Question

For the complete redox reactions given here, break down each reaction into its half-reactions, identify the oxidizing agent, and identify the reducing agent. (a) \(2 \mathrm{Sr}+\mathrm{O}_{2} \longrightarrow 2 \mathrm{Sr} \mathrm{O}\) (b) \(2 \mathrm{Li}+\mathrm{H}_{2} \longrightarrow 2 \mathrm{LiH}\) (c) \(2 \mathrm{Cs}+\mathrm{Br}_{2} \longrightarrow 2 \mathrm{CsBr}\) (d) \(3 \mathrm{Mg}+\mathrm{N}_{2} \longrightarrow \mathrm{Mg}_{3} \mathrm{~N}_{2}\)

Short answer

(a) \(\mathrm{O}_2\) is oxidizing; \(\mathrm{Sr}\) is reducing. (b) \(\mathrm{H}_2\) is oxidizing; \(\mathrm{Li}\) is reducing. (c) \(\mathrm{Br}_2\) is oxidizing; \(\mathrm{Cs}\) is reducing. (d) \(\mathrm{N}_2\) is oxidizing; \(\mathrm{Mg}\) is reducing.

Step-by-step solution

Break Down Reaction (a) into Half-Reactions

The given reaction is \(2 \mathrm{Sr} + \mathrm{O}_2 \rightarrow 2 \mathrm{SrO}\). First, identify the elements that undergo change.- **Oxidation**: \(\mathrm{Sr} \rightarrow \mathrm{Sr^{2+}} + 2e^-\)- **Reduction**: \(\mathrm{O}_2 + 4e^- \rightarrow 2\mathrm{O^{2-}}\)Now, combine the half-reactions by balancing the electrons exchanged, resulting in the complete balanced reaction.

Identify Agents for Reaction (a)

In the reaction \(2 \mathrm{Sr} + \mathrm{O}_2 \rightarrow 2 \mathrm{SrO}\):- **Oxidizing Agent**: \(\mathrm{O}_2\) because it gains electrons.- **Reducing Agent**: \(\mathrm{Sr}\) because it loses electrons.

Break Down Reaction (b) into Half-Reactions

The reaction is \(2 \mathrm{Li} + \mathrm{H}_2 \rightarrow 2 \mathrm{LiH}\).- **Oxidation**: \(\mathrm{Li} \rightarrow \mathrm{Li^{+}} + e^-\)- **Reduction**: \(\mathrm{H}_2 + 2e^- \rightarrow 2\mathrm{H^-}\)These half-reactions are balanced in terms of electron transfer.

Identify Agents for Reaction (b)

In the reaction \(2 \mathrm{Li} + \mathrm{H}_2 \rightarrow 2 \mathrm{LiH}\):- **Oxidizing Agent**: \(\mathrm{H}_2\) because it gains electrons.- **Reducing Agent**: \(\mathrm{Li}\) because it loses electrons.

Break Down Reaction (c) into Half-Reactions

The reaction is \(2 \mathrm{Cs} + \mathrm{Br}_2 \rightarrow 2 \mathrm{CsBr}\).- **Oxidation**: \(\mathrm{Cs} \rightarrow \mathrm{Cs^{+}} + e^-\)- **Reduction**: \(\mathrm{Br}_2 + 2e^- \rightarrow 2\mathrm{Br^-}\)Balance the electrons exchanged to ensure the reaction is complete.

Identify Agents for Reaction (c)

In the reaction \(2 \mathrm{Cs} + \mathrm{Br}_2 \rightarrow 2 \mathrm{CsBr}\):- **Oxidizing Agent**: \(\mathrm{Br}_2\) because it gains electrons.- **Reducing Agent**: \(\mathrm{Cs}\) because it loses electrons.

Break Down Reaction (d) into Half-Reactions

The reaction is \(3 \mathrm{Mg} + \mathrm{N}_2 \rightarrow \mathrm{Mg}_3\mathrm{N}_2\).- **Oxidation**: \(\mathrm{Mg} \rightarrow \mathrm{Mg^{2+}} + 2e^-\)- **Reduction**: \(\mathrm{N}_2 + 6e^- \rightarrow 2\mathrm{N^{3-}}\)Make sure to balance the total electrons to stabilize both reactions.

Identify Agents for Reaction (d)

In the reaction \(3 \mathrm{Mg} + \mathrm{N}_2 \rightarrow \mathrm{Mg}_3\mathrm{N}_2\):- **Oxidizing Agent**: \(\mathrm{N}_2\) because it gains electrons.- **Reducing Agent**: \(\mathrm{Mg}\) because it loses electrons.

Key concepts

Oxidation

Oxidation is a key process in redox reactions. It involves the loss of electrons by a species. In simpler terms, when a molecule, atom, or ion loses electrons, it is said to be oxidized. This loss of electrons increases the oxidation state of the element. For example, in the reaction \(2 \mathrm{Sr} + \mathrm{O}_2 \rightarrow 2 \mathrm{SrO}\), strontium (Sr) undergoes oxidation. It starts as a neutral element and becomes \(\mathrm{Sr^{2+}}\) by losing electrons.
This process is identified as oxidation due to the electron loss.The concept of oxidation is crucial because it allows us to track electron flow in chemical reactions. Observing which species loses electrons helps in understanding the chemical behavior and transformations during reactions.
Having a clear idea of oxidation goes hand in hand with its counterpart, reduction.

Reduction

Reduction is the other half of the redox reaction. Unlike oxidation, reduction involves the gain of electrons by a species, decreasing its oxidation state. This means the number of electrons associated with the atom or element increases. A practical example is seen in \(\mathrm{O}_2 + 4e^- \rightarrow 2\mathrm{O^{2-}}\), where oxygen is reduced as it gains electrons to form oxide ions.
Reduction is inseparable from oxidation; they occur simultaneously in redox reactions.Understanding reduction is key to mastering redox processes. It involves monitoring which species gains electrons and becomes reduced. This balance of electron transfer is essential for maintaining a complete and stable reaction.
Reduction is the partner to oxidation, together forming the fundamental foundation of many chemical reactions.

Oxidizing Agent

An oxidizing agent plays a critical role in redox reactions by accepting electrons. It is the compound that enables oxidation to occur by being reduced itself. In the reaction \(2 \mathrm{Li} + \mathrm{H}_2 \rightarrow 2 \mathrm{LiH}\), for instance, hydrogen \(\mathrm{H}_2\) acts as the oxidizing agent.
It takes in electrons during the process, allowing lithium to oxidize.The oxidizing agent is pivotal because it drives the transfer of electrons. Identifying it requires looking for the species that gains electrons during the reaction. These agents are essential in various chemical reactions, including combustion, rusting, and even in biological systems like respiration.
An oxidizing agent is an indispensable component of redox reactions and is crucial for the flow of chemical processes.

Reducing Agent

The reducing agent is the substance that donates electrons in a redox reaction. By losing electrons, it causes another species to be reduced and undergoes oxidation itself. For example, in the reaction \(2 \mathrm{Cs} + \mathrm{Br}_2 \rightarrow 2 \mathrm{CsBr}\), cesium \(\mathrm{Cs}\) acts as the reducing agent as it loses electrons to bromine.
This donation of electrons is what drives the reduction of another substance.Reducing agents are fundamental to the flow of electrons in chemical reactions. By examining which species loses electrons, one can identify the reducing agent. These agents are incredibly important in both industrial processes and biological systems, such as in photosynthesis.
Understanding reducing agents is crucial for a comprehensive grasp of redox dynamics and the mechanisms behind chemical transformations.