Problem 93 - Chemistry Matter and Change

Question

What are the oxidation numbers for the elements in the ion \(\mathrm{PO}_{4}^{3-?}(\text { Chapter } 19)\)

Step-by-Step Solution

Answer

The oxidation numbers for the elements in the \(\mathrm{PO}_4^{3-}\) ion are: Phosphorus (P) has an oxidation number of +5, and Oxygen (O) has an oxidation number of -2.

Step 1: Identify the elements and their general oxidation states

In a \(\mathrm{PO}_4^{3-}\) ion, there is one phosphorus atom (P) and four oxygen atoms (O). Typically, the oxidation state of phosphorus ranges from -3 to +5, and the oxidation state of oxygen is usually -2.

Step 2: Apply the oxidation rules to find the oxidation state of phosphorus

According to the oxidation rules, the sum of oxidation numbers on the atoms in a polyatomic ion should equal the charge on that ion. Since the charge on the \(\mathrm{PO}_4^{3-}\) ion is -3, we can set up the following equation: \(x + 4(-2) = -3\) Here, x is the oxidation state of phosphorus and -2 is the oxidation state of each oxygen atom.

Step 3: Solve the equation for the oxidation state of phosphorus

Now, we can solve the equation for x to find the oxidation state of phosphorus: \(x - 8 = -3\) Add 8 to both sides of the equation: \(x = 5\) So, the oxidation state of phosphorus (P) in the \(\mathrm{PO}_4^{3-}\) ion is +5.

Step 4: Verify that the sum of oxidation numbers equals the ion's charge

To make sure we have found the correct oxidation numbers for the elements in the ion \(\mathrm{PO}_4^{3-}\), we must verify that the sum of the oxidation numbers equals the total charge on the ion: (+5) + 4(-2) = -3 Simplifying the equation, we get: 5 - 8 = -3 Indeed, our oxidation numbers do add up to the total charge on the ion (-3), confirming that the oxidation numbers for the elements in the \(\mathrm{PO}_4^{3-}\) ion are as follows: Phosphorus (P): +5 Oxygen (O): -2

Key Concepts

Oxidation State

The oxidation state, also known as oxidation number, is a concept used to indicate the degree of oxidation of an atom in a chemical compound. Understanding oxidation states is essential in determining how electrons are distributed among the atoms in various chemical structures.

The oxidation state is a hypothetical charge that an atom would have if all bonds to atoms of different elements were fully ionic. This means each atom is either losing, gaining, or sharing electrons depending on the compound it forms.

General rules help determine an element's oxidation state in a molecule or ion:

• The oxidation state of an element in its natural state (like O₂ or H₂) is always zero.
• The oxidation state of a monatomic ion is equal to its charge.
• Oxygen almost always has an oxidation state of -2 in its compounds, except in peroxides where it is -1.
• In a neutral molecule, the sum of the oxidation states is zero; in a polyatomic ion, the sum is equal to the charge of the ion.

Phosphorus

Phosphorus is a versatile non-metal element that can display a range of oxidation states. Its oxidation states can widely vary from -3 to +5, depending on the types of bonds it forms in different compounds. In the \( \mathrm{PO}_4^{3-} \) ion, phosphorus takes on an oxidation state of +5. This suggests that phosphorus is interacting with oxygen in such a way that it loses electrons or shares them to a lesser extent.

Being a member of group 15 in the periodic table, phosphorus can exhibit varying valency, allowing it to engage in multiple oxidation states. This versatility enables phosphorus to form a variety of compounds, particularly useful in biological, chemical, and industrial processes.

• In phosphates, such as \( \mathrm{PO}_4^{3-} \), phosphorus is typically found in its highest oxidation state of +5.
• In other compounds, such as phosphine \( \mathrm{PH}_3 \), phosphorus may have an oxidation state of -3, representing its reduced form.

Oxygen

Oxygen is one of the most common elements in the Earth's crust and an essential part of many compounds. Typically, the oxidation state of oxygen in compounds is -2. This indicates that oxygen tends to gain electrons when forming bonds, reflecting its high electronegativity.

In most oxides, like the phosphate ion \( \mathrm{PO}_4^{3-} \), each oxygen atom contributes an oxidation state of -2. However, there are exceptions to this rule:

• In peroxides, like hydrogen peroxide \( \mathrm{H}_2\mathrm{O}_2 \), oxygen has an oxidation state of -1.
• In superoxides, such as \( \mathrm{KO}_2 \), it bears an oxidation state of -0.5.

Oxygen's ability to form strong covalent bonds with many elements explains its presence in several oxidation states and numerous compounds.

Polyatomic Ion

Polyatomic ions are ions that consist of two or more atoms bonded together that carry a charge, both positive or negative. These ions are notable for their complex structures and involvement in various chemical reactions.

For a polyatomic ion like \( \mathrm{PO}_4^{3-} \), the sum of the oxidation states of the atoms should equal the overall charge of the ion. This rule helps in determining the oxidation states of individual atoms within the ion.

Key Characteristics of Polyatomic Ions:

• They can combine with other ions, forming ionic compounds.
• They possess unique geometrical arrangements, impacting their chemical properties.
• Common polyatomic ions include sulfate \( \mathrm{SO}_4^{2-} \), nitrate \( \mathrm{NO}_3^{-} \), and carbonates \( \mathrm{CO}_3^{2-} \).

Understanding how these ions function and interact is crucial for mastering chemistry concepts, especially in reaction mechanisms and stoichiometry.