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Non Life Sciences => Chemistry => Topic started by: chris on 13/02/2019 19:45:18

Title: Is ethanol an acid or a base?
Post by: chris on 13/02/2019 19:45:18

Hello everyone.

While helping my daughter with her chemistry homework this evening I was asked a question that gave me pause for thought. Having thought about it, I'm still not sure, so now I'm asking you all!

Is ethanol a weak acid, or a weak base?

My intuition was that the H could be donated, under the right conditions, as a hydrogen ion, making ethanol a weak acid. But another source I happened upon said that ethanol was mildly basic and the pH of the pure alcohol is 7.33

Please explain!

Title: Re: Is ethanol an acid or a base?
Post by: chiralSPO on 13/02/2019 20:51:38

Alcohol, like water, is both a weak base and a weak acid (it is slightly weaker as an acid, and slightly weaker as a base*).

EtOH + H⁺ → EtOH₂⁺ (EtOH is a base)

EtOH → EtO^– + H⁺ (EtOH is an acid)

*The pKa of protonated ethanol is –2.4, while protonated water has a pKa of –1.7, and the pKa of ethanol is 16, while the pKa of water is 15.7 (according to this reference: http://cactus.dixie.edu/smblack/chem2310/summary_pages/pka_chart.pdf )

Title: Re: Is ethanol an acid or a base?
Post by: chiralSPO on 13/02/2019 21:04:14

Alcohol is unlikely to act as a base in aqueous solution, and I'm not sure how it would influence pH. However, recalling that pH is the negative log of the concentration of protons, I think the reason pure ethanol has a higher pH than water is that it has a significantly lower dielectric constant than water (about 24 vs 80), so any acid is less likely to dissociate when in ethanol than when in water (causing a lower concentration of protons, and thus a higher pH).

Title: Re: Is ethanol an acid or a base?
Post by: evan_au on 14/02/2019 10:50:09

Quote from: chris

Is ethanol a weak acid, or a weak base?

If I follow the above discussion, the answer is "yes"!