Oxidation-Reduction Reactions and Oxidation Numbers

Vocabulary flashcards covering oxidation numbers, redox terminology, common redox reaction types, and the half-reaction method for balancing equations.

Oxidation Number

The actual charge of the atom if it exists as a monatomic ion, or a hypothetical charge assigned to the atom in a substance using simple rules to keep track of the movement of electrons.

Oxidation Number of Free Elements Rule

The oxidation number of an atom in an uncombined element is always $0$.

Oxidation Number of Monatomic Ions Rule

The oxidation number of an atom in a monatomic ion equals the charge on the ion.

Oxidation Number of Oxygen Rule

The oxidation number of oxygen is $-2$ in most of its compounds, except in peroxides such as $H_2O_2$, where its oxidation number is $-1$.

Oxidation Number of Hydrogen Rule

The oxidation number of hydrogen is $+1$ in most of its compounds, but it is $-1$ in binary compounds with a metal, such as $CaH_2$.

Oxidation Number of Halogens Rule

Fluorine always has an oxidation number of $-1$. Each of the other halogens ($Cl$, $Br$, $I$) has an oxidation number of $-1$ in binary compounds, except when bonded to oxygen or another halogen above it in the periodic table.

Oxidation Number Sum Rule

The sum of the oxidation numbers of all atoms in a neutral compound is $0$, whereas the sum of the oxidation numbers in a polyatomic ion equals the charge of the ion.

Half-Reaction

One of two parts of an oxidation-reduction reaction, one part of which involves a loss of electrons (or an increase in oxidation number) and the other part of which involves a gain of electrons (or a decrease in oxidation number).

Oxidation

The half-reaction in which there is a loss of electrons by a species, corresponding to an increase in oxidation number.

Reduction

The half-reaction in which there is a gain of electrons by a species, corresponding to a decrease in oxidation number.

Reducing Agent

A species that reduces another species by donating electrons; it is itself oxidized during the reaction.

Oxidizing Agent

A species that oxidizes another species by accepting electrons; it is itself reduced during the reaction.

Combination Reaction

A reaction in which two elements combine to form a third substance, such as $2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)$.

Decomposition Reaction

A reaction in which a single compound reacts to yield two or more substances, such as $2HgO(s) \rightarrow 2Hg(l) + O_2(g)$.

Displacement Reaction

A reaction in which an element reacts with a compound and displaces another element from it, such as $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$.

Combustion Reaction

A reaction in which a substance reacts with oxygen, usually accompanied by the rapid release of heat to produce a flame, such as $2C_4H_{10}(g) + 13O_2(g) \rightarrow 8CO_2(g) + 10H_2O(g)$.

Half-Reaction Method

A method for balancing simple redox reactions consisting of three steps: (1) determine oxidation numbers to identify what is oxidized and reduced, (2) write out and balance each half-reaction including electron count, and (3) combine the balanced half-reactions to produce the overall reaction.