06:43:36 AM

Title: Ksp of CaSO4
Post by: BHAVESH on January 29, 2020, 06:43:36 AM
Hi Everyone,
I found an equation from Google about Ksp of CaSO4 which is as follows

   Ksp CaSO4  = 10^[1/(-0.0222XLog(Ionic Strength of Water)+2.2954)2-0.2478]

I just wanted to know how these numeric figures have been derived in above equation.

Thanks....
Title: Re: Ksp of CaSO4
Post by: mjc123 on January 29, 2020, 07:02:42 AM
Did it give a reference? Can you look up that reference?
Title: Re: Ksp of CaSO4
Post by: Corribus on January 29, 2020, 09:46:51 AM
Just by eye it looks like a empirical equation, determined by fitting to a series of experimental data points. If that's the case, the terms and coefficients probably don't have any specific scientific meaning.
Title: Re: Ksp of CaSO4
Post by: BHAVESH on January 31, 2020, 02:58:34 AM
They have just mentioned - (Dupont 1992)
They have mentioned - The solubility products for sparingly soluble salts are based on expressions derived from published data on solubility as a function of the ionic strength of the solution at a given temperature. The equation for calculating Ksp for CaSO4 at 25 Degree C.
And when I checked in Dupont document for Ksp, they have given a graph, it seems it must have been derived from that.

So, still the question remains is that how to calculate Ksp of CaSO4, BaSO4, SrSO4 at any given temp.? - Can I apply Van't Hoff equation here as well???
Title: Re: Ksp of CaSO4
Post by: Corribus on January 31, 2020, 08:28:38 AM
So, still the question remains is that how to calculate Ksp of CaSO4, BaSO4, SrSO4 at any given temp.? - Can I apply Van't Hoff equation here as well???
Sure... bearing in mind sources of error.
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 01, 2020, 06:24:59 AM
Hi Corribus,
I didn't understand about sources of error. Can you please explain.
Title: Re: Ksp of CaSO4
Post by: Corribus on February 01, 2020, 09:37:47 AM
For one, the accuracy and presumed temperature independence of thermodynamic quantities (e.g., standard enthalpy change).
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 02, 2020, 01:24:09 AM
Ksp of CaSO4 @ 25°C is 4.93 X 10-5. When calculated using Van't Hoff Equation @ 40°C, surprisingly reduction in solubility is 4.52 X 10-17 using standard enthalpy of CaSO4 (-1433 KJ mol-1) at 25°C.

Such a drastic drop in solubility!!!
Title: Re: Ksp of CaSO4
Post by: Borek on February 02, 2020, 04:30:29 AM
All that means is that you are doing something wrong.
Title: Re: Ksp of CaSO4
Post by: Corribus on February 02, 2020, 10:05:14 AM
Check units in general but especially on your literature value for standard enthalpy of calcium sulfate. That seems way too high (like, by several orders of magnitude).
Title: Re: Ksp of CaSO4
Post by: mjc123 on February 05, 2020, 04:59:59 AM
What is the value of ΔH that you must use in the van't Hoff equation? Is that what you have used?
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 07, 2020, 06:46:21 AM
Yes I used -1433 KJ mol-1. This I took from Wikipedia Data Standard Enthalpy. Can you please help me correct my mistake?
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 07, 2020, 06:49:50 AM
However, it has been observed that when Temp. increases, solubility of CaSO4 increases. In this case, I feel that I am doing some GROSS MISTAKE in calculating solubility using Van't Hoff Equation.
Title: Re: Ksp of CaSO4
Post by: mjc123 on February 07, 2020, 07:14:11 AM
-1433 kJ/mol is the enthalpy of formation of solid CaSO4. Is that what you need to use in the van't Hoff equation for the solubility equilibrium?
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 07, 2020, 11:55:00 PM
yes, this is because I want to calculate the prediction of CaSO4 precipitation at given concentration in water. I mean when water gets concentrated by means of either evaporation or by reverse osmosis process, CaSO4 tends to precipitate depending upon concentration of individual ions and comes out as solid from water.
Title: Re: Ksp of CaSO4
Post by: Corribus on February 08, 2020, 12:29:40 AM
No - what is the definition of enthalpy of formation? You either need to look up the enthalpy of dissolution, or calculate it from the enthalpies of formation of the solid sulfate and the dissolved ions.
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 10, 2020, 06:45:03 AM
you mean to say based on Lattice & Hydration Energy should I calculate? As mentioned below

[1ΔHf(Ca+2 (aq)) + 1ΔHf(SO4-2 (aq))] - [1ΔHf(CaSO4 (s))]
[1(-542.83) + 1(-909.27)] - [1(-1434.5)] = -17.5999999999999 kJ
-17.60 kJ     (exothermic)
And use this as ΔH° KJ mol1-
Title: Re: Ksp of CaSO4
Post by: mjc123 on February 10, 2020, 08:26:31 AM
Yes. I have told you before - the ΔH you should use is that of the equilibrium reaction under consideration.. In this case it is
CaSO4(s)  ::equil:: Ca2+(aq) + SO42-(aq)
This is not the formation reaction of CaSO4, so ΔH°f(CaSO4) is not the thing to use.
Generally ΔH°reaction = ΣΔH°f(products) - ΣΔH°f(reactants).
Next time you have a van't Hoff equation problem, write down the equilibrium reaction and determine what ΔH you need to use.
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 16, 2020, 03:39:26 AM
Dear mjc123,
Thank you very much for your guidance. However, the reaction ΔH°reaction = ΣΔH°f(products) - ΣΔH°f(reactants) you have given but the product is CaSO4 and reactants are Ca2+ & SO42- in saturated water.
when it is calculated it comes to 17.6 KJ/mol (Endothermic)??
Title: Re: Ksp of CaSO4
Post by: mjc123 on February 17, 2020, 04:54:45 AM
No, in the equilibrium for the dissolution of CaSO4, which is what Ksp refers to, CaSO4(s) is the reactant and Ca2+(aq) + SO42-(aq) are the products, as I wrote the equation above.
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 17, 2020, 10:21:49 AM
Thank you very much dear for making me understand (sometimes even silly things!!!) the conceptual thing. :) :) :)
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 17, 2020, 11:47:37 AM
How to calculate Lattice Enthalpy of Formation of individual element like Ca2+, Ba2+ etc...
Normally, that we use in enthalpy calculation. For an example we use -537.6 kj mol-1 for Ba2+. So how do we calculate this???

probably, the reaction is Ba(s)<----> Ba2+ + 2e-
Title: Re: Ksp of CaSO4
Post by: mjc123 on February 17, 2020, 12:48:28 PM
There is no such thing as lattice enthalpy of formation. Lattice energy and enthalpy of formation are different things.
-537.6 kJ/mol is the enthalpy of formation of Ba2+(aq). This is not calculated; it is derived from experiment. It can't be directly measured, but it can be derived from things that can be measured, by Hess's law, and some necessary conventions (e.g. that ΔHf° of H+(aq) ≡ 0).
Title: Re: Ksp of CaSO4
Post by: BHAVESH on February 19, 2020, 02:55:10 AM
Thank you very much. One Lesson over!!!