Question

Explain why oxidation and reduction must always occur together.

Short answer

Oxidation and reduction must always occur together because they are linked by electron transfer in redox (oxidation-reduction) reactions. Oxidation involves losing electrons, leading to an increase in oxidation state, while reduction involves gaining electrons, leading to a decrease in oxidation state. Electrons cannot be created or destroyed, so when one species loses electrons (oxidizing agent), another species must gain those electrons (reducing agent). This relationship ensures that oxidation and reduction always take place simultaneously in chemical reactions.

Step-by-step solution

Defining Oxidation and Reduction

Oxidation is the process in which an atom or molecule loses one or more electrons in a chemical reaction. During this process, the oxidation state of the atom or molecule increases. Reduction, on the other hand, is the process in which an atom or molecule gains one or more electrons in a chemical reaction, resulting in a decrease in the oxidation state.

The Role of Electron Transfer

Oxidation and reduction are always linked in reactions because they involve the transfer of electrons. When one species (an atom or molecule) loses electrons, it becomes oxidized. At the same time, another species must gain these electrons, undergoing reduction. This concept of coupled reactions is known as redox (oxidation-reduction) reaction. As a result of the electron transfer between the two species, the oxidizing agent is reduced, while the reducing agent is oxidized. This relationship shows that oxidation cannot take place without reduction and vice versa, as electrons cannot be created or destroyed but only transferred from one species to another.

Example of a Redox Reaction

Consider the example of the reaction between metallic zinc (Zn) and copper sulfate (CuSO4): \(Zn(s) + CuSO4(aq) \rightarrow ZnSO4(aq) + Cu(s)\) In this reaction, metallic zinc loses two electrons and becomes the zinc ion (Zn^(2+)). This is an example of oxidation: \(Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-\) At the same time, the copper ion (Cu^(2+)) in the copper sulfate solution gains two electrons and forms metallic copper: \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)\) This part of the reaction is an example of reduction. The overall reaction, which demonstrates the simultaneous occurrence of the oxidation of Zn and the reduction of Cu^(2+), is: \(Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)\) Thus, we can conclude that oxidation and reduction must always occur together because they are linked by electron transfer. One species loses electrons, and another species gains those electrons, completing the redox reaction.

Key concepts

Oxidation

Oxidation is a key concept in redox reactions. It involves the loss of electrons from a substance. When a molecule, atom, or ion undergoes oxidation, it loses one or more electrons. This results in an increase in its oxidation state.
For example, in a reaction involving zinc and copper sulfate, zinc atoms lose electrons and are oxidized to form zinc ions:

• The formula for this process is: \(Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-\)

Oxidation doesn't happen in isolation. It must be paired with reduction so that electrons have a destination to which they can be transferred.

Reduction

Reduction is the process that complements oxidation. It involves gaining electrons, resulting in the decrease of an atom's or a molecule's oxidation state. In a redox reaction, whenever oxidation occurs, reduction follows to balance the electron exchange.
For example, in the interaction between zinc and copper sulfate:

• Copper ions gain electrons, undergoing reduction to become copper atoms.
• The formula for this is: \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)\)

Reduction ensures that the electrons lost during oxidation are absorbed, maintaining equilibrium in the chemical reaction.

Electron Transfer

Electron transfer is the central theme of redox reactions. It is the movement of electrons from one species to another during a chemical reaction. This process is crucial because it connects oxidation and reduction.
Simply put, electrons cannot appear or disappear; they move from one entity to another.

• When zinc loses electrons, they must be transferred to something else — in this case, copper ions.
• This simultaneous electron transfer results in zinc's oxidation and copper's reduction.

Without electron transfer, redox reactions would not be possible, as one entity must lose electrons for another to gain them.

Oxidizing Agent

An oxidizing agent is a substance that enables oxidation by accepting electrons from another species. This means it becomes reduced itself during the reaction.
In the context of redox reactions, the oxidizing agent is crucial as it facilitates electron uptake.

• In the example of zinc and copper sulfate, copper ions \( (Cu^{2+}) \) act as the oxidizing agent.
• Copper ions accept the electrons lost by zinc, allowing the reduction process to occur.

Therefore, oxidizing agents play a vital role in completing the electron transfer cycle within redox reactions.

Reducing Agent

The reducing agent is the counterpart of the oxidizing agent in a redox reaction. It donates electrons to another species, thereby becoming oxidized itself in the process.
Reducing agents are essential for facilitating the transfer of electrons by providing them.

• For example, in the zinc and copper sulfate reaction, zinc acts as the reducing agent.
• Zinc donates electrons to copper ions, allowing them to be reduced to copper metal.

Reducing agents thus ensure that there are electrons available for transfer, enabling the essential processes of oxidation and reduction to occur.