Lily P.

asked • 01/16/21

pH of organic acids

Hi!


I have started writing a lab report about a specific lab that I will be conducting on Wednesday, but before I do that lab I was hoping to finish writing down some background information in the report so that I can write the result and discussion part after the lab.


Anyway, I would like to write about benzoic acid and stearic acid, amongst other things. I have started writing about organic acids in general and about the chemical properties of benzoic acid and stearic acid. Of course, benzoic acid is more acidic than stearic acid, but I kinda stopped there because I cant really understand why. I hope that someone could help me in explaining why benzoic acid is more acidic, because I really feel I can't move on without understanding that.


Thanks!

2 Answers By Expert Tutors

By:

Robert S. answered • 01/16/21

Tutor
4.9 (113)

Dr Bob Loves Science (especially chemistry and math)
About this tutor ›
About this tutor ›

Hello, Lily,


Nice to hear you are thinking ahead. I don't have a definitive (i.e., I didn't look for a technical publication) answer to why benzoic acid is more acidic than stearic acid, but if we look at the two structures, we can justify a reason.


First, benzoic acid is a much smaller molecule than stearic acid:


Benzoic: C6H5COOH molar mass: 122 g/mole pH around 4 (in water)

Stearic: CH3(CH2)16CO2H molar mass: 284 g/mole pH around 6 (in water)


Both molecules have just one carboxylic acid group (-CO2H). That fact alone puts stearic acid at a "disadvantage" when competing for the world acidity championship. The fact that is a long chain of 16 CH2 groups adds to it's misery, since most of the molecule wants to avoid water.


A second reason is that benzoic acid is composed of a benzene ring (the C6H5 in the formula) to which a carboxylic acid group (-COOH) group is attached. The ring of alternating double-bonded carbons creates what's know as a resonance structure, (termed "aromatic") where the electrons flow freely between the ring's carbon valence electrons. This stabilizes the structure, but also creates a slight attraction for the valence electrons in the oxygen on the carboxylic acid group. This makes it slightly easier for a H+ ion to leave, making it slightly more acidic.


I hope this helps,

Bob

Upvote • 1 Downvote
More
Report

Lily P.

Thank you so much for your explanation :)
Report

01/17/21

Robert S.

tutor
I'm happy you liked it. J.R. S. makes several additional excellent points. -Bob
Report

01/17/21

Lily P.

Thank you! I appreciate all the help from both of you :)
Report

01/17/21

J.R. S. answered • 01/16/21

Tutor
5.0 (145)

Ph.D. University Professor with 10+ years Tutoring Experience
About this tutor ›
About this tutor ›

The fundamental characteristic of an acid is to produce H+ ions. This is possible only when it is soluble in water. So, you may have a slight problem with stearic acid. But, as far as what makes one acid stronger than another, in general, would be the ease with which the H can dissociate. This is indicated by the Ka for weak acids. For example, the Ka for benzoic acid is about 6x10-5. I don't know what the Ka for stearic acid would be, if it can even be determined because of the limited solubility. Good luck with this.

Upvote • 1 Downvote
More
Report

Lily P.

Thank you for your answer! So, can I exemplify this point by saying that for example if we mix stearic acid with water, the pH would not change so much and stick to 7, because stearic acid will have a difficulty to be soluble in water and produce hydrogen ions?
Report

01/16/21

J.R. S.

tutor
Well, I guess you could say that but according to Dr. Bob's answer, the pH would be around 6. Not sure where that value comes from, but if correct, then your statement would be incorrect. Also, pH will depend on the concentration of the acid that is dissolved in water. And I'm not sure what it proves to put stearic acid in water, have it probably just float on top, and then say the pH is 7. I'm not sure what your exact assignment is, but if it's only to compare organic acids, perhaps it would be easier to pick another soluble organic acid. Probably not the answer you wanted.
Report

01/16/21

Lily P.

Hi! Thanks for your answer! I have been assigned to specifically write about benzoic acid vs stearic acid, and I thought of writing about differences in acidity. But can I instead say that since stearic acid is less soluble in water, less of it's molecules will react and produce hydrogen ions and therefore the change in pH will not be so much and therefore be 6, in comparison to benzoic acid?
Report

01/17/21

Robert S.

tutor
Yes, Lily - stearic acid's low solubility accounts for much of it's inability to be a good acid. Stearic acid is produced by the "saponification" of animal fat, a process used to convert animal fats into a variety of products. The long chain carboxylic acids were found to be good surfactants whose discovery can be traced back to ancient Greece. The story is that the Greeks tribes who cooked over campfires discovered mixing the wood ash with the animal fat would create what we now call soaps. The discovery was near Mount Sapo, and thus the process took on its name. Soaps help water become more effective in washing away dirt and oils. They reduce the surface tension of water so that it can dissolve oils and dirt. The surfactant bonds to water with its carboxylic head and the oily stain with its carbon-rich tail. Eventually the soap allows the water to surround the dirt or oil and carry it away with agitation, as an emulsion.
Report

01/17/21

Lily P.

Ok, thanks for the answer and interesting history of stearic acid
Report

01/18/21

Still looking for help? Get the right answer, fast.

Ask a question for free

Get a free answer to a quick problem.
Most questions answered within 4 hours.

OR

Find an Online Tutor Now

Choose an expert and meet online. No packages or subscriptions, pay only for the time you need.