AP Chemistry 4.9: Oxidation-Reduction Reactions

Reducing agent versus oxidizing agent

A reducing agent donates electrons and is itself oxidized. An oxidizing agent accepts electrons and is itself reduced.

Half-reaction

An equation showing either the oxidation or reduction portion of a redox reaction. For example, sodium oxidation is $2Na\rightarrow2Na^++2e^-$, while chlorine reduction is $Cl_2+2e^-\rightarrow2Cl^-.$

Oxidation number (oxidation state)

The hypothetical charge an atom would have if the compound were completely ionic. Oxidation numbers allow redox processes to be identified even when electrons are not transferred as discrete particles.

What is the oxidation number of an element in its elemental form?

The oxidation number is $0$. This applies to elements such as $Na(s)$, $H_2(g)$, $O_2(g)$, and $Cl_2(g)$.

What is the oxidation number of a monatomic ion?

It equals the ion's charge. For example, $Na^+$ has oxidation number $+1$, while $S^{2-}$ has oxidation number $-2$.

Common oxidation-number rules for hydrogen, oxygen, and halogens

Hydrogen is usually $+1$ with nonmetals and $-1$ in metal hydrides. Oxygen is usually $-2$, but it is $-1$ in peroxides. Fluorine is always $-1$; other halogens are usually $-1$ except when bonded to oxygen or a more electronegative halogen.

What is the oxidation-number sum rule for a compound or polyatomic ion?

The sum of all atomic oxidation numbers equals the overall charge of the species: $0$ for a neutral compound and the ion charge for a polyatomic ion.

How do you calculate the oxidation number of sulfur in $SO_3^{2-}$?

Let sulfur be $x$ and use oxygen's usual value of $-2$: $x+3(-2)=-2$. Therefore, $x=+4$.

What are the oxidation numbers of sulfur in $H_2S$ and $Na_2SO_4$?

In $H_2S$, sulfur is $-2$: $2(+1)+x=0$. In $Na_2SO_4$, sulfur is $+6$: $2(+1)+x+4(-2)=0$.

How can you identify a redox reaction from a chemical equation?

Assign oxidation numbers to relevant elements in reactants and products. If at least one element's oxidation number increases and at least one decreases, the reaction is redox.

What happens to oxidation numbers in $2Na+Cl_2\rightarrow2NaCl$?

Sodium increases from $0$ to $+1$ and is oxidized; chlorine decreases from $0$ to $-1$ and is reduced. Sodium is the reducing agent, and chlorine is the oxidizing agent.

Why is $BaCl_2(aq)+K_2SO_4(aq)\rightarrow BaSO_4(s)+2KCl(aq)$ not a redox reaction?

No element changes oxidation number: the reaction is an ion exchange that produces an insoluble precipitate. It is classified as a precipitation reaction rather than redox.

Combustion reaction as a redox subclass

A combustion reaction is a vigorous redox reaction between a fuel (reductant) and an oxidant, often $O_2$, that releases substantial heat and frequently light.

Single-displacement reaction

A redox reaction in which an elemental metal is oxidized and displaces an ion of another element from solution. For example, $Zn(s)+2HCl(aq)\rightarrow ZnCl_2(aq)+H_2(g)$.

Oxidation-reduction (redox) reaction

A reaction in which one or more elements undergo a change in oxidation number. Oxidation is an increase in oxidation number, while reduction is a decrease.

Oxidation and reduction in terms of electrons

Oxidation is loss of electrons, and reduction is gain of electrons. The mnemonic OIL RIG summarizes this relationship: Oxidation Is Loss, Reduction Is Gain.

What occurs when copper metal is placed in aqueous silver nitrate?

Copper is oxidized to $Cu^{2+}$ and silver ions are reduced to silver metal: $Cu(s)+2Ag^+(aq)\rightarrow Cu^{2+}(aq)+2Ag(s)$. Silver deposits on the wire, and dissolved $Cu^{2+}$ gives the solution a blue color.

Disproportionation reaction

A redox reaction in which the same element in the same reactant is both oxidized and reduced. In $2H_2O_2\rightarrow2H_2O+O_2$, oxygen changes from $-1$ to both $-2$ and $0$.