As you know, when ethanoic acid (CH3COOH) is dissolved in water, the solution formed will be acidic due to the presence of the dissociated H+ ions. But what happens if you dissolve a CH3COO– ion by itself into water. Will the solution be acidic/alkaline/or neutral?
Tell me why! (Ain’t nothing but…)
Again, we need to make some reference to your existing knowledge. As you know, ethanoic acid is a weak acid and dissociates partially in water to produce H+ ions and CH3COO– ions.
The fact that it dissociates only partially implies that the CH3COOH is very unwilling to give up its H+ yes? Put another way, the CH3COO– ion has relatively strong attraction to the H+ and is unwilling to give it up.
So, when you dissolve the CH3COO– ion by itself into water it is attracted to the H atom of the water (Remember that water is a polar molecule with permanent partial positive charges on the H and partial negative charge on the O).
It is then able to hydrolyse (break apart) the water molecule, stealing the hydrogen for itself to form a CH3COOH molecule, leaving behind an OH- ion causing the solution to be alkaline!
In equation form:
CH3COO– (aq) + H2O(l) ⇌ CH3COOH(aq) + OH– (aq)
Ah ha! I know what you guys are thinking… Since I form CH3COOH, the solution will be acidic no? Imagine this situation…
I dissolve 100 CH3COO- ions into water.
According to the equation above, I should get 100 CH3COOH molecules and 100 OH- ions. (Stage1)
As you know, CH3COOH is a weak acid, so it dissociates partially in water. So out of 100 CH3COOH molecules, maybe 3 will dissociate to give me 3 CH3COO– and 3H+. (Stage2)
So, now, the solution contains 3 H+, 3 CH3COO–, 97 CH3COOH molecules (from stage 2) and 100 OH– ions (from stage 1). Since there are more OH– ions than H+ ions, the solution is alkaline! Mind reading complete!
Based on feedback from T1, I realize that perhaps, the table form isn’t the best presentation.. Perhaps the diagram below would help you understand better?
Thank you Ms S (Sec3, 2013) for asking such an interesting question & T1 for her feedback!