Problem 34 - Chemistry The Molecular Nature of Matter

Question

Assign oxidation numbers to the elements in the following: (a) \(\operatorname{Sr}\left(\mathrm{BrO}_{3}\right)_{2}\) (b) \(\mathrm{Cr}_{2} \mathrm{~S}_{3}\) (c) \(\mathrm{OF}_{2},\) and (d) \(\mathrm{HOF}\)

Step-by-Step Solution

Answer

The oxidation numbers are: (a) Sr\textsuperscript{2+}, Br\textsuperscript{5+}, O\textsuperscript{2-}, (b) Cr\textsuperscript{3+}, S\textsuperscript{2-}, (c) O\textsuperscript{2+}, F\textsuperscript{1-}, (d) H\textsuperscript{1+}, O\textsuperscript{0}, F\textsuperscript{1-}.

Step 1: Determine the Oxidation Number of Strontium in Sr(BrO3)2

Strontium is an alkaline earth metal and typically has an oxidation number of +2. Since it is in group 2 of the periodic table, it loses two electrons when forming compounds.

Step 2: Determine the Oxidation Numbers for Bromine and Oxygen in Sr(BrO3)2

Oxygen generally has an oxidation number of -2. There are three oxygens, contributing a total of -6. To balance the +2 charge from strontium, the oxidation number of bromine must be +5 (since 2 * (+1 Br) = +10 and +10 - 6 = +2, which balances with the +2 of Sr).

Step 3: Determine the Oxidation Number of Chromium in Cr2S3

Sulfur generally has an oxidation number of -2. There are three sulfurs, contributing a total of -6. Since the molecule is neutral, chromium must balance this out with a +6 charge. With two chromium atoms, each must have an oxidation number of +3 (+3 * 2 = +6, balancing the -6 from sulfur).

Step 4: Determine the Oxidation Numbers for Oxygen and Fluorine in OF2

Fluorine has an oxidation number of -1 in all of its compounds because it is the most electronegative element. There are two fluorines, contributing a total of -2. To balance this, oxygen must have an oxidation number of +2.

Step 5: Determine the Oxidation Numbers for Hydrogen, Oxygen, and Fluorine in HOF

Hydrogen typically has an oxidation number of +1, and fluorine has an oxidation number of -1. Since the molecule is neutral, oxygen must have an oxidation number of 0 to balance the +1 from hydrogen and -1 from fluorine.

Key Concepts

Redox Chemistry

Redox chemistry, or reduction-oxidation chemistry, is the branch of chemistry that focuses on reactions involving the changes in oxidation states of atoms. During these reactions, one species undergoes oxidation (loss of electrons) and another species undergoes reduction (gain of electrons). It's important to understand that the total number of electrons is conserved; electrons are simply transferred from one species to another.

Understanding redox chemistry is key because it underpins many chemical processes, from the rusting of iron to the metabolic pathways in our bodies. Furthermore, assigning correct oxidation numbers is essential in balancing redox equations and in identifying the oxidizing and reducing agents in a reaction. For instance, in the provided exercise, various elements were assigned with oxidation numbers to comprehend their chemical behavior in compounds.

Oxidation States Assignment

Assigning oxidation numbers or states to elements within a compound is a foundational skill in chemistry. An oxidation number is a theoretical charge that an atom would have if the compound was composed of ions. It helps chemists keep track of electron flow in a reaction.

Here are some guidelines for assigning oxidation states:

• The oxidation state of a pure element is always zero.
• For monatomic ions, the oxidation state is equal to the charge of the ion.
• Oxygen usually has an oxidation number of -2, except in peroxides or when bonded to fluorine.
• Hydrogen is typically +1 when bonded to nonmetals and -1 when bonded to metals.
• Fluorine always has an oxidation number of -1, because it is the most electronegative element.
• The sum of oxidation numbers in a neutral compound must be zero, while in a polyatomic ion, it must equal the charge of the ion.

Applying these rules to the provided exercise, students can methodically determine the oxidation states of the elements in various compounds, leading to a greater understanding of their chemical properties.

Periodic Table Elements

The periodic table is a tabular arrangement of chemical elements, organized by increasing atomic number, electron configuration, and recurring chemical properties. Elements are grouped into periods (rows) and groups (columns) that share similar characteristics. Learning the layout of the periodic table is crucial in predicting the behavior of elements, including how they combine and interact in chemical reactions.

To effectively assign oxidation numbers, familiarity with the periodic table is essential. For example, elements in group 1 (alkali metals) typically have an oxidation state of +1, while group 2 (alkaline earth metals) have an oxidation state of +2. Transition metals can have multiple oxidation states, which adds complexity to determining the correct numbers. Furthermore, nonmetals generally have negative oxidation states, although there can be exceptions, as seen with the oxygen in the compound OF₂ from the exercise. By utilizing the periodic table as a tool, students can discern various oxidation states and apply them to solve redox chemistry problems.