Question

Assign oxidation states to all of the atoms in each of the following: a. \(\mathrm{MnO}_{2}\) b. \(\mathrm{BaCrO}_{4}\) c. \(\mathrm{H}_{2} \mathrm{SO}_{3}\) d. \(\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}\)

Short answer

a. In \(\mathrm{MnO}_{2}\), Mn has an oxidation state of +4 while O has an oxidation state of -2. b. In \(\mathrm{BaCrO}_{4}\), Ba has an oxidation state of +2, Cr has an oxidation state of +6, and O has an oxidation state of -2. c. In \(\mathrm{H}_{2} \mathrm{SO}_{3}\), H has an oxidation state of +1, S has an oxidation state of +4, and O has an oxidation state of -2. d. In \(\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}\), Ca has an oxidation state of +2, P has an oxidation state of +5, and O has an oxidation state of -2.

Step-by-step solution

Rule 1: Oxidation state of free elements

For all elements in their free state, the oxidation state is 0.

Rule 2: Oxidation state of monatomic ions

For monatomic ions, the oxidation state is equal to the charge of the ion.

Rule 3: Oxidation state of oxygen

In most compounds, the oxidation state of oxygen is usually -2. Exceptions are in peroxides (such as H2O2) where its oxidation state is -1, and in compounds with fluorine (such as OF2) where its oxidation state is +2.

Rule 4: Oxidation state of hydrogen

In most compounds, the oxidation state of hydrogen is +1. When bound to metals in metal hydrides, the oxidation state is -1.

Rule 5: Oxidation state of fluorine

In all compounds, the oxidation state of fluorine is always -1.

Rule 6: Oxidation state of other halogens

In most compounds, the oxidation state of other halogens (such as chlorine, bromine, and iodine) is -1, unless they are combined with oxygen or a group 1 metal.

Rule 7: Sum of oxidation states in a compound

The sum of oxidation states of all atoms in a compound is equal to its overall charge. In neutral compounds, the sum of oxidation states will be 0. Now, let's apply these rules to the given compounds: a. \(\mathrm{MnO}_{2}\) - Oxygen: -2 (Rule 3) - Mn: +4 (Rule 7) Assign oxidation states: Mn(+4) and O(-2) b. \(\mathrm{BaCrO}_{4}\) - Oxygen: -2 (Rule 3) - Ba: +2 (Rule 2) - Cr: +6 (Rule 7) Assign oxidation states: Ba(+2), Cr(+6), and O(-2) c. \(\mathrm{H}_{2} \mathrm{SO}_{3}\) - Hydrogen: +1 (Rule 4) - Oxygen: -2 (Rule 3) - S: +4 (Rule 7) Assign oxidation states: H(+1), S(+4), and O(-2) d. \(\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) - Calcium: +2 (Rule 2) - Oxygen: -2 (Rule 3) - Phosphorus: +5 (Rule 7) Assign oxidation states: Ca(+2), P(+5), and O(-2)

Key concepts

Oxidation State Rules

Understanding oxidation state rules is crucial for students as it forms the foundation of many concepts in chemistry.

At its core, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. These rules are a set of guidelines that help us determine the oxidation number of an atom.

Key Rules Simplified

• Free Elements Rule: Atoms in their elemental form have an oxidation state of 0.
• Monatomic Ions Rule: The oxidation state is the charge of the ion. For example, Na+ has an oxidation state of +1.
• Oxygen Rule: Typically, oxygen has an oxidation state of -2, except in peroxides or when bonded to fluorine.
• Hydrogen Rule: Hydrogen is usually +1 except in metal hydrides where it is -1.
• Fluorine Rule: Fluorine always has an oxidation state of -1 in compounds.
• Halogens Rule: Other halogens are usually -1, except when combined with oxygen or group 1 metals.
• Compound Rule: The sum of all oxidation states in a neutral compound is 0, aligning with its overall charge.

These succinct guidelines allow students to systematically approach and solve problems involving chemical compounds.

Chemical Compounds

A chemical compound is a substance composed of two or more different types of atoms chemically bonded together. The properties of compounds often differ from those of their constituent elements.

Understanding Compound Composition

When analyzing compounds like \( \mathrm{MnO}_{2} \), \( \mathrm{BaCrO}_{4} \), \( \mathrm{H}_{2} \mathrm{SO}_{3} \) or \( \mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2} \), it's important to appreciate the ratio in which atoms combine and how this determines the overall properties of the compound.

The concept of oxidation states helps explain the transfer and sharing of electrons in the formation of these compounds, shedding light on chemical bonding and reactions. For instance, in \( \mathrm{MnO}_{2} \), manganese and oxygen come together with a specific oxidation state, contributing to the overall characteristics of this compound.

Assigning Oxidation Numbers

Assigning oxidation numbers to atoms within a compound involves a straightforward application of the rules we've discussed. This skill is essential in balancing chemical equations, analyzing redox reactions, and understanding the electron transfer processes.

Applying the Rules

For example, in the compound \( \mathrm{MnO}_{2} \), applying the rules helps deduce that manganese (Mn) has an oxidation state of +4, balancing the -2 charge of each oxygen atom.

It is important when learning these concepts to work through examples step by step, as seen with \( \mathrm{H}_{2} \mathrm{SO}_{3} \), where hydrogen has an oxidation state of +1, sulfur +4, and oxygen -2.

By mastering the assignment of oxidation numbers, students can better understand the interplay of elements within compounds and the fundamental principles that govern chemical reactions.