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May 6 2012 1 06 /05 /May /2012 03:04


Hydrolysis is the reaction of a compound or ion with water. Most hydrolysis reactions in inorganic chemistry result in the donation or removal of a proton by water, forming either hydroxide or hydronium ions. Two sample hydrolysis reactions are:

NH4(+) + H2O à NH3 + H3O(+) forms an acidic solution


CN(-) + H2O à HCN + OH(-) forms an alkaline solution

Here, ammonium ion reacts with water, generating ammonia and hydronium, which makes an acidic solution. Below, cyanide ion reacts with water as well, generating hydrogen cyanide and hydroxide. Learn more by taking a General Chemistry II course.

These reactions happen with salts of weak acids or weak bases, which is what gives the salts their pH. Many other chemical compounds form their unique hydrolysis products which can be interesting. Most metal oxides are weak bases, making their salts prone to hydrolysis. The less basic the oxide is, the stronger the hydrolysis of the salt and the more acidic the resulting solution. Below several metal and semimetal ions are listed, along with their hydrolysis properties and any uses of such properties.


Aluminium: Aluminium forms a stable complex with water, but solutions of aluminium with strong acids are highly acidic, showing the weakness of aluminium(III) hydroxide as a base.


Antimony(III) and arsenic: This ion is stable in strongly acidic solutions. When diluted with water, it produces a white precipitate of antimony oxychloride in a similar manner to bismuth. However, the hydrolysis happens at an even lower pH. Arsenic(III) hydrolyzes even more easily than antimony(III) in water. Arsenic pentoxide, however, is insoluble in concentrated hydrochloric acid, preferring instead to dissolve in water, forming its own acidic solution. When the nonmetals are reached, the oxides no longer keep up any pretense of basicity.


Bismuth(III): When a colorless acidic (pH 1 or so) solution is bismuth(III) chloride is diluted with water, a white precipitate of bismuth oxychloride is instantly formed, even though the pH remains around 2. Bismuth(III) oxide is a very weak base and so its salts are highly acidic and prone to hydrolysis. This is how bismuth oxychloride, a substance used in cosmetics, is created.


Monovalent ions: Monovalent ions hardly even have an acidic pH in most cases, showing that they do not hydrolyze at all. The alkali metal salts with strong acids are completely neutral. Other monovalent ions (thallium, silver, copper) can have different properties because of their greater proximity to the right side of the periodic table, but none are easily hydrolyzed AFAIK.


Most divalent ions:  Copper shows a very small tendency toward hydrolysis. The production of an acidic pH of 3 and higher is the most hydrolysis that occurs. Therefore, aqueous solutions of metals which form divalent ions (cobalt, copper, manganese, iron, nickel, zinc, cadmium, mercury, lead, alkaline earth metals) are often the easiest to study in amateur chemistry due to ease of dissolution in water.


Niobium(V): Niobium pentachloride is a yellow solid that hydrolyzes completely in water and in moist air. Even in the most acidic solutions it slowly hydrolyzes, depositing white niobium pentoxide.


Phosphorus: Although not technically an ion, simple phosphorus compounds are highly prone to hydrolysis. Phosphorus trichloride, a colorless liquid, fumes upon contact with water, forming phosphorous acid and hydrochloric acid. Phosphorus pentachloride also has a vigorous reaction. The reaction between phosphorus triiodide and water (which creates hydriodic acid) is used in illegal drug manufacture.


Silicon compounds: Silicon tetrachloride, a typical binary silicon compound, hydrolyzes rapidly with water, releasing silicic acid (hydrated sand, essentially) as a colorless gel along with hydrogen chloride fumes. Because of silicon tetrachloride's unique properties, it is used to produce high-purity silica gel by mixing with water.


Sulfuryl chloride: This compound hydrolyzes much more slowly in water. I have heard that it can take weeks for a layer of sulfuryl chloride to completely react with water.


Thionyl chloride: This compound reacts vigorously with water, releasing sulfur dioxide gas and hydrogen chloride fumes in large amounts, as I accidentally experienced upon opening a lithium battery containing the substance. The high acidity of this complex is used to prevent sensitive metal chlorides (e.g. rare earth chlorides) from hydrolyzing as they are dehydrated. The thionyl chloride reacts with any released water, releasing a cloud of acidic gases that keep the anhydrous chloride stable.


Tin(II): A solution of tin(II) chloride is stable in hydrochloric acid, but slowly hydrolyzes when diluted, forming a white precipitate of tin(II) oxychloride. This is not desired in most circumstances, and is prevented by using hydrochloric acid.


Tin(IV): Solutions of tin tetrachloride in water are generally turbid to some degree. Because tin dioxide is a weak base, the solution hydrolyzes easily, forming white insoluble tin dioxide. The anhydrous form of tin tetrachloride fumes upon contact with air or water. This was used in the past as a naval smokescreen.


Titanium(IV): Titanium tetrachloride, one of the common tetravalent titanium compounds, hydrolyzes very strongly. When the anhydrous compound is sprayed into air, it forms a dense white smoke as a result of reacting with the water vapor in the air. This is used to determine air flow in a room or to test smoke detectors for effectiveness.




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