Question

Assign oxidation numbers to each element in the following compounds: (a) \(\mathrm{VOCl}_{3}\) (b) \(\mathrm{CuSO}_{4}\) (c) \(\mathrm{CH}_{2} \mathrm{O}\) (d) \(\mathrm{Mn}_{2} \mathrm{O}_{7}\) (e) \(\mathrm{OsO}_{4}\) (f) \(\mathrm{H}_{2} \mathrm{PtCl}_{6}\)

Short answer

(a) V: +3, O: 0, Cl: -1; (b) Cu: +2, S: +6, O: -2; (c) C: 0, H: +1, O: -2; (d) Mn: +7, O: -2; (e) Os: +8, O: -2; (f) H: +1, Pt: +4, Cl: -1.

Step-by-step solution

Assign Known Oxidation Numbers

Start by assigning oxidation numbers to elements based on common rules: Oxygen (O) is usually -2, and hydrogen (H) is usually +1. For chlorine (Cl), it is typically -1 unless it is bonded to oxygen or fluorine.

Calculate Oxidation Numbers for VOCl₃

In VOCl₃, assume Cl is -1. This compound is neutral, so the sum of oxidation numbers must be zero. Let the oxidation number of V be x: \[x + 3(-1) = 0 \] Thus, \[x = +3\]. The oxidation numbers are V: +3, O: 0, Cl: -1.

Calculate Oxidation Numbers for CuSO₄

In CuSO₄, assume O is -2. S is typically +6 when bonded with oxygen in sulfates. The oxidation state of Cu will balance the sulfate to make the compound neutral: \[ x - 2(4) + 6 = 0 \] Thus, \[ x = +2 \]. The oxidation numbers are Cu: +2, S: +6, O: -2.

Calculate Oxidation Numbers for CH₂O

In CH₂O, assume H is +1 and O is -2. Carbon's oxidation state will be computed to balance the compound: \[x + 2(+1) - 2 = 0\] \[x = 0\]. The oxidation numbers are C: 0, H: +1, O: -2.

Calculate Oxidation Numbers for Mn₂O₇

In Mn₂O₇, assume O is -2. The total oxidation for 7 oxygens is -14. Let the oxidation number for Mn be x: \[2x - 14 = 0\] \[2x = +14 \rightarrow x = +7\]. The oxidation numbers are Mn: +7, O: -2.

Calculate Oxidation Numbers for OsO₄

In OsO₄, assume O is -2. Let the oxidation number of Os be x: \[x + 4(-2) = 0\] \[x = +8\]. The oxidation numbers are Os: +8, O: -2.

Calculate Oxidation Numbers for H₂PtCl₆

In H₂PtCl₆, assume H is +1, and Cl is -1. The compound is neutral, so let Pt's oxidation number be x:\[ 2(+1) + x + 6(-1) = 0 \] \[x = +4\]. The oxidation numbers are H: +1, Pt: +4, Cl: -1.

Key concepts

Redox Reactions

Redox reactions are a fascinating and essential part of chemistry. They occur when there is a change in the oxidation states of the elements involved during the reaction. In simple terms, redox reactions involve the transfer of electrons between substances. When an atom or compound loses electrons, it is said to be oxidized. Conversely, when an atom or compound gains electrons, it is reduced.

The term 'redox' is a combination of the words reduction and oxidation, signifying the interdependence of these processes. A classic example is the rusting of iron, where iron reacts with oxygen (a redox reaction) to form iron oxide. During this process, iron (Fe) loses electrons and gets oxidized, while oxygen (O) gains electrons and is reduced.

• Reduction: Gain of electrons, resulting in a decrease in oxidation number.
• Oxidation: Loss of electrons, resulting in an increase in oxidation number.
• Oxidizing agent: The substance that gets reduced and, in turn, allows another substance to be oxidized.
• Reducing agent: The substance that gets oxidized and allows another substance to be reduced.

Understanding redox reactions is crucial as they underpin many biological processes and technological applications.

From the operation of batteries and fuel cells to metabolic processes in living organisms, redox reactions play a pivotal role. Most importantly, mastering redox reactions will provide insights into balancing chemical equations and predicting reaction outcomes.

Chemical Compounds

Chemical compounds are substances composed of two or more different elements that are chemically bonded together. Each compound has specific properties and composition. The properties of a compound are often different from the properties of the individual elements that make it up.

Compounds are usually classified based on the types of bonds holding the atoms together:

• Ionic compounds: Formed by the transfer of electrons from one atom to another, resulting in positive and negative ions. An example is sodium chloride (NaCl).
• Covalent compounds: Formed by the sharing of electrons between atoms. An example is water (H₂O).

Understanding chemical compounds involves recognizing their molecular formula and determine element ratios. These provide clues about their properties and behavior.

Knowing the oxidation numbers of each element in a compound is critical. It helps in identifying the types of reactions they can undergo, especially in the context of redox reactions. For example, in compounds like \( \text{CuSO}_4 \), identifying the oxidation states can help predict how copper might be extracted or utilized in reactions. Thus, learning about chemical compounds is foundational to the study of chemistry, emphasizing the interconnected or distinct roles each element plays within compound structures.

Element Oxidation States

The oxidation state (or oxidation number) of an element in a chemical compound provides a way to keep track of electrons in redox reactions. It is a numerical representation that indicates the degree of oxidation or reduction of an element.

The rules for determining oxidation states are straightforward:

• Elements in their pure form have an oxidation state of zero (e.g., \( \text{O}_2 \), \( \text{N}_2 \)).
• The oxidation state of a monoatomic ion is the same as its charge (e.g., \( \text{Na}^+ \) has an oxidation state of +1).
• Oxygen usually has an oxidation state of -2 in compounds (exceptions include peroxides).
• Hydrogen usually has an oxidation state of +1 when bonded to non-metals and -1 when bonded to metals.
• Alkali metals (group 1) have an oxidation number of +1; alkaline earth metals (group 2) have +2.

Understanding oxidation states allows chemists to balance chemical equations and determine how electrons are transferred during chemical reactions.

For example, in \( \text{Mn}_2\text{O}_7 \), each manganese (Mn) has an oxidation state of +7, which indicates it is highly oxidized. Recognizing these states aids in predicting chemical behavior and reaction pathways. Consequently, mastering the determination of oxidation states is essential for anyone studying chemistry.