# The Acidity of the Hydrated Ferric Ion

1. May 10, 2014

### Qube

Why is $Fe(OH_{2})_{6}^{3+}$ fairly acidic? This iron has six water molecules coordinated to it.

In other words, water itself is a very weak acid. But when water is coordinated to iron, it becomes more acidic. I can think of a few reasons; can you think of more?

1) Induction. The negative charge generated through loss of the hydrogen proton is stabilized by the coordinated water molecules. Plus the center of the square planar molecule is greatly positively charged (and therefore can handle the negative charge, despite the fact that iron itself is actually electropositive rather than electronegative - at least in iron's ground state).

2) Size of the ferric ion. The ferric ion is rather big (it is, after all, able to coordinate six water molecules, as opposed to only four for some other metal ions). So whatever positive charge it has after being ionized once is spread out over a large surface area and that makes entire compound relatively stable. This is an assessment of conjugate stability.

3) Oxidation state of the central ferric ion - positive 3. This indicates a high degree of ionic character in the coordination bonds. Withdrawal of electron density from the water molecules makes the hydrogens even more positively charged and thus even more electrophilic.

4) Positive charge density of the central ferric atom. Coulomb's law is in (significant) effect here. Positive and positive. Like charges repel. Kicking off a hydrogen proton - no problem.

2. May 10, 2014

### AGNuke

Try to look the acidity of Ferric ion in terms of Lewis Acid. It is striped of 3 electrons, so it can accept electron.

1) I don't know what to say here, except try to evaluate the acidic strength of the ferric ion in terms of ferric ion alone.

2) Size of ferric ion is quite less than ferrous ion, yet it is far more acidic than the latter. Hence you need to reverse your statement here.

3) Higher oxidation states do help, no doubt.

4) Positive charge density is a valid point, causing it to leech out hydroxyl group from water and thus producing protons and thus acidity.

3. May 10, 2014

### Qube

True, I should have been more clear in my original statement rather than have simply tacked on a sentence at the end that I am assessing conjugate stability there.

I was trying to say that the conjugate base of the hydrated ferric ion, the ferrous ion, is bigger than the ferric ion. Cations get progressively smaller with increasing positive charge because the stripping of the electrons increases $Z_{eff}$, thereby causing the nucleus to pull the electron cloud closer and making the atom smaller.

By nature of the ferrous ion being bigger and having less positive charge, the ferrous ion has to be more stable - we have both an increase in size and a decrease in charge - leading to a definite decrease in positive charge density - and therefore a definite increase in stability. The more stable the conjugate base, the stronger the acid.

4. May 10, 2014

### AGNuke

True that. We can determine the direction of the reaction (Acid to conjugate base in the example) by roughly judging the stability of the products in the test condition.

5. May 10, 2014

### Qube

Sweet. Thank you again for helping me, a new to organic chem student. I feel well prepared thanks to you and my gen chem teacher (I think I got my gen chem basically down).

6. May 10, 2014

### Qube

This was what I was getting at; did you cover it in your chem classes? Just curious.

http://en.wikipedia.org/wiki/Inductive_effect

7. May 10, 2014

### AGNuke

We used that series to determine the acidic strength of organic cations/anions/radicals, but I never once used it to test the strength of complexes. For me, these two things were covered in a very different ways.

8. May 10, 2014

### Qube

Ah. Okay. My gen chem teacher actually didn't use induction to characterize the acidity of hydrated metal ions but I think it would be valid; after all, the water molecules are bound somewhat together.

9. May 13, 2014

### yolo123

My general chemistry teacher emphasized on the induction effect. My chemistry level is unfortunately not high enough to go any further.