Question

What is a half-reaction? What does each of the two half-reactions that make up an overall process represent?

Short answer

A half-reaction is a chemical reaction that shows either the oxidation or reduction process separately, involving the transfer of electrons between chemical species. In a redox (reduction-oxidation) reaction, the overall process is accomplished by combining two half-reactions: an oxidation half-reaction, which represents a chemical species losing electrons and increasing its oxidation state, and a reduction half-reaction, which represents a chemical species gaining electrons and decreasing its oxidation state. When combined, the electrons lost in the oxidation half-reaction equal the electrons gained in the reduction half-reaction, ensuring that the overall process is balanced with respect to both mass and charge.

Step-by-step solution

Definition of Half-Reaction

A half-reaction refers to a chemical reaction that shows either oxidation or reduction process separately. It involves the transfer of electrons between chemical species. Half-reactions are often used in redox (reduction-oxidation) reactions, which involve both oxidation and reduction taking place. In a redox reaction, one reactant is oxidized (loses electrons), and another reactant is reduced (gains electrons). So, overall redox reaction can be separated into two half-reactions: one for oxidation and the other for reduction.

Oxidation Half-Reaction

The oxidation half-reaction represents the process where a chemical species loses one or more electrons and increases its oxidation state. This species is known as the reducing agent, as it donates electrons to another species. A common example of an oxidation half-reaction is the oxidation of solid zinc to zinc ions in an aqueous solution: \[ Zn(s) \rightarrow Zn^{2+}(aq) + 2e^{-} \]

Reduction Half-Reaction

The reduction half-reaction represents the process where a chemical species gains one or more electrons and decreases its oxidation state. This species is known as the oxidizing agent, as it accepts electrons from another species. A common example of a reduction half-reaction is the reduction of hydrogen ions to hydrogen gas in an aqueous solution: \[ 2H^{+}(aq) + 2e^{-} \rightarrow H_{2}(g) \]

Connecting the Two Half-Reactions

The overall process in a redox reaction is accomplished by combining the oxidation and reduction half-reactions. When the half-reactions are added together, the electrons lost in the oxidation half-reaction equal the electrons gained in the reduction half-reaction. This ensures that the overall process is balanced with respect to both mass and charge. In the example given above, the two half-reactions can be combined to form an overall reaction as follows: \[ Zn(s) + 2H^{+}(aq) \rightarrow Zn^{2+}(aq) + H_{2}(g) \] In this overall process, solid zinc (reducing agent) loses 2 electrons through oxidation and hydrogen ions (oxidizing agent) gain 2 electrons through reduction.