Problem 66 - Chemistry Matter and Change

Question

Label each half-reaction as reduction or oxidation. a. \(\mathrm{Fe}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{e}^{-}\) b. \(\mathrm{MnO}_{4}-5 \mathrm{e}^{-}+8 \mathrm{H}^{+} \rightarrow \mathrm{Mn}^{2+}+4 \mathrm{H}_{2} \mathrm{O}\) c. \(2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2}\) d. \(\mathrm{F}_{2} \rightarrow 2 \mathrm{F}^{-}+2 \mathrm{e}^{-}\)

Step-by-Step Solution

Answer

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

Step 1: Identify the change in oxidation states

To determine whether a reaction is reduction or oxidation, you need to identify the change in oxidation states for the elements involved in the reactions. a. $\mathrm{Fe}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{e}^{-}$ b. $\mathrm{MnO}_{4}-5 \mathrm{e}^{-}+8 \mathrm{H}^{+} \rightarrow \mathrm{Mn}^{2+}+4 \mathrm{H}_{2} \mathrm{O}$ c. \(2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2}\) d. \(\mathrm{F}_{2} \rightarrow 2 \mathrm{F}^{-}+2 \mathrm{e}^{-}\)

Step 2: Determine if the change represents oxidation or reduction

Now that the change in oxidation states has been identified, use the following rules to determine if the half-reaction is a reduction or oxidation process. 1. If the oxidation state increases, then the half-reaction represents oxidation. 2. If the oxidation state decreases, then the half-reaction represents reduction. a. In this half-reaction, the oxidation state of Fe increases from +2 to +3. Therefore, this is an oxidation process. Label: Oxidation b. In this half-reaction, the oxidation state of Mn decreases from +7 in \(\mathrm{MnO}_{4}^-\) to +2 in \(\mathrm{Mn}^{2+}\). Therefore, this is a reduction process. Label: Reduction c. In this half-reaction, the oxidation state of H decreases from +1 in \(\mathrm{H}^{+}\) to 0 in \(\mathrm{H}_{2}\). Therefore, this is a reduction process. Label: Reduction d. In this half-reaction, the oxidation state of F increases from 0 in \(\mathrm{F}_{2}\) to -1 in \(2\mathrm{F}^{-}\). Therefore, this is an oxidation process. Label: Oxidation

Key Concepts

Oxidation

Oxidation is a fundamental concept in redox reactions, where an atom or ion loses electrons. This is characterized by an increase in the oxidation state.
This process can be observed in many chemical reactions, particularly when metals react with non-metals. Some key points about oxidation:

• Oxidation involves the loss of electrons. For example, in the half-reaction \(\mathrm{Fe}^{2+} \rightarrow \mathrm{Fe}^{3+} + \mathrm{e}^{-}\), iron loses an electron, which increases its oxidation state from +2 to +3.
• When oxidation occurs, the substance becomes oxidized. It's important to remember the mnemonic OIL RIG: Oxidation Is Loss (of electrons).

Understanding oxidation helps in identifying how elements interact and transform in chemical reactions. It plays a crucial role in everyday processes such as combustion and metabolism.

Reduction

Reduction is the counterpart to oxidation. In this process, an atom or ion gains electrons, resulting in a decrease in its oxidation state. Reduction is as critical as oxidation in forming a complete redox reaction.
Here's what you should know about reduction:

• Reduction involves the gain of electrons. For example, in the reaction \(2 \mathrm{H}^{+} + 2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2}\), hydrogen ions gain electrons, reducing their oxidation state from +1 to 0.
• When reduction occurs, the substance is said to be reduced. This is easily remembered by the remaining part of the acronym OIL RIG: Reduction Is Gain (of electrons).

Reduction is vital for processes like photosynthesis, where carbon dioxide is reduced to glucose. It also occurs in batteries during the discharge of energy.

Oxidation States

Oxidation states, also known as oxidation numbers, are a way to keep track of the electron transfer in redox reactions. They represent the degree of oxidation or reduction of an element in a compound.
Understanding oxidation states allows chemists to determine how electrons are shared or transferred in a reaction. Here are some insights:

• Assigning oxidation states involves using a set of rules, such as assigning -2 for oxygen and +1 for hydrogen in most compounds.
• For example, in the compound \(\mathrm{MnO}_{4}^{-}\), the oxidation state of manganese is +7. This is crucial in understanding a half-reaction like \(\mathrm{MnO}_{4}^{-} + 5 \mathrm{e}^{-} + 8 \mathrm{H}^{+} \rightarrow \mathrm{Mn}^{2+} + 4 \mathrm{H}_{2} \mathrm{O}\), where manganese's oxidation state drops to +2, indicating reduction.
• Oxidation states help identify how electrons are transferred between species, aiding in classifying reactions as either oxidative or reductive.

Grasping the concept of oxidation states is essential for anyone studying chemistry, as it underpins the understanding of reaction mechanisms and chemical behavior.