How hydrolysis can take place to form an acidic solution when AlCl3 reacts with water?

Question

this was what my notes says:

AlCl3(s) + 6H2O(l) --> [Al(H2O)6]3+(aq) + 3Cl-(aq)



[Al(H2O)6]3+ (aq) --> [Al(H2O)5(OH)]2+ (aq) + H+ (aq)

Al3+ has high charge density, able to polarise and weaken O-H in H2O of [Al(H2O)6]3+(aq) to release acidic H+.

!!!!! can someone kindly elaborate more?

also... how Al3+ is produced? i cant see it in the equations...
what is high charge density?
what is hydrolysis?

thx... thx...

Answer 1

I can see where this might confuse you a litte, here. First off, I just want to point out that this is NOT a reaction that you want to do in the lab. AlCl3 reaction very violently with water (much the same way pure alkali metals do). [safety rant over]

Firstly, when the AlCl3 hits the water, it does what most salts do in a aqueous medium, they dissociates. This means that they become ionic and are free to do a variety of pretty chemistry that you might find in a gen chem course. Thusly, Al3+ if formed.

I am not sure how much you know about basic inorganic chemistry, but whenever most metallic ions becomes aqueous they like to suck up whatever free electrons are floatly around. Being removed from their counter ions (as in salts), they seek free electrons to complete there outer shell. Al3+ is an interesting ion since it is the smallest (hands down) metal with a large positive ionic form (+3). This is where the term "high charge density" comes from, a very small element with a very high charge.

When this ion sees the water molecules floating around, it does whatever all positively charged, metallic, aqueous ions do: it binds to the water. Water has free electrons, which account for waters bent geometry, and Al3+ pulls it towards the two opposing charges (positive ionic charge + free electrons). Thus an aquaphor is formed, a species which has bound water molecules (called "ligands") surrounding a metallic ionic core. All metals do this and it's these aquaphors which account for the pretty colors you see in salt solutions. This process, of attracting waters to a metallic ion, is called "hydrolysis."

Lastly, why acid is formed. Since Al3+ has SUCH a high charge density, it can actually break the bonds in water and create counter ionic (H+ and OH-). Now there are negative charges floating around, which the metal would rather be bonded to, so it does, thus lowering it's charge and leaving an excess of protons (H+) floating around making the water most acidic.

So, the aquatic species of any metals can be anywhere from:

[Al(H2O)6]3+ ......to...... [Al(H2O)3(OH)3]

and everywhere in between.

Answer 2

The Al in AlCl3 is considered to be Al3+ because the Cl ligand always carries a 1- charge (it's actually Cl-, not Cl).

High charge density means that Al3+ has a large positive charge in a relatively small area, so it's very good at pulling electrons away from other atoms or molecules.

Hydrolysis literally means "water-splitting." Here it refers to splitting water into OH- (which stays on the Al), and H+ (which enters the solution, making it acidic).

As for the reaction itself, in the first step, H2O molecules come in and surround the Al3+, pointing the O ends toward it and the H ends away. Water is polar--it has a partial negative charge on the oxygen--so that extra electron density helps to stabilize the Al3+ ion. It doesn't hold on to the Cl- ions as tightly anymore, so it releases them into solution. At the end of this step, you have [Al(H2O)6]3+ and 3Cl-.

Now the Al3+ REALLY wants those electrons on O, and it pulls them closer to itself. That means the electrons from water get pulled even further toward the O end and away from the H end. (That's what it means when it says it polarizes the water.) But the O is losing elecrton density too, because the electrons are going right past it and into the Al3+. So the O pulls the electrons from one of the O-H bonds into it's orbit, making it O- (half of OH-). The H leaves as H+, making the solution acidic. At that point, the Al3+ has five neutral water molecules and one OH- molecule bound to it, so the overall complex is [Al(H2O)5OH]2+.

Answer 3

ok, an uncomplicated one........... Calcium and sodium oxides are alkaline anhydrides...this is, they form alkalis in water...and copper (II) oxide, whilst a transition metallic oxide, isn't at risk of offer an acid in water. Which leaves NO2. sturdy success!