Problem 74 - Chemistry Matter and Change

Question

Identify each of the following changes as either oxidation or reduction. \begin{equation} \begin{array}{ll}{\text { a. } 2 \mathrm{Cl}^{-} \rightarrow \mathrm{Cl}_{2}+2 \mathrm{e}^{-}} & {\text { c. } \mathrm{Ca}^{-2}+2 \mathrm{e}^{-} \rightarrow 2 \mathrm{Ca}} \\ {\text { b. } \mathrm{Na} \rightarrow \mathrm{Na}^{+}+\mathrm{e}^{-}} & {\text { d. } \mathrm{O}_{2}+4 \mathrm{e}^{-} \rightarrow 2 \mathrm{O}^{2-}}\end{array} \end{equation}

Step-by-Step Solution

Answer

Reactions a and b are oxidation, while reactions c and d are reduction.

Step 1: Analyze Reaction a

The reaction given is \( 2 \text{Cl}^- \rightarrow \text{Cl}_2 + 2 e^- \). Here, \( \text{Cl}^- \) loses electrons to form \( \text{Cl}_2 \). The loss of electrons is known as oxidation, so reaction (a) is an oxidation.

Step 2: Analyze Reaction b

The reaction given is \( \text{Na} \rightarrow \text{Na}^+ + e^- \). Sodium \( \text{Na} \) loses an electron to form \( \text{Na}^+ \). This electron loss indicates the reaction is an oxidation process, thus reaction (b) is oxidation.

Step 3: Analyze Reaction c

The reaction given is \( \text{Ca}^{2-} + 2 e^- \rightarrow 2 \text{Ca} \). Calcium \( \text{Ca}^{2-} \) gains electrons to form \( \text{Ca} \). The gain of electrons is termed as reduction, so reaction (c) is a reduction.

Step 4: Analyze Reaction d

The reaction given is \( \text{O}_2 + 4 e^- \rightarrow 2 \text{O}^{2-} \). Oxygen \( \text{O}_2 \) gains electrons to form \( \text{O}^{2-} \). The gain of electrons defines reduction, indicating that reaction (d) is a reduction.

Key Concepts

Oxidation

In chemistry, oxidation is a fundamental concept often associated with redox reactions. It refers to the process where a substance loses electrons. During oxidation, the oxidation state of an element increases.
For example, in the reaction \( \text{Na} \rightarrow \text{Na}^+ + e^- \), sodium \( \text{Na} \) loses an electron to become \( \text{Na}^+ \). This loss of an electron is what we call oxidation.
When you hear the term 'oxidation', think of loss and increase.

• **Loss of electrons:** Electrons are removed from a substance during oxidation.
• **Increase in oxidation number:** The substance undergoing oxidation usually has an increase in its oxidation state or number.

Remember, oxidation doesn't always involve oxygen. It's all about the transfer of electrons, even in reactions where oxygen isn't present.

Reduction

Reduction is the counterpart to oxidation, forming the other half of a redox (reduction-oxidation) reaction. Reduction occurs when a substance gains electrons, leading to a decrease in its oxidation state.
Consider the reaction \( \text{O}_2 + 4 e^- \rightarrow 2 \text{O}^{2-} \). Oxygen \( \text{O}_2 \) gains electrons to form \( \text{O}^{2-} \). Here, the gain of electrons is termed reduction.

• **Gain of electrons:** Electrons are added to a substance during reduction.
• **Decrease in oxidation number:** When a substance is reduced, its oxidation state decreases.

Reduction is essentially the opposite of oxidation, and both processes are always paired in a redox reaction. One substance gets oxidized while the other gets reduced.

Electrons Transfer

The transfer of electrons is at the heart of redox reactions. It is the movement of electrons from one substance to another. This transfer is what characterizes and drives the processes of oxidation and reduction.
In any redox reaction, when one molecule loses electrons (oxidized), another molecule must gain those electrons (reduced).

• **Electrons as currency:** Think of electrons as a sort of currency in chemical reactions, being traded from one atom or molecule to another.
• **Simultaneous nature:** Oxidation and reduction occur simultaneously because electrons are not created or destroyed in a reaction, only transferred.

This coupling ensures that the overall number of electrons is conserved in a chemical reaction. Understanding how electrons transfer is crucial to mastering redox reactions and their various applications across chemistry.