SunriseSunset
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HCl + H2O Gibbs free energy Question
I calculated Gibbs free energy of HCl(aq) -> H+(aq) & Cl-(aq)
And got a positive number. (38650.6) Which would indicate that it's not a favorable reaction.. Yet we know that HCl dissolves readily with water.
So I calculated gibbs free energy of
NaOH(s) -> Na+(aq) & OH-(aq)
and got -37.8, which indicates a favorable reaction. Which is true, dissolving NaOH in water definitely gives off heat. So I wanted to see what would
happen when I calculated dissolving HCl in water, since I know no real heat is given off, yet it's definitely a strong acid. Wanted to see what the
gibbs free energy calculation would look like, and for some reason, it's saying it's not favorable. Why is that?
Nvm. I didn't convert J/mole for entropy to KJ/mole... I wish I could delete my own topic. My mistake!
[Edited on 2-11-2015 by SunriseSunset]
Why do chemists call helium, curium and barium the medical elements?
because if you cant helium or curium, you barium! - Heimerdinger
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blogfast25
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How did you calculate ΔG here?
Hint: H<sup>+</sup>(aq) doesn't really exist, protons is water are fully solvated as
H<sub>3</sub>O<sup>+</sup>(aq).
The dissolution of HCl(g) into water is exothermic, by the way.
[Edited on 1-11-2015 by blogfast25]
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SunriseSunset
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I did originally attempt to use H3O+ ion, but every thermodynamic data chart that I could find only had H+ values.
Ultimately for HCl(aq) + H2O -> H+ & Cl-, I got..
delta G* = -38.65 KJ/mole
| Quote: | | The dissolution of HCl(g) into water is exothermic, by the way. |
After just thinking about it some more for a second, that could make sense bcuz HCl aqueous solns. are already pre-dissolved and diluted. If I were to
take 99% HCl and add it to water, things would be quiet obvious... Lol
[Edited on 1-11-2015 by SunriseSunset]
Why do chemists call helium, curium and barium the medical elements?
because if you cant helium or curium, you barium! - Heimerdinger
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blogfast25
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You chose a particular example that probably deserves explaining.
In the electrochemical series the 'centre point' (so to speak) the half reaction H<sup>+</sup>/1/2 H<sub>2</sub> has been
chosen conventionally to be +0 V. This means that in accordance with Nernst we have to put the Standard ΔG for the
H<sup>+</sup>/1/2 H<sub>2</sub> reaction (oxidation or reduction makes no difference here: + 0 V = - 0 V) also at 0 kJ/mol.
And since as the Standard ΔG of Formation for H<sub>2</sub> is also conventionally set to 0 kJ/mol, the Standard ΔG of Formation for
H<sup>+</sup> must also be zero.
Now, as ALL SRPs are relative to H<sup>+</sup>/1/2 H<sub>2</sub>, it matters not one iota: we really are interested in
changes in Gibbs Free Energy of actual reactions and these the system used predicts perfectly well.
[Edited on 1-11-2015 by blogfast25]
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SunriseSunset
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This might be an obviously bad question, but what's the way to find out the delta "G", delta "H", or "S"'s at other varying temperatures besides
standard room temp data from the charts?
If it requires calculus, i'll be biting the dust for weeks. lol
[Edited on 2-11-2015 by SunriseSunset]
Why do chemists call helium, curium and barium the medical elements?
because if you cant helium or curium, you barium! - Heimerdinger
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blogfast25
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Take a simple reaction:
A + B === > C
You already know how to calculate these values at STP (298 K), so how do we calculate it at any T different from 298 (Kelvin)?
You need to calculate the increase in ΔG (or ΔH or S) to get A and B from 298 K to T and simply add these increases to their STP
values.
Then calculate the increase in ΔG (or ΔH or S) to get C from 298 K to T and simply subtract that increase from C's STP value.
Then carry out the summation as you would at 298 K.
NIST webbook provides a large database of data of ΔH or S in function of temperature for many compounds.
For watery species these data are harder to find but how often do you watery chemistry at 200 C? 
[Edited on 2-11-2015 by blogfast25]
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SunriseSunset
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This looks nice and useful to calculate Gibbs free energies of reactions at other temperatures than STP. Too bad it's calculus. I'll see if I can't
learn it real quick tho
Gibbs–Helmholtz equation
OH! I understand what you were saying. Basically, T variable can be changed, and it works fine to figure out a new delta G according to any
temperature.
[Edited on 2-11-2015 by SunriseSunset]
Why do chemists call helium, curium and barium the medical elements?
because if you cant helium or curium, you barium! - Heimerdinger
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