Problem 49 - Chemistry and Chemical Reactivity

Question

Name each of the following ionic compounds: (a) \(\mathrm{K}_{2} \mathrm{S}\) (b) \(\cos 0_{4}\) (c) \(\left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4}\) (d) \(\mathrm{Ca}(\mathrm{ClO})_{2}\)

Step-by-Step Solution

Answer

(a) Potassium sulfide; (b) Cobalt sulfate; (c) Ammonium phosphate; (d) Calcium hypochlorite.

Step 1: Identify the Cations and Anions

To name ionic compounds, first identify the cation (positive ion) and the anion (negative ion) in each formula. For example, in \( \mathrm{K}_{2} \mathrm{S} \), \( \text{K}^{+} \) is the cation (potassium) and \( \text{S}^{2-} \) is the anion (sulfide). Repeat this process for all compounds.

Step 2: Name the Cation and Anion

For each compound, use the periodic table or list of common ions to name the cation and anion. For example:- (a) \( \mathrm{K}_{2}\mathrm{S} \): potassium sulfide- (b) \( \mathrm{CoSO_{4}} \): cobalt sulfate- (c) \( \left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4} \): ammonium phosphate- (d) \( \mathrm{Ca}(\mathrm{ClO})_{2} \): calcium hypochlorite

Step 3: Correcting the Formula Identifications

Re-evaluate identification for compound (b), noting that the formula \( \cos 0_{4} \) seems incorrect, and assume a likely typographic error such as the compound \( \mathrm{CoSO}_{4} \), cobalt sulfate, due to context.

Step 4: Verify and Finalize Compound Names

Double-check each name for standard conventions. Ensure each name correctly represents the composition of the compounds. For example, compound (b) with formula \( \mathrm{CoSO}_{4} \) is best known as cobalt(II) sulfate, and similarly finalize names as appropriate.

Key Concepts

Cation Identification

In an ionic compound, the cation is the positively charged ion. Identifying the cation is essential because it forms the first part of the compound's name. Cations are often metal ions, and their charge is typically determined from their position in the periodic table. For instance, in the compound \( \mathrm{K}_{2}\mathrm{S} \), the cation is \( \text{K}^+ \), which represents potassium. Potassium, a Group 1 element, typically forms a +1 charge.
When naming the cation:

• Use the element name as it appears on the periodic table.
• If the cation can have more than one charge (as with transition metals), indicate the charge using Roman numerals in parentheses. For example, cobalt can be \( \text{Co}^{2+} \) or \( \text{Co}^{3+} \).

Anion Identification

Identifying the anion, which is the negatively charged ion in an ionic compound, is just as crucial as identifying the cation because it forms the second part of the compound's name. Typically, anions are nonmetals or polyatomic ions. In \( \mathrm{K}_{2}\mathrm{S} \), the anion is \( \text{S}^{2-} \), which corresponds to the element sulfur in its sulfide form.
For simple anions:

• Change the element's name ending to "-ide." For example, chlorine becomes chloride and oxygen becomes oxide.

In cases of polyatomic ions, use specific names such as sulfate or phosphate as seen in compounds like \( \mathrm{CoSO}_4 \) and \( \left(\mathrm{NH}_4\right)_{3} \mathrm{PO}_{4} \). Remember that polyatomic ions retain their specific names regardless of the element they are bonded to.

Periodic Table

The periodic table serves as a valuable tool not only for identifying cations and anions but also for predicting the charge of ions. Groups or columns of the periodic table often have characteristic charges when these elements become ions. Elements in the same group typically form ions with the same charge due to similar electronic configurations.
For example:

• Alkali metals in Group 1 like potassium form \( +1 \) ions.
• Alkaline earth metals in Group 2 like calcium form \( +2 \) ions.
• Halogens in Group 17, such as chlorine, form \( -1 \) ions.

The periodic table also helps in identifying the correct names of ions, ensuring you follow standard naming conventions. For transition metals, commonly found in the central block, the table can hint at multiple possible charges.