Chemguide: Support for CIE A level Chemistry

Learning outcome 16: Hydroxy compounds

16.1: Alcohols

Statement 16.1.5

This statement asks you to describe and explain the relative acidities of ethanol and water.

Before you go on, you should find and read the statement in your copy of the syllabus.

You should read parts of the page explaining the acidity of organic acids.

This page also talks about the acidity of carboxylic acids and phenol, and eventually you will need to know all the information on this page. For the moment, read the introductory section down to, but not including, where it starts talking about ethanoic acid in detail. Then jump to the very short comment about ethanol about 2/3 of the way down the page.

The page doesn't, however, mention the acidity of water.

In fact, the equilibrium here is exactly the same as all the other ones discussed on that page. All I have done in the following equation is to replace the X in an equation on the page you have just read by an H.

Again, the extent of this ionisation depends on the stability of the hydroxide ion formed. This is very similar to the ion formed when an alcohol ionises. All the negative charge is located on the oxygen atom, and isn't spread around at all.

That means that you would expect the pKa of water to be similar to that of ethanol - and it is.

The pKa of ethanol is 16; that of water is 15.7.

There is a slight difference. Remember that the higher the value of pKa, the weaker the acid. So ethanol is a slightly weaker acid than water.

Note:  If you have done any work on Ka and Kw, you may be slightly worried about the 15.7 value of pKa for water.

This comes from the different ways in which Ka and Kw are defined. There is a fairly simple explanation of this on this site.

I am wary of links to other sites because they change. If this link doesn't work, please contact me via the address on the about this site page.

If ethanol is a slightly weaker acid than water, why is that?

In their syllabus support material, CIE offers an explanation of this in terms of the positive inductive effect of the alkyl group.

A positive inductive effect is often described as an "electron pushing effect". So an ethyl group will push electrons away from itself onto the oxygen atom in the ethoxide ion. That makes the oxygen even more negative, and so even more likely to pick up a hydrogen ion again to return to the alcohol.

Note:  If you aren't happy about this inductive effect of alkyl groups then you could look at this page about carbocations (not exactly what we are talking about here, but similar enough to be useful), and perhaps also this page about electronegativity (read the bit about inductive effects about half-way down the page).

Incidentally, I'm not too happy about this explanation because, although it works for most alcohols, it doesn't work for methanol. Methanol has a pKa of 15.5, which makes it a slightly stronger acid than water. I have no idea why that is, but any decent explanation should be able to account for exceptions like this.

For CIE purposes, for ethanol and water, you only need to learn the bit just above this box.

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