Not logged in [Login ]
 Sciencemadness Discussion Board » Fundamentals » Beginnings » Help with Benzoic acid pKa&Gibbs free energy Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues

Author: Subject: Help with Benzoic acid pKa&Gibbs free energy
yoyoils
Harmless

Posts: 22
Registered: 6-8-2011
Member Is Offline

Mood: positive

Help with Benzoic acid pKa&Gibbs free energy

Book says Benzoic acid, C6H5CO2H, undergoes dissociation in water at 25*C with the following thermodynamic parameters:
H* = -67 cal mol-1
S* = -19.44 cal deg-1 mol-1
Write the equation for this dissociation process. What kind of reaction is it? Calculate Ka and pKa for benzoic acid. How does Benzoic acid compare in strength with acetic acid, whose pKa = 4.7
---------------------------------------------------------------------------

C6H5CO2H + H2O <-----> C6H5CO2(-) + H3O(+)

This process is a typical Bronsted-Lowry acid-base reaction. In order to calculate the equilibrium constant, we must first determine G* at 25*C, or 298 Kelvins:

So basically my answer is G* = +5.726 kcal mol-1

I've got that far, but I don't know how to find Ka using that.

I know how to change Ka into pKa and compare Benzoic acid to acetic acid whose pKa=4.7, but how do I get Benzoic acid's K or Ka with only what they've given me. Organic Chemistry Structure and Function fourth edition by K.Peter C. Vollhardt & Neil E. Schore isn't a bad book but it's mostly vague on any lessons. It doesn't go into comprehensible details like some of the first books I've read to get this far, nore does it include enough exercises for practice. Thank you for your time!

It just says Using G* = -1.36 log K, we obtain log Ka = -4.2,

How did they get "-1.36 log K" from
G* = +5.726 kcal mol-1
and how did they obtain the Ka from it?

sparkgap
International Hazard

Posts: 1234
Registered: 16-1-2005
Location: not where you think
Member Is Offline

Mood: chaotropic

From here, we have the formula

ΔG°=-RT*ln Ka

You've computed ΔG°, so it's a matter of determining the proper units to use for the gas constant R, and then solving for Ka. As for pKa, the thing to know about p-functions in general is that

pX=-log10 X

(e.g. pH is the negative common logarithm of the hydrogen ion concentration, pKsp is the negative common logarithm of the solubility product, etc.)

I should hope these hints should be sufficient.

sparky (~_~)

P.S. Some logarithm identities might also be of use; in particular, logb x = (loga x)/(loga b)...

[Edited on 3-12-2012 by sparkgap]

"What's UTFSE? I keep hearing about it, but I can't be arsed to search for the answer..."
yoyoils
Harmless

Posts: 22
Registered: 6-8-2011
Member Is Offline

Mood: positive

can you tell me how to answer it please or give me an example with how to use the calculator. Because I learn better that way

yoyoils
Harmless

Posts: 22
Registered: 6-8-2011
Member Is Offline

Mood: positive

You said I've computed G*, and so it's a matter of determining the proper units to use for the gas constant R.

With that example, I will do my best to assume

ΔG°=-RT*ln Ka
ΔG°=[(negative gas constant)](multiplied?)[Absolute Temp in Kelvins] *(multiply?) ln Ka

I see ΔG°= negative number multiplied by absolute temp, but i don't know what else. Do I use LN or eX buttons on the calculator?

I learned how to do logarithms because it was simply the log button and negative log button to compute pH,pOH,Ka,pKa,Kb,pKb. It's just difficult for me without help on how to use the calculator.

watson.fawkes
International Hazard

Posts: 2793
Registered: 16-8-2008
Member Is Offline

Mood: No Mood

 Quote: Originally posted by yoyoils help on how to use the calculator.
Here's some help: http://lmgtfy.com/?q=how+do+I+use+a+scientific+calculator%3F

 Sciencemadness Discussion Board » Fundamentals » Beginnings » Help with Benzoic acid pKa&Gibbs free energy Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues