Problem 40 - Chemistry and Chemical Reactivity

Question

In the following reactions, decide which reactant is oxidized and which is reduced. Designate the oxidizing agent and the reducing agent. (a) \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(\mathrm{aq})+3 \mathrm{Sn}^{2+}(\mathrm{aq})+14 \mathrm{H}^{+}(\mathrm{aq}) \longrightarrow\) \(2 \mathrm{Cr}^{3+}(\mathrm{aq})+3 \mathrm{Sn}^{4+}(\mathrm{aq})+7 \mathrm{H}_{2} \mathrm{O}(\ell)\) (b) \(\mathrm{FeS}(\mathrm{s})+3 \mathrm{NO}_{3}^{-}(\mathrm{aq})+4 \mathrm{H}^{+}(\mathrm{aq}) \longrightarrow\) \(3 \mathrm{NO}(\mathrm{g})+\mathrm{SO}_{4}^{2-}(\mathrm{aq})+\mathrm{Fe}^{3+}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\ell)\)

Step-by-Step Solution

Answer

(a) Oxidized: Sn, Reduced: Cr; Oxidizing Agent: \(\mathrm{Cr}_{2}\mathrm{O}_{7}^{2-}\), Reducing Agent: \(\mathrm{Sn}^{2+}\). (b) Oxidized: Fe and S, Reduced: N; Oxidizing Agent: \(\mathrm{NO}_{3}^{-}\), Reducing Agent: \(\mathrm{FeS}\).

Step 1: Assign Oxidation Numbers for Reaction (a)

For the reaction \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} + 3\mathrm{Sn}^{2+} + 14\mathrm{H}^{+} \to 2\mathrm{Cr}^{3+} + 3\mathrm{Sn}^{4+} + 7\mathrm{H}_{2}\mathrm{O} \), assign oxidation numbers to each element. Chromium (Cr) in \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} \) starts at +6, and ends at +3 in \( \mathrm{Cr}^{3+} \). Tin (Sn) starts at +2 in \( \mathrm{Sn}^{2+} \) and ends at +4 in \( \mathrm{Sn}^{4+} \).

Step 2: Determine Oxidation and Reduction for Reaction (a)

Identify which element is oxidized and which is reduced by comparing their oxidation numbers. Chromium is reduced from +6 to +3, and tin is oxidized from +2 to +4.

Step 3: Identify Oxidizing and Reducing Agents for Reaction (a)

The substance that is reduced (gains electrons) is the oxidizing agent, while the one that is oxidized (loses electrons) is the reducing agent. Therefore, \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} \) is the oxidizing agent, and \( \mathrm{Sn}^{2+} \) is the reducing agent.

Step 4: Assign Oxidation Numbers for Reaction (b)

For the reaction \( \mathrm{FeS} + 3 \mathrm{NO}_{3}^{-} + 4 \mathrm{H}^{+} \to 3 \mathrm{NO} + \mathrm{SO}_{4}^{2-} + \mathrm{Fe}^{3+} + 2 \mathrm{H}_{2} \mathrm{O} \), iron (Fe) in \( \mathrm{FeS} \) starts at +2 and ends at +3 in \( \mathrm{Fe}^{3+} \), sulfur (S) in \( \mathrm{FeS} \) starts at -2 and becomes +6 in \( \mathrm{SO}_{4}^{2-} \), and nitrogen (N) in \( \mathrm{NO}_{3}^{-} \) starts at +5 and ends as +2 in \( \mathrm{NO} \).

Step 5: Determine Oxidation and Reduction for Reaction (b)

Identify which element is oxidized and which is reduced. Iron and sulfur in \( \mathrm{FeS} \) are oxidized; sulfur from -2 to +6, and iron from +2 to +3. Nitrogen in \( \mathrm{NO}_{3}^{-} \) is reduced from +5 to +2.

Step 6: Identify Oxidizing and Reducing Agents for Reaction (b)

\( \mathrm{NO}_{3}^{-} \), which is reduced, is the oxidizing agent. \( \mathrm{FeS} \), which contains both iron and sulfur that are oxidized, acts as the reducing agent.

Key Concepts

Redox Chemistry

Redox chemistry, short for oxidation-reduction chemistry, is a core concept in chemical reactions where the transfer of electrons between substances occurs.
This concept is crucial for understanding processes ranging from biological systems to industrial applications. Redox reactions involve two interconnected processes: oxidation and reduction.
In oxidation, a substance loses electrons, increasing its oxidation number, whereas reduction involves a substance gaining electrons, thus reducing its oxidation number. To determine the changes in oxidation states, it is often helpful to first assign oxidation numbers to each element in a reaction.

• An element that loses electrons during the reaction is said to have undergone oxidation.
• Conversely, an element that gains electrons is considered to have undergone reduction.

These processes occur simultaneously, making them essential components of a redox reaction. Many everyday phenomena, such as rusting iron or metabolizing food, are driven by redox reactions.
Through these events, elements and compounds constantly exchange electrons, keeping the world's chemistry in motion.

Oxidation Numbers

Assigning oxidation numbers is a fundamental step in analyzing redox reactions.
Oxidation numbers help chemists track where electrons are moving during the reaction. Oxidation numbers are hypothetical charges assigned to atoms within a molecule or ion that represent the gain or loss of electrons:

• The oxidation number of a free element is always zero.
• For monoatomic ions, the oxidation number is equal to the charge of the ion.
• In covalent compounds, more electronegative elements are typically assigned the electron pairs in shared bonds.

Understanding these rules allows for the identification of which atoms have changed their oxidation states during a reaction. This identification process was illustrated in the exercise provided, where elements like chromium, tin, and nitrogen showed shifts in oxidation states.
For example, in part (a) of the exercise, chromium changes from +6 to +3, and tin from +2 to +4, indicating their respective reduction and oxidation. These changes help pinpoint the part each atom plays in the overall redox process.

Oxidizing and Reducing Agents

Oxidizing and reducing agents play crucial roles in redox reactions.
These agents determine the direction of electron flow in the chemical process.An oxidizing agent is a substance that causes another substance to lose electrons.
As it accepts electrons, it gets reduced in the process.

• In the earlier example (a), \(\mathrm{Cr}_{2} \\mathrm{O}_{7}^{2-}\), reduces to \(2 \\mathrm{Cr}^{3+}\), acting as the oxidizing agent.

On the flip side, a reducing agent donates electrons to another substance and gets oxidized while doing so.
In the same example, \( \\mathrm{Sn}^{2+}\) is the reducing agent as it converts to \( \\mathrm{Sn}^{4+}\).Determining oxidizing and reducing agents is a vital part of mastering redox reactions.
These agents are responsible for propelling reactions forward by transferring electrons, making them an integral feature in both laboratory and real-world chemical processes.