Question

Write half-reactions for the oxidation and the reduction of water.

Short answer

Oxidation half-reaction: \(2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-\). Reduction half-reaction: \(O_2(g) + 4H^+(aq) + 4e^- \rightarrow 2H_2O(l)\).

Step-by-step solution

Identify Oxidation and Reduction

To write the half-reactions for the oxidation and reduction of water, we need to identify the elements that are being oxidized and reduced. In water, hydrogen can be oxidized, and oxygen can be reduced.

Write the Oxidation Half-Reaction

The oxidation half-reaction involves the loss of electrons. For water, the oxidation of hydrogen can be represented as: \(2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-\). Hydrogen is going from an oxidation state of +1 to the oxygen in \(O_2\), which has an oxidation state of 0.

Write the Reduction Half-Reaction

The reduction half-reaction involves the gain of electrons. For water, the reduction of oxygen can be represented as: \(O_2(g) + 4H^+(aq) + 4e^- \rightarrow 2H_2O(l)\). Oxygen is going from an oxidation state of 0 in \(O_2\) to an oxidation state of -2 in \(H_2O\).

Key concepts

Oxidation and Reduction

Understanding oxidation and reduction is crucial when studying chemical reactions, particularly redox (reduction-oxidation) reactions.
Simply put, oxidization refers to the loss of electrons by a molecule, atom, or ion. The substance that loses electrons becomes more positive and is said to be oxidized. Conversely, reduction is the gain of electrons, making the substance more negative, indicating it has been reduced. An easy way to remember this process is by the mnemonic 'OIL RIG' - Oxidation Is Loss, Reduction Is Gain of electrons.
For instance, when water undergoes electrolysis, a decomposition process, we see both these reactions happening simultaneously. Hydrogen atoms in water lose electrons (are oxidized) to form positive hydrogen ions (protons), while oxygen atoms gain electrons (are reduced) when the water is converted back to its gaseous state.
This duality is the fundamental aspect of redox reactions. Any redox reaction can be split into two parts, or half-reactions, each representing either the oxidation process or the reduction process.

Electron Transfer in Redox Reactions

Diving deeper into redox reactions, we find that they're all about the exchange of electrons.
In any redox reaction, there is an electron transfer from the substance that gets oxidized to the substance that gets reduced. These reactions are hand-in-hand occurrences: one substance can't be oxidized unless another is reduced. This mutual exchange is the driving force behind a wide variety of chemical processes, including energy production in batteries and cells.

Half-Reactions in the Electron Transfer Process

In the context of water's oxidation and reduction, remember that the two half-reactions represent the paths of electron transfer. Oxidation shows electrons being donated, as with the hydrogen in water becoming ionized, while reduction shows electrons being accepted, as with the oxygen in water re-forming into the liquid state. Together, these half-reactions illustrate the full story of how electrons move during the redox reaction.

Oxidation States

To fully grasp redox reactions, one must be familiar with oxidation states (also known as oxidation numbers).
An oxidation state is a theoretical charge that an atom would have if all bonds to atoms of different elements were fully ionic. This notion is a bit like an accountant's ledger for electrons: it helps chemists keep track of where electrons are and how they move during a reaction.
In water (\( H_2O \)), the hydrogen has an oxidation state of +1, while oxygen has an oxidation state of -2. When hydrogen is oxidized, its electrons are taken away, effectively increasing its oxidation state. Conversely, when oxygen is reduced, electrons are added to it, thereby decreasing its oxidation state.

Interpreting Changes in Oxidation States During Half-Reactions

Looking at the half-reactions for water oxidation and reduction, you'll notice how the oxidation state for each element changes. As hydrogen goes from an oxidation state of +1 in water to zero in gaseous hydrogen, it gets oxidized. The electrons that hydrogen loses are gained by oxygen, bringing it from zero in gaseous oxygen to -2 in water—hence, oxygen is reduced. Observing these shifts is integral to understanding the electron flow in redox reactions.