AP Chemistry Notes: Redox Reactions, Oxidation Numbers, and Half Reactions

Redox Reactions

• Definition of Redox Reactions: Redox reactions are chemical processes involving the transfer of electrons between two or more substances. One substance is oxidized (loses electrons) while another is reduced (gains electrons).

• Example of a Redox Reaction:

  • Reaction: $ext{Na} + ext{Cl}_2 <br>ightarrow ext{NaCl}$

  • Sodium (Na) loses an electron to become sodium ion (Na+).

    • Oxidation: Na is oxidized because it loses an electron.

  • Chlorine (Cl) gains an electron from sodium to become chloride ion (Cl-).

    • Reduction: Cl is reduced because it gains electrons.

Oxidation Numbers

• Definition: Oxidation numbers are assigned to elements in a compound to indicate the degree of oxidation (loss of electrons) or reduction (gain of electrons) of the atoms.

• Rules for Assigning Oxidation Numbers:

  • An element in its free state has an oxidation number of 0. Examples include: $ext{Ca}$, $ext{N}_2$, $ext{Ag}$.

  • Ions have oxidation numbers equal to their charge:

  • For example: $ext{Zn}^{2+}$, $ext{Br}^-$ have oxidation numbers of +2 and -1 respectively.

  • Group 1 metals have an oxidation number of +1.

  • Group 2 metals have an oxidation number of +2.

  • Fluorine always has an oxidation number of -1.

  • Hydrogen generally has an oxidation number of +1 except when combined with metals, where it can be -1.

  • Oxygen has an oxidation number of -2 except in peroxides, where it is -1.

  • Chlorine has an oxidation number of -1 unless it is bonded to oxygen or fluorine.

Half Reactions

• Definition of Half Reactions: Half reactions are used to illustrate the transfer of electrons in redox reactions, separating the oxidation process from the reduction process.

• Mnemonic Device: Remember the phrase "LEO says GER" to distinguish oxidation and reduction.

  • LEO = Loss of Electrons is Oxidation

  • GER = Gain of Electrons is Reduction

• Example of Half Reactions:

  • Consider the overall reaction:
    $2 ext{Al}(s) + 3 ext{Zn(NO}<em>3 ext{)}</em>2 (aq) <br>ightarrow 2 ext{Al(NO}<em>3 ext{)}</em>3 (aq) + 3 ext{Zn}(s)$

  • Oxidation half-reaction:

  • $ext{Al}^0 <br>ightarrow ext{Al}^{3+} + 3 e^-$ (
    Loss of electrons: Oxidation)

  • Reduction half-reaction:

  • $ext{Zn}^{2+} + 2 e^- <br>ightarrow ext{Zn}^0$ (Gain electrons: Reduction)

• Observation of Oxidation and Reduction:

  • Upon oxidation, an element’s oxidation number increases.

  • Upon reduction, an element’s oxidation number decreases.