Problem 64 - Chemistry: The Central Science

Question

Give the names and charges of the cation and anion in each of the following compounds: (a) CuS, (b) \(\mathrm{Ag}_{2} \mathrm{SO}_{4}\), (c) \(\mathrm{Al}\left(\mathrm{ClO}_{3}\right)_{3}\), (d) \(\mathrm{Co}(\mathrm{OH})_{2}\), \((\mathrm{e}) \mathrm{PbCO}_{3}\)

Step-by-Step Solution

Answer

(a) CuS: Cation - Copper(II) (Cu²⁺), Anion - Sulfide (S²⁻) (b) \(\mathrm{Ag}_{2} \mathrm{SO}_{4}\): Cation - Silver (Ag⁺), Anion - Sulfate (SO₄²⁻) (c) \(\mathrm{Al}\left(\mathrm{ClO}_{3}\right)_{3}\): Cation - Aluminum (Al³⁺), Anion - Chlorate (ClO₃⁻) (d) \(\mathrm{Co}(\mathrm{OH})_{2}\): Cation - Cobalt(II) (Co²⁺), Anion - Hydroxide (OH⁻) (e) \(\mathrm{PbCO}_{3}\): Cation - Lead(II) (Pb²⁺), Anion - Carbonate (CO₃²⁻)

Step 1: (a) Identify the cation and anion in CuS (Copper(II) sulfide).

CuS is composed of copper (Cu) and sulfur (S) ions. Copper has a charge of +2 in this compound, which is denoted as Copper(II) (Cu²⁺). Sulfur has a charge of -2 (S²⁻). Therefore, the cation is Copper(II) with a charge of +2, and the anion is Sulfide with a charge of -2.

Step 2: (b) Identify the cation and anion in \(\mathrm{Ag}_{2} \mathrm{SO}_{4}\) (Silver sulfate).

\(\mathrm{Ag}_{2} \mathrm{SO}_{4}\) is composed of silver (Ag) and sulfate (SO₄) ions. In this compound, silver has a charge of +1 (Ag⁺), and there are two silver ions. Sulfate is a polyatomic ion with a charge of -2 (SO₄²⁻). So, the cation is Silver (Ag) with a charge of +1, and the anion is Sulfate (SO₄) with a charge of -2.

Step 3: (c) Identify the cation and anion in \(\mathrm{Al}\left(\mathrm{ClO}_{3}\right)_{3}\) (Aluminum chlorate).

\(\mathrm{Al}\left(\mathrm{ClO}_{3}\right)_{3}\) is composed of aluminum (Al) and chlorate (ClO₃) ions. Aluminum has a charge of +3 (Al³⁺). Chlorate is a polyatomic ion with a charge of -1 (ClO₃⁻), and there are three chlorate ions in the compound. Hence, the cation is Aluminum (Al) with a charge of +3, and the anion is Chlorate (ClO₃) with a charge of -1.

Step 4: (d) Identify the cation and anion in \(\mathrm{Co}(\mathrm{OH})_{2}\) (Cobalt(II) hydroxide).

\(\mathrm{Co}(\mathrm{OH})_{2}\) is composed of cobalt (Co) and hydroxide (OH) ions. Cobalt has a charge of +2 in this compound, which is denoted as Cobalt(II) (Co²⁺). Hydroxide is a polyatomic ion with a charge of -1 (OH⁻), and there are two hydroxide ions in the compound. As a result, the cation is Cobalt(II) with a charge of +2, and the anion is Hydroxide (OH) with a charge of -1.

Step 5: (e) Identify the cation and anion in \(\mathrm{PbCO}_{3}\) (Lead(II) carbonate).

\(\mathrm{PbCO}_{3}\) is composed of lead (Pb) and carbonate (CO₃) ions. Lead is in the +2 oxidation state here, which is shown as Lead(II) (Pb²⁺). Carbonate is a polyatomic ion with a charge of -2 (CO₃²⁻). Thus, the cation is Lead(II) with a charge of +2, and the anion is Carbonate (CO₃) with a charge of -2.

Key Concepts

Cation and Anion Identification

Identifying cations and anions is a fundamental skill when working with ionic compounds. A cation is a positively charged ion, whereas an anion carries a negative charge. To determine which component of an ionic compound is the cation and which is the anion, you can often look at the periodic table.
Elements from groups 1 and 2 typically form cations, because they lose electrons easily, achieving a positive charge. For example, in CuS (Copper(II) sulfide), Cu represents Copper(II) which has a charge of +2.
Conversely, non-metals and certain groups, like halogens, form anions. In CuS, S stands for sulfide, carrying a -2 charge. Understanding these basic rules is key to figuring out the roles cations and anions have in wider chemistry context.

Polyatomic Ions

Polyatomic ions consist of multiple atoms bonded together, carrying an overall charge. They differ from simple ions that contain only one atom. Common polyatomic ions that you might encounter include sulfate \(\mathrm{SO}_4^{2-}\), nitrate \(\mathrm{NO}_3^{-}\), and phosphate \(\mathrm{PO}_4^{3-}\).
These compounds behave as single entities in a reaction, despite containing more than one atom. For instance, \(\mathrm{SO}_4^{2-}\) is the polyatomic ion in \(\mathrm{Ag}_2 \mathrm{SO}_4\), acting as the anion, while silver (Ag) serves as the cation.
Knowing the common polyatomic ions and their charges is crucial as it helps simplify understanding reactions and the behavior of more complex ionic compounds.

Oxidation States

Oxidation states, or oxidation numbers, are key to understanding the electronic structure of compounds and the flow of electrons in chemical reactions. They represent the number of electrons an atom gains, loses, or shares when forming bonds. For metals, oxidations states are often positive, as metals tend to lose electrons.
For instance, in aluminum chlorate \(\mathrm{Al}(\mathrm{ClO}_3)_3\), aluminum is in the +3 oxidation state, meaning it has lost three electrons. On the other hand, the chlorate ion, \(\mathrm{ClO}_3^-\), represents the -1 oxidation state of the entire polyatomic ion.
Keeping track of oxidation states helps predict compound formation, balancing equations, and identifying redox reactions.

Charge Balancing

Charge balancing is crucial for the stability of ionic compounds. It ensures that the total positive charge equals the total negative charge, resulting in a neutrally charged compound.
In \(\mathrm{Co}(\mathrm{OH})_2\), Cobalt(II) (\(\mathrm{Co}^{2+} \)) provides a +2 charge which is balanced by two hydroxide ions (\(\mathrm{OH}^-\)), each carrying a -1 charge. This results in a net charge of zero for the compound.
Understanding charge balancing is important as it underpins the composition of neutral ionic compounds. Without charge balancing, compounds would be electrically unstable and could not form crystalline structures, impacting their function and application in real-world scenarios.