Question

State what an oxidation half-reaction shows. What does a reduction half- reaction show?

Short answer

An oxidation half-reaction shows the process of a chemical species losing electrons, resulting in an increase in its oxidation state. For example, \(Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-\). A reduction half-reaction shows the process of a chemical species gaining electrons, resulting in a decrease in its oxidation state. For example, \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)\). In redox reactions, these half-reactions occur simultaneously, and the number of electrons lost and gained must be equal to maintain balance.

Step-by-step solution

Definition of Oxidation and Reduction

Oxidation is the process of losing electrons by a chemical species, while reduction is the process of gaining electrons by a chemical species. In a redox (reduction-oxidation) reaction, one chemical species is oxidized, and another is reduced.

Half-Reactions

A half-reaction is a representation of one part of a redox reaction, either the oxidation or reduction process. By dividing a redox reaction into two half-reactions, we can analyze each process separately. In each half-reaction, the number of electrons lost or gained must be equal to maintain a balanced redox reaction.

Oxidation Half-Reaction

An oxidation half-reaction shows the process of a chemical species losing electrons. It represents the part of the redox reaction where the species undergoes oxidation. This means that the chemical species will give up one or more electrons and will increase its oxidation state. Example of an oxidation half-reaction: \(Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-\) In this oxidation half-reaction, zinc (Zn) is oxidized by losing two electrons, forming a zinc ion (Zn^2+) with a positive charge.

Reduction Half-Reaction

A reduction half-reaction shows the process of a chemical species gaining electrons. It represents the part of the redox reaction where the species undergoes reduction. This means that the chemical species will take up one or more electrons and will decrease its oxidation state. Example of a reduction half-reaction: \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)\) In this reduction half-reaction, the copper ion (Cu^2+) is reduced from a positive charge of +2 by gaining two electrons, forming elemental copper (Cu) in its solid form.

Key concepts

Oxidation Half-Reaction

Understanding an oxidation half-reaction is pivotal in the study of redox reactions. This process highlights the loss of electrons from a substance. It is the 'losing side' of an electron tug-of-war. In the context of a redox reaction, when a species gets oxidized, it essentially donates electrons and undergoes an increase in its oxidation state, indicating it has a higher positive charge post-reaction.

An oxidation half-reaction is written by placing the reactant that loses electrons on the left and the resultant ion plus the freed electrons on the right. For clarity, take the example of zinc transforming into zinc ion:
\begin{center}\(Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-\)Here, metallic zinc loses two electrons to become positively charged zinc ions. This transformation represents an increase in oxidation state from 0 in metallic zinc to +2 in zinc ions.

Reduction Half-Reaction

The reduction half-reaction is the counterbalance to oxidation in redox processes, showcasing the electron acquisition by a substance. Students learning redox reactions often hear the mnemonic 'LEO says GER' — Lose Electrons Oxidation, Gain Electrons Reduction. A reduction half-reaction exactly portrays this gain of electrons and is thus known as the 'gaining side' of the reaction.

Reduction involves the decrease in oxidation state, suggesting an element becomes less positively charged due to the gained electrons:
\begin{center}\(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)\)As highlighted by this example, copper ions gain electrons to form elemental copper, thereby undergoing reduction. The oxidation state drops from +2 in copper ions to 0 in solid copper. Isolating and studying these half-reactions separately facilitates a better grasp of the individual processes occurring within the broader context of a redox reaction.

Electron Transfer

Electron transfer is the core of redox reactions, as it involves the movement of electrons from one species to another. It is this very exchange that defines oxidation and reduction processes. When considering redox reactions, one must understand that electrons cannot simply disappear; they must be conserved. As such, electrons lost by one substance in an oxidation half-reaction are the same ones gained by another substance in a reduction half-reaction.

This process highlights the fundamental law of charge conservation in chemical reactions. Overall neutrality must be maintained, which is why the total number of electrons lost in oxidation must equal those gained in reduction.

• Oxidation releases electrons into the reaction environment.
• Reduction species capture these free electrons.
• The net charge before and after the reaction remains constant.

Students are encouraged to carefully balance these electron transfers to ensure accurate stoichiometry in redox equations.

Oxidation State

Oxidation state, or oxidation number, is a conceptual charge that an atom would have if the compound was composed entirely of ions. It helps chemists keep track of electron movement in redox reactions. In a neutral molecule, the sum of all oxidation states equals zero, while in ions, it equals the ion's charge. Understanding the changes in oxidation state during a reaction helps to identify which species are oxidized and which are reduced.

An increase in oxidation state corresponds to a loss of electrons (oxidation), and a decrease in oxidation state corresponds to a gain of electrons (reduction). For instance, in the oxidation of zinc, zinc starts with an oxidation state of 0 and ends with +2 after losing two electrons. Alternatively, copper's reduction starts at +2, decreasing to 0 when electrons are gained. Students can use the oxidation states to verify that charge and mass are balanced in the redox reaction and to grasp the underlying electron shifts.