Sciencemadness Discussion Board

Predicting Hydrolysis

AndersHoveland - 10-5-2012 at 21:33

Predicting whether a compound will hydrolyse (react) with water can be complicated for those with less experience in chemistry. There are several factors which determine this, including atomic radius size, electronegetivity, double bonds, steric hindrance, and even physical solubility. Without knowing these factors, the reactivity of the following selected compounds could seem inexplicable.


Compounds that hydrolyse in water:

COCl2 (phosgene)
SF4
SCl2
CSO (carbonyl sulfide)
CH3Br (bromomethane, insecticide)
CH3I


Compounds that do not hydrolyse in water:

CCl4 (carbon tetrachloride*, used in some fire extinguishers)
SF6 (sulfur hexafluoride, non-toxic)


Compounds that can reversibly hydrolyse with water:

CO2
SO2
NCl3 (dangerously unstable explosive)


Compounds that only very slowly hydrolyse with water:

CS2 (half-life 1.1 years at pH 9)
CHCl3 (chloroform, half-life of 15 months)
CH3NO3 (methyl nitrate, explosive ester)
SO2F2 (sulfuryl fluoride, used as termite fumigant, solubility in water about 1g/L, hydrolysis half-life 3 days at pH 7, but hydrolyses much faster under alkaline conditions, half-life 10 minutes at pH 8.3)
* actually CCl4 does does hydrolyse, but the reaction rate is neglibable under ordinary conditions, a half life of 7000 years has been calculated)


Hopefully this topic will provoke a discussion.

[Edited on 11-5-2012 by AndersHoveland]

woelen - 10-5-2012 at 22:34

I think that one can state that high oxidation state oxides and chloro-oxides hydrolyse to form the associated acid(s). E.g. SOCl2 gives HCl and H2SO3 (which further decomposes to SO2 and H2O in an equilibrium reaction). CrO2Cl2 gives chromic acid and hydrochloric acid.

The same is true for covalent oxides and halides in the right hand part of the periodic table (e.g. Cl2O7, NO2, PCl5, PCl3).

Lambda-Eyde - 10-5-2012 at 22:44

Quote: Originally posted by woelen  
I think that one can state that high oxidation state oxides and chloro-oxides hydrolyse to form the associated acid(s). E.g. SOCl2 gives HCl and H2SO3 (which further decomposes to SO2 and H2O in an equilibrium reaction). CrO2Cl2 gives chromic acid and hydrochloric acid.

But why are the high oxidation states important when there's no redox occuring in a hydrolysis?

woelen - 10-5-2012 at 23:13

The high oxidation state of the central atom (e.g. "Mn(7+)" in Mn2O7 or "Cr(6+)" in CrO2Cl2) means higher concentration of positive charge near the central atom (better: lack of concentration of negative charge, which is more strongly drawn to the oxygen or halogen atoms).

Water has two lone pairs on its oxygen atom and one of these easily can attach (form a coordinating bond) to the relatively strongly positively charged central atom. When this occurs, then immediately a charge redistribution over the entire object occurs and usually this results in splitting off of one hydrogen ion from the water, which can connect to another oxygen or chlorine atom on the molecule. With a chlorine atom it recombines to form HCl, which splits off, with an oxygen atom it form a OH-group, which remains attached to the central high oxidation state atom.

Example of reaction of SO3 and H2O to H2SO4:

O3S + :OH2 --> O3S:OH2 --> [O3S:OH](-) + H(+) <---> O2S(OH)2



For the same reason, aqua ions of metal ions in high oxidation states easily hydrolyse, giving acidic solutions.

E.g. [Fe(OH2)6](3+) has, due to the relatively highly positively charged central iron atom its electrons drawn inside relatively strongly. The :OH2 groups attached to it are bound relatively strongly due to this effect and hence the Fe:O bonds are quite strong. The O-H bonds are weakened due to this shift of charge and there is fairly easy splitting off of H(+). So, in solution you get [Fe(OH2)5(OH)](2+) and H(+). With Bi(3+) this effect is even stronger and with Sn(4+) and Ti(4+) this effect is so strong that these ions hardly can exist in aqueous solution, except maybe at very very low pH.

In the example above with O3S:OH2 the same effect occurs and the electrons of :OH2 are drawn inside so much that the S:O bond becomes much stronger than the O-H bonds and one of the H atoms is expelled as H(+) completely.

AJKOER - 11-5-2012 at 14:26

Another way of predicting that a salt is subject to 'sudden' hydrolysis is whether it is a heavy metal salt. This may be a function of atomic radius size and/or bonding.

For example, FeCl3 on standing for week(s) breaks down into Fe2O3.xH2O and HCl. Aqueous Silver acetate behaves simarily. Even more rapidly, Zinc hypochlorite and also Lead hypochlorite in water (which upon heating forms PbO2 and HCl).

CHRIS25 - 13-5-2012 at 17:24

========Another way of predicting that a salt is subject to 'sudden' hydrolysis is whether it is a heavy metal salt.========

So Ferric Nitrate would break down to Ferrous oxide (2) but not nitric acid, instead NO2 nitrogen dioxide would continue to evolve? Oh thinking about it yes the water would contain the nitric oxide and therefore become nitric acid?

Hey look, I know I am interfering with your discussion but just checking my thoughts with what I am reading.

weiming1998 - 17-5-2012 at 06:29

Quote: Originally posted by CHRIS25  
========Another way of predicting that a salt is subject to 'sudden' hydrolysis is whether it is a heavy metal salt.========

So Ferric Nitrate would break down to Ferrous oxide (2) but not nitric acid, instead NO2 nitrogen dioxide would continue to evolve? Oh thinking about it yes the water would contain the nitric oxide and therefore become nitric acid?

Hey look, I know I am interfering with your discussion but just checking my thoughts with what I am reading.


Fe(III) nitrate will not break down into Fe(II) oxide. Instead, it breaks down into NO2, Fe2O3 and O2. The actual process is as: 2Fe(NO3)3===>Fe2O3+3N2O5, the anhydride of HNO3. N2O5 is unstable, especially at such high temperatures, and immediately breaks down into NO2 and O2. This depends on the equilibrium. The mildly acidic Fe3+ reacts with H2O at high temperatures to drive off NO2 as a gas. This is why you can make HCl from SiO2, a chloride salt and water vapour. Link: http://www.sciencemadness.org/talk/viewthread.php?tid=11875#...
But in aqueous this hardly occurs, unless you try and drive this equilibrium ([Fe(H2O)6]3+<===>[FeOH(H2O)5]2(+)+ H+) to the right hand side with the addition of a base. In a neutral solution, this hydrolysis makes less than 10^-5 moles of nitric acid/mole of iron nitrate. Salts like Al2(CO3)3, Fe2(CO3)3 and even AlCl3 and FeCl3 are unstable and irreversibly hydrolyzes because of this equilibrium. The H+ ions generated in the hydrolysis reacts with the CO3(2-) ions to form carbonic acid, which bubbles out as CO2. The removal of H+ causes the equilibrium to be driven to the right, continuing liberating CO2 until one of the reactants are exhausted, creating a hydroxide/oxide precipitate as a product.

CHRIS25 - 17-5-2012 at 07:01

Hi Welming, thanks for that exhaustive explanation. I have just been realising what complex reaction Fe and Nitric acid is so still working on that one.

weiming1998 - 18-5-2012 at 02:04

Anyway, I would think that hydrolysis of an inorganic, mostly ionic compound depends on the Lewis acidity of the cation in the compound. If the Lewis acidity of the cation is high enough, paired with a conjugate base that forms/decomposes into a volatile (or just a gas), weak acid when acidified, the compound is most likely to hydrolyze upon contact with water.

Examples of this are mostly aluminum salts like Al2S3, Al2(CO3)3, Al2(S2O3)3, Al halides (heated to dryness), etc. Apart from Al salts, Fe salts (especially Fe3+), Zn salts, etc of volatile/gaseous acids, should hydrolyze as well.