Question

Assign oxidation number to the underlined elements in each of the following species: (a) \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) (b) \(\mathrm{NaH} \underline{\mathrm{S} O}_{4}\) (c) \(\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{7}\) (d) \(\mathrm{K}_{2} \underline{\mathrm{Mn}} \mathrm{O}_{4}\) (e) \(\mathrm{Ca} \underline{\mathrm{O}}_{2}\) (f) \(\mathrm{NaB} \mathrm{H}_{4}\) (g) \(\mathrm{H}_{2} \underline{\mathrm{S}}_{2} \mathrm{O}_{7}\) (h) \(\operatorname{KAl}\left(\underline{\mathrm{S}} \mathrm{O}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\)

Short answer

(a) +5, (b) +6, (c) +5, (d) +6, (e) -1, (f) +3, (g) +6, (h) +6.

Step-by-step solution

Assign Oxidation Number in NaH2PO4

The compound is \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\). Sodium (Na) is +1 as it's an alkali metal. Hydrogen (H) is +1. The oxidation number of oxygen (O) is -2. We need to find the oxidation number of phosphorus (P). Set up the equation: \[ 1 + 2(+1) + \mathrm{P} + 4(-2) = 0 \]Solve for \(\mathrm{P}\):\[ 1 + 2 + \mathrm{P} - 8 = 0 \]\[ \mathrm{P} = +5 \]

Assign Oxidation Number in NaHSO4

The compound is \(\mathrm{NaHSO}_{4}\). Sodium (Na) has an oxidation number of +1 as an alkali metal. Hydrogen (H) has +1. Oxygen (O) has -2. Solve for sulfur (S):\[ 1 + 1 + \mathrm{S} + 4(-2) = 0 \]\[ \mathrm{S} = +6 \]

Assign Oxidation Number in H4P2O7

The compound is \(\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{7}\). Hydrogen (H) is +1. Oxygen (O) is -2. Solve for phosphorus (P):\[ 4(+1) + 2\mathrm{P} + 7(-2) = 0 \]\[ 2\mathrm{P} = +10 \]\[ \mathrm{P} = +5 \]

Assign Oxidation Number in K2MnO4

The compound is \(\mathrm{K}_{2} \mathrm{MnO}_{4}\). Potassium (K) is +1. Oxygen (O) is -2. Solve for manganese (Mn):\[ 2(+1) + \mathrm{Mn} + 4(-2) = 0 \]\[ \mathrm{Mn} = +6 \]

Assign Oxidation Number in CaO2

The compound is \(\mathrm{CaO}_{2}\). Calcium (Ca) is +2 as an alkaline earth metal. The total oxidation number for oxygen in peroxides like \(\mathrm{O}_{2}\) is -1 each. Therefore, solve for the single oxygen atom:\[ +2 + 2(-1) = 0 \]The oxidation number of oxygen is -1.

Assign Oxidation Number in NaBH4

The compound is \(\mathrm{NaBH}_{4}\). Sodium (Na) is +1. Hydrogen (H) here is -1 as it's part of a metal hydride. Solve for boron (B):\[ 1 + \mathrm{B} + 4(-1) = 0 \]\[ \mathrm{B} = +3 \]

Assign Oxidation Number in H2S2O7

The compound is \(\mathrm{H}_{2} \mathrm{S}_{2} \mathrm{O}_{7}\). Hydrogen (H) is +1. Oxygen (O) is -2. Solve for sulfur (S):\[ 2(+1) + 2\mathrm{S} + 7(-2) = 0 \]\[ 2\mathrm{S} = +12 \]\[ \mathrm{S} = +6 \]

Assign Oxidation Number in KAl(SO4)2 · 12H2O

The compound is \(\operatorname{KAl}\left(\underline{\mathrm{S}} \mathrm{O}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\). Focus on \(\mathrm{SO}_{4}^{2-}\). Potassium (K) and aluminum (Al) are not relevant to sulfur. Each \(\mathrm{SO}_{4}^{2-}\) has sulfur (S) + X and oxygen (O) -8. Solve for X:\[ \mathrm{S} + 4(-2) = -2 \]\[ \mathrm{S} = +6 \]

Key concepts

Redox Reactions

Redox reactions are a crucial concept in chemistry, involving the transfer of electrons between atoms. The term "redox" stands for reduction and oxidation, which are complementary processes that occur simultaneously. In these reactions:

• Oxidation refers to the loss of electrons by a molecule, atom, or ion. When this occurs, the oxidation number of the substance increases.
• Reduction is the gain of electrons, resulting in a decrease in the oxidation number.

This electron exchange can affect a substance's overall charge and chemical properties. A common example of a redox reaction is the reaction between hydrogen and oxygen to form water:\[2H_2 + O_2 \rightarrow 2H_2O\]In this reaction, hydrogen is oxidized as it loses electrons, whereas oxygen is reduced as it gains electrons. Understanding redox reactions is vital for interpreting various chemical processes, including combustion, respiration, and even metabolism.
Assigning oxidation numbers is a pivotal step in identifying which components undergo oxidation or reduction during a reaction.

Chemical Compounds

Chemical compounds are substances that consist of two or more different elements chemically bonded together. These compounds have properties distinct from the individual elements that compose them. For example, water (H₂O) shows very different characteristics compared to the elements hydrogen and oxygen.

• Ionic compounds: Formed through the transfer of electrons from one element to another, resulting in the formation of charged ions. Examples include sodium chloride (NaCl).
• Covalent compounds: Formed through the sharing of electrons between atoms. This type of bonding is prevalent in organic molecules such as methane (CH₄).

Each compound can be described by its chemical formula, which indicates the elements present and their relative proportions, as in NaH₂PO₄. The chemical structure and molecular geometry of compounds greatly influence their chemical reactivity and physical properties. Assigning oxidation numbers within compounds is critical for understanding their reactions, especially in identifying redox processes.

Valence States

Every chemical element can exist in different valence states, which describe an atom's ability to combine with other atoms. The valence state is defined by the number of bonding electrons an atom can share or transfer. This can also be illustrated through the oxidation number of an atom within a molecule or compound.

• Oxidation Number: A positive or negative integer that represents the total number of electrons an atom gains or loses to form a chemical bond. For example, the phosphorus in NaH₂PO₄ has an oxidation number of +5.
• Variables Affecting Valence States: Environmental conditions, chemical context, and the oxidation state of adjacent atoms all influence an element's valence state.

Valence states are essential for understanding chemical reactions and bonding, as they dictate how atoms interact and bond with each other. These states influence the stability, reactivity, and electron configuration of the molecules involved. Assigning the correct oxidation number to each atom in a compound is a fundamental skill that helps determine the possible redox changes a molecule may undergo.
Understanding valence states allows one to predict how different elements might interact in a given chemical reaction.