Question

Assign oxidation numbers to each atom in the following: (a) \(\mathrm{Na} \mathrm{OBr}\) (b) \(\mathrm{NaBrO}_{2}\), (c) \(\mathrm{NaBrO}_{3},\) and (d) \(\mathrm{NaBrO}_{4}\)

Short answer

The oxidation numbers for each atom are: (a) Na(+1), O(-2), Br(+1), (b) Na(+1), Br(+3), O(-2), (c) Na(+1), Br(+5), O(-2), and (d) Na(+1), Br(+7), O(-2).

Step-by-step solution

Understanding oxidation numbers

Oxidation numbers (or states) represent the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic. A set of rules is used to assign oxidation numbers: (1) The oxidation number of a free element is always 0. (2) The oxidation number of a monatomic ion equals the charge of the ion. (3) Hydrogen has an oxidation number of +1 when bonded to non-metals, and -1 when bonded to metals. (4) Oxygen has an oxidation number of -2 in most compounds, except in peroxides. (5) The oxidation numbers of all atoms in a neutral compound must add up to 0, while in a polyatomic ion, they must add up to the charge of the ion. (6) Group 1 elements (alkali metals) always have an oxidation number of +1, and group 2 (alkaline earth metals) have +2 in their compounds.

Assigning Oxidation Numbers for NaOBr

For NaOBr: Sodium (Na) as a Group 1 element has an oxidation number of +1. Oxygen (O) typically has an oxidation number of -2. To find the oxidation number of bromine (Br), we set up the equation +1 + (-2) + x = 0, where x is the oxidation number of Br. Solving for x gives: x = +1. Therefore, the oxidation numbers are Na(+1), O(-2), Br(+1).

Assigning Oxidation Numbers for NaBrO2

For NaBrO2: Sodium (Na) has an oxidation number of +1. Oxygen (O) has an oxidation number of -2, and there are two oxygens, contributing a total of -4. To find the oxidation number of bromine (Br), we set up the equation +1 + x + (-4) = 0. Solving for x gives: x = +3. Therefore, the oxidation numbers are Na(+1), Br(+3), O(-2).

Assigning Oxidation Numbers for NaBrO3

For NaBrO3: Sodium (Na) has an oxidation number of +1. Oxygen (O) has an oxidation number of -2, and there are three oxygens, contributing a total of -6. To find the oxidation number of bromine (Br), we set up the equation +1 + x + (-6) = 0. Solving for x gives: x = +5. Therefore, the oxidation numbers are Na(+1), Br(+5), O(-2).

Assigning Oxidation Numbers for NaBrO4

For NaBrO4: Sodium (Na) has an oxidation number of +1. Oxygen (O) has an oxidation number of -2, and there are four oxygens, contributing a total of -8. To find the oxidation number of bromine (Br), we set up the equation +1 + x + (-8) = 0. Solving for x gives: x = +7. Therefore, the oxidation numbers are Na(+1), Br(+7), O(-2).

Key concepts

Assigning Oxidation States

Understanding how to assign oxidation states to atoms within molecules and ions is a foundational skill in chemistry, especially in analyzing redox reactions. Oxidation states are theoretical charges assigned based on a set of rules.

Steps to Assign Oxidation States

Here is a simplified process for assigning oxidation states:

• Identify and apply the oxidation state conventions, such as for group 1 and group 2 elements, and the usual states for hydrogen and oxygen.
• Use known oxidation states to determine the unknown ones, ensuring that the sum corresponds to the compound's overall charge.
• Remember that multiple oxidation states are possible for transition and post-transition metals.

Following these guidelines can help clarify the charge distribution within a compound, laying a foundation to understand reactions these molecules may undergo.

Redox Chemistry

Redox chemistry is a branch of chemistry that studies the transfer of electrons between species in a chemical reaction. These processes are critical for many biological and industrial applications.

Electron Transfer and Energy

In redox reactions, one species will lose electrons (oxidation) while another gains electrons (reduction). The movement of electrons also means the transfer of energy, which is why redox reactions are often associated with batteries and cellular respiration.

• Identify the species gaining electrons (reduced) and those losing electrons (oxidized).
• Analyze the change in oxidation states to determine which reactants and products are involved in the electron transfer.

Understanding redox chemistry is vital for areas ranging from electrochemistry to metabolic pathways in biology.

Chemical Bonding

Chemical bonding pertains to the forces holding atoms together in molecules and compounds. It's the glue of chemistry, with bonds forming and breaking being central to chemical reactions.

Types of Chemical Bonds

Three primary types of bonds include:

• Covalent bonds: sharing of electron pairs between atoms.
• Ionic bonds: transfer of electrons from one atom to another, creating charged ions that attract each other.
• Metallic bonds: a sea of electrons shared among a lattice of metal cations.

The oxidation state concept is an idealization that is most accurate for ionic compounds, less so for covalent species where electron sharing doesn't result in full charges on atoms but rather in partial charges.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, commonly known as redox reactions, are processes where oxidation state changes occur due to the transfer of electrons.

Identifying Redox Reactions

To recognize a redox reaction, look for the following signs:

• A change in the oxidation state of two or more elements.
• The movement of electrons is implicit in the process.
• The presence of an oxidizing agent (gaining electrons) and a reducing agent (losing electrons).

Understanding the interchange of electrons in redox reactions is crucial for interpreting biochemical pathways, industrial processes, and environmental chemistry.