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Author Topic: Explain difference between Enthalpy of Solution and Delta G of Solution ?.  (Read 872 times)

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happychappie1700

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Dear Colleagues,

As I understand it, the Enthalpy of Solution determines whether or not an ionic solid dissolves in water ( assuming the enthalpy of hydration is greater than the lattice energy ).

1) Then, what is the purpose of Delta G of solution ? - assuming that the Enthalpy of Solution tells us if a solid will dissolve - meaning mixing occurs ( a spontaneous reaction ), where does ΔG of Solution fit in to all this and what is ΔG of Solution's relationship to the Enthalpy of Solution ?.

Thank you.
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mjc123

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Re: Explain difference between Enthalpy of Solution and Delta G of Solution ?.
« Reply #1 on: December 12, 2018, 02:53:03 AM »

What determines whether the solid dissolves is ΔG of solution, not ΔH. You can easily verify that you can get both exothermic and endothermic dissolutions, so it's not ΔH that determines what dissolves.
What is the relationship of ΔG of solution to enthalpy? ΔGsoln = ΔHsoln - TΔSsoln
The entropy of solution is positive, so even quite endothermic dissolutions can occur.
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happychappie1700

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Re: Explain difference between Enthalpy of Solution and Delta G of Solution ?.
« Reply #2 on: December 12, 2018, 06:25:08 AM »

mjc123 thank you for replying.

Am I correct in the following:
1) ΔH of solution tells us only if there is a exothermic or endothermic dissolution.
2) ΔG of Solution tells us if the solid will form a 'solution' at room temperature .
( presuming that if an ionic substance does dissolve in water, it does not mean that it will form a solution ? eg. a suspension or a partial dissolution ).

In response to your previous answer:
Quote
What is the relationship of ΔG of solution to enthalpy? ΔGsoln = ΔHsoln - TΔSsoln
The sum of ΔHsoln comes from the ΔH(Lattice Energy) and ΔH hydration. Where does the figure for the TΔSsoln come from ?.


« Last Edit: December 12, 2018, 07:10:20 AM by happychappie1700 »
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mjc123

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Re: Explain difference between Enthalpy of Solution and Delta G of Solution ?.
« Reply #3 on: December 12, 2018, 07:02:49 AM »

1. Yes
2. Yes
3. Do they? Can you quote one?
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happychappie1700

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Re: Explain difference between Enthalpy of Solution and Delta G of Solution ?.
« Reply #4 on: December 12, 2018, 07:13:07 AM »

apologies, I amended my previous post to:
In response to your previous answer:

Quote
What is the relationship of ΔG of solution to enthalpy? ΔGsoln = ΔHsoln - TΔSsoln
The sum of ΔHsoln comes from the ΔH(Lattice Energy) and ΔH hydration. Where does the figure for the TΔSsoln come from ?.
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mjc123

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Re: Explain difference between Enthalpy of Solution and Delta G of Solution ?.
« Reply #5 on: December 12, 2018, 10:25:13 PM »

T is the temperature. ΔS is analogous to ΔH. ΔSsoln = -ΔSlattice + ΔShydration
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happychappie1700

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Re: Explain difference between Enthalpy of Solution and Delta G of Solution ?.
« Reply #6 on: December 13, 2018, 08:46:04 AM »

mjc123, I've been trying to find example questions and answers specifically on ΔGsoln, but unfortunately no luck on google or on youtube. .Can you point me in the right direction ?. Thank you
« Last Edit: December 13, 2018, 08:59:28 AM by happychappie1700 »
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Corribus

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Re: Explain difference between Enthalpy of Solution and Delta G of Solution ?.
« Reply #7 on: December 14, 2018, 04:53:13 AM »

If you understand the Gibbs energy concept in general, then it is no different for the specific case of solubility. The spontaneity (ΔG) of any reaction/process depends on both the overall enthalpy change and the entropy change. In the case of dissolution of a crystalline solid, the ΔG will depend on (1) whether net heat is released or absorbed during breaking of the crystalline lattice and formation of new intermolecular interactions between dissolved components and the solvent; and (2) whether a net entropy is positive or negative during breaking of the ordered crystalline lattice and formation of hydration spheres around the dissolve solute components. All of these factors are not always easy to predict, and which ever one wins out will determine whether dissolution happens spontaneously at a given temperature.

If you do not understand the Gibbs energy concept at all, then that would be a good place to start, ignoring for a moment the specific case of dissolution.

It's really important to also understand that even if ΔG is negative, this only means that the process will eventually proceed in the forward direction toward an equilibrium point that favors formation of products. This says nothing about how long it takes to get there. The world is full of thermodynamically favorable processes that never happen over reasonable timescales because they aren't kinetically favorable.
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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

happychappie1700

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Re: Explain difference between Enthalpy of Solution and Delta G of Solution ?.
« Reply #8 on: December 24, 2018, 04:29:53 AM »

Corribus,

I hope I'mnot straying off topic here, but wanted to askyou about an example calculation about the entropy change of the universe in regards to whether water will freeze to ice at 5°c ( 278 K ):

ΔSuniv=ΔSsurr+ΔSsys

If we already know that the entropy change of the system for water converting to ice is -22.0 J K-1mol-1 and that the entropy change of surroundings for water converting to ice at 5°c ( 278K ) is 21.6 J K-1 mol-1

Then ΔSuniv=ΔSsurr+ΔSsys:

                  = 21.6 - 22.0
                  = -0.4 J K-1mol-1(meaning entropy is reduced in the
                                                                               universe )

The question is where did the - negative sign come from in deducting the 21.6 - 22.0 ?
(when the equation ΔSuniv=ΔSsurr+ΔSsys involves addition )
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