Hydrates are chemical compounds that incorporate water molecules directly into their crystalline structures. This article explains the systematic method used to name these substances, providing a clear understanding of how chemists identify and communicate about them.
Understanding Hydrates
Hydrates are ionic compounds that contain a definite number of water molecules within their crystal lattice. These water molecules are not chemically bonded to the ions but are associated through weaker intermolecular forces. They are integral to the compound’s structure, filling specific sites within the crystal framework.
The presence of these water molecules distinguishes a hydrated compound from its anhydrous form, which lacks water. For example, anhydrous copper(II) sulfate is a white powder, but when it forms a hydrate, copper(II) sulfate pentahydrate, it appears as a blue crystal. This change in physical properties highlights the importance of the water molecules in the hydrate’s overall characteristics. The water molecules can typically be removed by heating, leaving behind the anhydrous compound.
Core Naming Principles
Naming hydrates involves combining the name of the ionic compound with a term that indicates the number of water molecules. The first part identifies the ionic compound, following standard inorganic naming rules. This involves naming the cation first, then the anion, using a Roman numeral for transition metals if needed.
The second part of the name specifies the amount of water using a Greek prefix that corresponds to the number of water molecules, followed by the word “hydrate.” Common Greek prefixes include “mono-” for one, “di-” for two, “tri-” for three, “tetra-” for four, “penta-” for five, “hexa-” for six, “hepta-” for seven, “octa-” for eight, “nona-” for nine, and “deca-” for ten. By combining these two parts, the complete name of the hydrate is formed, providing a clear and specific identification.
Step-by-Step Naming Examples
To name MgSO₄·7H₂O, first identify the ionic compound. MgSO₄ is magnesium sulfate, where magnesium is a fixed-charge metal and sulfate is a polyatomic anion. Next, count the seven water molecules. The prefix for seven is “hepta-.” Combining these parts, the full name is magnesium sulfate heptahydrate.
For CuSO₄·5H₂O, the ionic compound is copper(II) sulfate, as copper is a transition metal that can have varying charges. There are five water molecules present. The prefix for five is “penta-.” The full name for this hydrate is copper(II) sulfate pentahydrate.
For BaCl₂·2H₂O, the ionic compound is barium chloride, since barium is a fixed-charge metal. The formula shows two water molecules. The prefix for two is “di-.” Thus, the complete name for this hydrate is barium chloride dihydrate.
From Name to Formula
Converting a hydrate’s name back into its chemical formula requires reversing the naming process. Begin by identifying the ionic compound’s formula from its name. For instance, if the name is calcium chloride dihydrate, “calcium chloride” indicates the ionic compound. Calcium (Ca²⁺) and chloride (Cl⁻) combine to form CaCl₂, ensuring the charges balance.
Next, determine the number of water molecules from the Greek prefix and the “hydrate” suffix. “Dihydrate” signifies two water molecules. Finally, combine the ionic compound formula with the water molecules using a dot (·) to show their association within the crystal structure. Therefore, calcium chloride dihydrate becomes CaCl₂·2H₂O.
Similarly, for iron(III) chloride hexahydrate, the ionic compound is iron(III) chloride. Iron(III) means Fe³⁺, and chloride is Cl⁻, so the formula is FeCl₃. “Hexahydrate” indicates six water molecules. Combining these parts results in the formula FeCl₃·6H₂O. This systematic approach allows for the accurate representation of hydrate structures from their names.