VeritasC&E
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Is the dissociation of acetic acid dependent on temperature?
Does the percentage of acetate ions in an acetic acid solution show a marked increase with temperature?
Would you know the approximate % of dimer in solution at 100C compared to the % at 20C? (just to get a general idea of the impact of temperature, if
any, on the concentration of acetate ions in solutions).
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clearly_not_atara
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Off the top of my head, autoionization results in an increase in potential energy (charge separation) paid for by an increase in entropy (charges in
different places). So it should increase with rising temperatures. How much is not obvious, but I'd figure it's a very small change, since
heating electrolytes is just not usually necessary in practice.
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Texium
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Thread Moved
29-7-2022 at 12:40 |
VeritasC&E
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Quote: Originally posted by clearly_not_atara  | | Off the top of my head, autoionization results in an increase in potential energy (charge separation) paid for by an increase in entropy (charges in
different places). So it should increase with rising temperatures. How much is not obvious, but I'd figure it's a very small change, since
heating electrolytes is just not usually necessary in practice. |
Thank you!
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j_sum1
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You could always measure pH as a function of temperature.
From the pH, calculating the amount of dissociation is trivial.
Even measuring with pH paper at two or three different temperatures at the concentration you intend to use would give you a sense of what is going on.
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VeritasC&E
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| Quote: Originally posted by j_sum1 | You could always measure pH as a function of temperature.
From the pH, calculating the amount of dissociation is trivial.
Even measuring with pH paper at two or three different temperatures at the concentration you intend to use would give you a sense of what is going on.
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I didn't think about this but it should have been a reflex. That way I'll get exact numbers and see for myself. Thanks a lot for your input!
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Chalo
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Delta S for acetic acid dissociation is 0.094 Kjoules per mole,
Delta H is -0.41 kJoules/mole
so the difference in delta-G between 25 degrees C (27.154 kjoules/mole)
and say 65 degrees is only 40x.09 or 3.6 kjoules/mole, so 23.6 kJ/mole.
using pKa=deltaG/(2.3RT), and pKa at 25C of 4.76 I calculate a pK at 65 degrees of 4.2. Not a large change at all. Dissociation is highly unfavorable
to begin with (why only 1 molecule in 10 thousand dissociated) heat helps a little but not much.
feel free to check my math.
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VeritasC&E
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| Quote: Originally posted by Chalo | Delta S for acetic acid dissociation is 0.094 Kjoules per mole,
Delta H is -0.41 kJoules/mole
so the difference in delta-G between 25 degrees C (27.154 kjoules/mole)
and say 65 degrees is only 40x.09 or 3.6 kjoules/mole, so 23.6 kJ/mole.
using pKa=deltaG/(2.3RT), and pKa at 25C of 4.76 I calculate a pK at 65 degrees of 4.2. Not a large change at all. Dissociation is highly unfavorable
to begin with (why only 1 molecule in 10 thousand dissociated) heat helps a little but not much.
feel free to check my math.
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Thanks a lot! Low dissociation really is a limiting thing with acetic acid. On the positive side it makes it safer to handle in many conditions.
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