Cobalt(II) iodide is an inorganic chemical compound formed from the transition metal cobalt and the halogen iodine. It exists as a solid salt at standard temperature and pressure. Determining its specific chemical formula requires examining the charges of its constituent ions and the principle of electrical neutrality.
Deriving the Formula
The systematic name “Cobalt(II) iodide” provides all the information necessary to determine the chemical formula. The name indicates that the compound is an ionic compound, consisting of a metal cation and a nonmetal anion.
The first part, “Cobalt(II),” refers to the cobalt ion (\(\text{Co}\)), and the Roman numeral (II) signifies its specific charge (\(\text{Co}^{2+}\)). This convention applies to transition metals like cobalt, which can form ions with multiple possible charges, such as \(+2\) and \(+3\). The second part, “iodide,” refers to the iodine ion (\(\text{I}\)), which typically forms an anion with a \(-1\) charge (\(\text{I}^{-}\)).
For an ionic compound to be stable, the overall positive charge must exactly balance the overall negative charge, resulting in a net charge of zero. To neutralize the \(+2\) charge of a single cobalt ion (\(\text{Co}^{2+}\)), two iodide ions (\(\text{I}^{-}\)) are required. Combining the symbols and their subscripts gives the final chemical formula: \(\text{CoI}_2\).
Basic Physical Characteristics
Cobalt(II) iodide is a solid material at room temperature, but its appearance depends on the presence of water molecules, a condition known as hydration. The anhydrous, or water-free, form of cobalt(II) iodide (\(\text{CoI}_2\)) is a dark blue, blue-black, or black crystalline solid. This contrasts sharply with its hydrated form, most commonly the hexahydrate (\(\text{CoI}_2 \cdot 6\text{H}_2\text{O}\)), which presents as a reddish or pink solid. The color difference results from the change in the coordination environment around the cobalt ion when water molecules are present.
The compound is notable for its high solubility in water, dissolving readily to form a solution that typically appears pink or red due to the formation of hydrated cobalt complexes. Beyond water, the compound is also soluble in certain polar organic solvents, including ethanol, and slightly soluble in acetone. The anhydrous form has a molar mass of approximately 312.74 grams per mole.
Practical Applications
The most recognized application of cobalt(II) iodide is its use as a colorimetric indicator for moisture and humidity. This function is directly related to its ability to undergo a reversible color change between its anhydrous (blue) and hydrated (red/pink) states. This property makes it useful in devices like humidity indicator cards, which provide a visual check on the moisture level within sealed packaging for moisture-sensitive goods.
Another significant role for cobalt(II) iodide is its application as a catalyst in various chemical reactions. Catalysts are substances that increase the rate of a chemical reaction without being consumed themselves. Specifically, it is employed in certain organic synthesis processes, aiding in the preparation of complex organic compounds.