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Name: Carl
Status: educator
Grade: 9-12
Location: CA
Country: USA
Date: November 2008


Question:
Sodium hydroxide is a strong base, what about iron (III) hydroxide? I know it is sparingly soluble, but the amount that is soluble should be 100% dissociated. Would this make it a strong base which is only slightly soluble. Texts that I have seen do not seem to address this.



Replies:
There are a couple of issues involved here. First, the term "strong" can refer to the degree of dissociation or solubility. The classic examples are carboxylic acids which are referred to as "weak" acids, meaning the degree of dissociation. Second, the term "strong" vs. "weak" can also refer to the concentration of the aqueous solution. Do not take for granted that Fe(III) is the molecular species present in solution. That would have to be determined experimentally. But your distinction of degree of dissociation and solubility is correct. They are not the same thing.

Many texts are careless about that distinction.

Vince Calder


Carl,

We have to make a distinction between solubility/basicity of a solution and acid or base strength. pH or pOH measures the amount of H+ or OH- in solution - this indicates how acidic or basic a solution is. ka or kb indicates how much a mole of substance will dissociate in water and is an indicator of the acid or base strength of the substance. Thus, whereas NaOH is strong base because it is fully dissociated in water, a 0.0000002M solution (pH = 7.3) of NaOH would have a near neutral pH and hardly be basic at all, but still does not suggest that NaOH is not a strong base.

Greg (Roberto Gregorius)


You raise the issue "strong" vs. "weak" and "soluble" vs. "insoluble" terminology. These terms refer to different properties of the substance. A classical example: acetic acid in miscible with water in all proportions -- therefore it is totally miscible, that is, soluble. However, the species in solution change. The same can be said for sulfuric acid or nitric acid. Potassium hydroxide is highly ionized at all concentrations (assuming normal lab environments), but it has limited solubility. So it is a "strong" base. In contrast, barium hydroxide is only slightly soluble, but it still produces the same kinds of hydroxyl ions as potassium hydroxide.

The distinction is the amount of solute dissolved versus the relative amount of ionized vs. unionized species dissolved in the solvent.

Vince Calder


Hi Carl,

As you say, iron (iii) hydroxide is sparingly soluble. I think your questions is, what happens to the small amount of Fe(OH)3 that does dissolve? It is difficult because the process of solvation involves the compounds acid-base chemistry.

You seem to assume that it completely ionizes as below:

Fe(OH)3 (aq) ---> Fe3+ (aq) + 3 OH- (aq)

Fe3+ has +3 charge and is relatively small in size. Consequently, it would be very difficult to break off even one OH- ion and the second and third ionizations would be orders of magnitude smaller in extent. As far as I can tell the little bit of Fe (OH)3 (s) which dissolves only does so because of the small amount of H+ available in pure water (10-7 M):

Fe(OH)3 (s) + H+ (aq) <---> [Fe(OH)2]+ (aq) + H2O (l)

The equilibrium constant for this reaction is 4.5 x 10-3 . However, there are other equilibria to consider (which I will not try to reproduce here!) and this is a highly complex system, so this is not enough to calculate the pH with great accuracy.

Incidentally, if you dissolve FeCl3 in water you get a highly acidic solution because of the following two reactions:

Fe3+ (aq) + 6 H2O (l) ---> [Fe(H2O)6]3+ (aq) (formation)

[Fe(H2O)6]3+ (aq) + H2O (l) ---> [Fe(H2O)5(OH)]2+ (aq) + H3O+ (aq) (acid/base)

The second reaction has a pretty large equilibrium constant because Fe3+ is a great Lewis acid. And of course this means that Fe3+ has a strong tendency to combine with strong Lewis bases (like OH-)...

I hope this is helpful.

Best, Dr. Topper



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