May 09, 2021, 12:12:34 PM
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### Topic: Ksp of CaSO4  (Read 2716 times)

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#### Corribus

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##### Re: Ksp of CaSO4
« Reply #15 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.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

#### BHAVESH

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##### Re: Ksp of CaSO4
« Reply #16 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-
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#### mjc123

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##### Re: Ksp of CaSO4
« Reply #17 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)  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.

#### BHAVESH

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##### Re: Ksp of CaSO4
« Reply #18 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)??
« Last Edit: February 16, 2020, 03:55:17 AM by BHAVESH »
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#### mjc123

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##### Re: Ksp of CaSO4
« Reply #19 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.

#### BHAVESH

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##### Re: Ksp of CaSO4
« Reply #20 on: February 17, 2020, 10:21:49 AM »
Thank you very much dear for making me understand (sometimes even silly things!!!) the conceptual thing.
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#### BHAVESH

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##### Re: Ksp of CaSO4
« Reply #21 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-
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#### mjc123

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##### Re: Ksp of CaSO4
« Reply #22 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).

#### BHAVESH

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##### Re: Ksp of CaSO4
« Reply #23 on: February 19, 2020, 02:55:10 AM »
Thank you very much. One Lesson over!!!
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