Naming Hydrates

 

Learning Objective

  • Generate the chemical formula and systematic name of a given inorganic hydrate

Key Points

    • Hydrates are named by the ionic compound followed by a numerical prefix and the suffix “-hydrate. ” The “· nH2O” notation indicates that “n” (described by a Greek prefix) number of loosely bonded water molecules are associated per formula unit of the salt.
    • An anhydride is a hydrate that has lost water. A substance that does not contain any water is referred to as anhydrous.
    • In organic chemistry, a hydrate is a compound of water, or its elements, with another molecule. Glucose, C6H12O6, was originally thought of as a carbohydrate (carbon and water), but this classification does not properly describe its structure and properties.

Terms

  • anhydrideAny compound formally derived from another (or from others) by the loss of a water molecule; a molecule with no water.
  • hydrateA solid compound containing or linked to water molecules.
  • carbohydrateA sugar, starch, or cellulose that is a food source of energy for an animal or plant; a saccharide

Inorganic Hydrates

“Hydrate” is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains loosely bonded water. The name of a hydrate follows a set pattern: the name of the ionic compound followed by a numerical prefix and the suffix “-hydrate.” For example, CuSO4 · 5 H2O is “copper(II) sulfate pentahydrate.” The notation of hydrous compound · nH2O, where n is the number of water molecules per formula unit of the salt, is commonly used to show that a salt is hydrated. The “[latex]\cdot[/latex]” indicates that the water is loosely bonded to the ionic compound. The “n” is usually a low integer though it is possible for fractional values to exist. The prefixes are the same Greek prefixes used in naming molecular compounds. Therefore, in a monohydrate “n” is one; in a hexahydrate “n” is 6, and so on.

The Greek prefixes used in naming hydrates for numbers 1/2 through 10 are as follows:

  • 1/2: hemi-
  • 1: mono-
  • 2: di-
  • 3: tri-
  • 4: tetra-
  • 5: penta-
  • 6: hexa-
  • 7: hepta-
  • 8: octa-
  • 9: nona-
  • 10: deca-

A hydrate that has lost water is referred to as an anhydride. An anhydride can normally lose water only with significant heating. A substance that no longer contains any water is referred to as anhydrous.

Organic Hydrates

In organic chemistry, hydrates tend to be rarer. An organic hydrate is a compound formed by the addition of water or its elements to another molecule. For example, ethanol, CH3–CH2–OH, can be considered a hydrate of ethene, CH2=CH2, formed by the addition of H to one C and OH to the other C. Another example is chloral hydrate, CCl3–CH(OH)2, which can be formed by the reaction of water with chloral, CCl3–CH=O.

Molecules have been labeled as hydrates for historical reasons. Glucose, C6H12O6, was originally thought of as C6(H2O)6 and was described as a carbohydrate, but this is a very poor description of its structure given what is known about it today. Methanol is often sold as “methyl hydrate,” implying the incorrect formula CH3OH2. The correct formula is CH3–OH.

Cobalt(II) chloride hexahydrateCoCl2·6H2O has the systematic name cobalt(II) chloride hexahydrate.