Dual-lye bar soap trial

[LisaBoBisa]

I teach gen chem and would love to have you in my class! It’s fun when students “connect the dots” to real life!

DeeAnna had a very good explanation. I will add that the electonegativity difference between Na and K is so small that it probably won’t make much difference since they are in solution with water. The ion that is closest to the citrate will grab it.

Awww, thanks! It's been fascinating to see how profs adapt to the pandemic; mine created a discord server for our class to ask questions, and has a channel in there for chem we run into outside of class. Soapmaking, NaOH, and bath bombs get mentioned a lot in there since I'm in the class...

Your specific "why" is so helpful! I'll just dissolve my citric acid first, then, then add the NaOH, let it react, then add the KOH.

 

[DeeAnna]

Remember that this is an ionic interaction, not a covalent reaction. In other words, you aren't making the molecules sodium citrate and/or potassium citrate. You're making sodium, potassium, and citrate ions.

Na or K is strong enough to get this dissociation reaction done. It doesn't greatly matter which one you start with, because, again, this is an ionic interaction between the acid and the alkalis to form ions, not molecules. After the citric acid breaks apart (dissociates), the citrate ions, K ions, Na ions, and OH ions go their merry way.

 

[LisaBoBisa]

Remember that this is an ionic interaction, not a covalent reaction. In other words, you aren't making the molecules sodium citrate and/or potassium citrate. You're making sodium, potassium, and citrate ions.

Na or K is strong enough to get this dissociation reaction done. It doesn't greatly matter which one you start with, because, again, this is an ionic interaction between the acid and the alkalis to form ions, not molecules. After the citric acid breaks apart (dissociates), the citrate ions, K ions, Na ions, and OH ions go their merry way.

That totally changes the way I'll think about this--thank you! Sodium laurate is covalently bonded, though, right?

 

[DeeAnna]

That totally changes the way I'll think about this--thank you! Sodium laurate is covalently bonded, though, right?

No, it's ionic too. Any soap, including sodium laurate, is a salt of a strong base and weak acids.

 

[LisaBoBisa]

No, it's ionic too. Any soap, including sodium laurate, is a salt of a strong base and weak acids.

Learned in our last chapter that soap salts are the weak conjugate base of a product-favored reaction (because the strong base+weak acid side is unfavorable, too high energy)! So here's the question that's been bothering me since learning that: if soapmaking is a product-favored reaction, why does adding heat in HP drive the reaction to the product (finished soap) side? Wouldn't adding heat drive it to the unfavorable/higher energy (reactant) side? Or does adding heat speed it up because there are so many triglyceride bonds to break first?

I know this is really getting off topic for the thread... thanks for taking so much time to answer questions--I know you have better things to do, and you guys have been insanely helpful.

 

[DeeAnna]

@LisaBoBisa -- "...does adding heat speed it up because there are so many triglyceride bonds to break first? ..."

Yep. Try making soap with just fatty acids and see the difference.

It takes a fair bit of energy to break the covalent bonds in the fat between the fatty acids and the glycerol backbone. That is why a strong hydroxide alkali (NaOH or KOH) and some warmth are helpful. But as long as you're using NaOH or KOH, the warmth only accelerates the triglyceride breakdown; heat isn't strictly required. That is why a cold process method also works.

Soap can also be made using weaker carbonate alkalis (sodium carbonate, potassium carbonate) or even bicarbonate alkali (sodium bicarbonate) but heat and a longer reaction time is required with these alkalis.