Question

Indicate each of the following as an oxidation or a reduction: a. \(\mathrm{O}_{2}(g)+4 e^{-} \longrightarrow 2 \mathrm{O}^{2-}(a q)\) b. \(\mathrm{Al}(s) \longrightarrow \mathrm{Al}^{3+}(a q)+3 e^{-}\) c. \(\mathrm{Fe}^{3+}(a q)+e^{-} \longrightarrow \mathrm{Fe}^{2+}(a q)\) d. \(2 \mathrm{Br}^{-}(a q) \longrightarrow \mathrm{Br}_{2}(l)+2 e^{-}\)

Short answer

a. Reduction, b. Oxidation, c. Reduction, d. Oxidation

Step-by-step solution

Identify Gain or Loss of Electrons

For each reaction, determine whether the substance is gaining or losing electrons. Gain of electrons indicates reduction, while loss of electrons indicates oxidation.

Analyze Each Reaction

Analyze each given reaction one by one to decide if it is an oxidation or a reduction.

Step 2.1: Reaction (a) Analysis

For \[\mathrm{O}_{2}(g)+4 e^{-} \longrightarrow 2 \mathrm{O}^{2-}(a q)\]Oxygen is gaining 4 electrons, hence this is a reduction reaction.

Step 2.2: Reaction (b) Analysis

For \[\mathrm{Al}(s) \longrightarrow \mathrm{Al}^{3+}(a q)+3 e^{-}\]Aluminum is losing 3 electrons, hence this is an oxidation reaction.

Step 2.3: Reaction (c) Analysis

For \[\mathrm{Fe}^{3+}(a q)+e^{-} \longrightarrow \mathrm{Fe}^{2+}(a q)\]Iron is gaining 1 electron, hence this is a reduction reaction.

Step 2.4: Reaction (d) Analysis

For \[2 \mathrm{Br}^{-}(a q) \longrightarrow \mathrm{Br}_{2}(l)+2 e^{-}\]Bromine is losing 2 electrons, hence this is an oxidation reaction.

Key concepts

Electron Transfer

Understanding electron transfer is key to mastering oxidation-reduction reactions. In these reactions, electrons move from one atom or molecule to another.

When an atom loses electrons, another atom must gain them. This movement is what drives oxidation and reduction reactions. Always remember:

• Loss of electrons = Oxidation
• Gain of electrons = Reduction

For example, consider the reaction \[\text{\text{O}_{2}(g) + 4 e^{-} \rightarrow 2 \text{O}^{2-}(a q)} \]. Oxygen (\text{\text{O}_{2}}) gains 4 electrons, illustrating a reduction reaction where electron transfer plays a vital role.

Understanding electron transfer helps you see the bigger picture of how oxidation and reduction are linked.

Oxidation

Oxidation involves the loss of electrons. In every oxidation reaction, an atom or molecule gives up electrons.

Consider the example \[\text{\text{Al}(s) \rightarrow \text{\text{Al}^{3+}(a q)} + 3 e^{-}} \]. Here, aluminum (Al) loses 3 electrons and transforms into \[\text{Al}^{3+}\]. This loss of electrons characterizes oxidation.

Remember, oxidation doesn’t always involve oxygen! It's all about the electrons being lost. Key points about oxidation:

• Losing electrons
• Increases oxidation state
• Common in metals like aluminum in the example above

Reduction

Reduction is the opposite of oxidation. It occurs when an atom or molecule gains electrons. In a reduction reaction:

• The oxidation state decreases
• Electrons are gained
• Often seen in non-metals and ions

For example, \[\text{\text{Fe}^{3+}(a q) + e^{-} \rightarrow \text{\text{Fe}^{2+}(a q)}}\]. Here, iron (\text{\text{Fe}^{3+}}) gains an electron to form \[\text{\text{Fe}^{2+}}\], marking a reduction process.

Remember this simple rule: Gain of electrons = Reduction. Focusing on the electron gain helps clearly see reduction moments in reactions.

Half-Reactions

Half-reactions are a great way to understand the details of oxidation-reduction reactions. Each half-reaction shows either the oxidation process or the reduction process alone.

Take the reaction \[\text{2 \text{\text{Br}^{-}}(a q) \rightarrow \text{\text{\text{Br}_{2}}(l)} + 2 e^{-}} \]. The half-reactions are:

• Oxidation: \[\text{2 \text{\text{Br}}^{-} \rightarrow \text{\text{Br}}_{2} + 2 e^{-}}\]
• Reduction is the corresponding process that would gain these electrons, not shown here

Half-reactions help identify what’s being oxidized and what’s reduced. They ensure that the entire electron transfer process is clear. Each half-reaction can be combined to see the full picture of the overall redox process, capturing both electron loss and gain.