[Aalz-7]

Hello. I have a lab due in a few days, and I need the theoretical value of the Ka of acetic acid at 70 degrees Celsius. If anyone has it, can they please list the value and the source? Or if anyone can tell me if there is a way to use an equation to find it out? I heard of something called the Van't Hoff equation, but I have no idea how to use it or where to start. Thanks

[Vidya]

Yes you are right ..use Van't  Hoff equation ...
plug in the constants and T at which you want to know ka ...you just need to do mathematics ...try it

[Aalz-7]

Yes you are right ..use Van't  Hoff equation ...
plug in the constants and T at which you want to know ka ...you just need to do mathematics ...try it

Ok thank you very much. I am sorry but the equation is completely alien to me. I am in grade 11 Chem IB HL, and the experiment im working on is my extended essay. The extended essay does require some chemistry that is above and beyond the syllabus, so I think this would be a good application. I think I understand the calculus behind it, but what is the "R" value in the equation mean. For example, in the wikipedia article of the equation, it says it is the ideal gas constant, but I have no idea what the gas constant has to do with an acid. Thanks again for any clarification.

[Vidya]

You know that you are using enthalpy and entropy physical quantities which are measured in kJ or J ...so take the value of R in Joules

[Aalz-7]

Ok so here is my go at it.
According to wikipedia, the equation is:

ln(K2/K1) = (-∆H/R)(1/T2 - 1/T1)

Now

K2 = unknown
K1 = 1.75E-5
T2 = 343.15K
T1 = 298.15K
R = 8.314862175J/Kmol
-∆H = -66.0kJ/mol = -66000J/mol

After plugging in all these terms into the equation, my Kelvin unit does not cancel out by unit analysis. The left hand side should be a unitless number right? But my Kelvin unit does not cancel out.

[mjc123]

Yes it does. ΔH is in J/mol, R in J/K mol, so ΔH/R has units K, and 1/T has units 1/K...

I have no idea what the gas constant has to do with an acid.

It happens to be called the gas constant for historical reasons, as it was the proportionality constant in the gas equation PV = constant*T. But in fact it crops up everywhere, because it is a fundamental constant relating energy and temperature. We continue to call it the gas constant, however, which sometimes causes confusion.

[Aalz-7]

I made a mistake with the 1/T expression... sorry

So are my substitutions correct? If so, I assume all there is left to do is to make both sides a power of e, and then multiply both sides by K1?

thanks again

[mjc123]

I think your value of ΔH is wrong. This ref http://en.wikipedia.org/wiki/Acid_dissociation_constant gives a value of -0.41 kJ/mol.
The value of 66 kJ/mol looks like the dissociation enthalpy of the acetic acid dimer, i.e. (AcOH)₂  2AcOH: http://en.wikipedia.org/wiki/Acetic_acid. Not an acid dissociation.