Electrochemistry and Oxidation Essay

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9.1 Oxidation and Reduction * The term reduction came to be associated with producing metals from their compounds. Red iron (III) oxide “reduced” by carbon monoxide gas to iron metal is a typical example of this process.
3 common metallurgical processes
Fe2O3(s) + 3 CO (g) (reducing agent) → 2 Fe(s) + 3 CO2 (g)
SnO2(s) + C(s) (reducing agent) → Sn(s) + CO2 (g)
CuS(s) + H2 (g) (reducing agent) → Cu(s) + H2S (g) * A reducing agent causes the reduction of a metal compound to an elemental metal * Corrosion reactions are similar to combustion reactions but they occur more slowly. They both involve the use of oxygen, and came to be called oxidation. * A substance that causes or promotes the oxidation of a metal to produce a metal compound is called an oxidizing agent.

2Mg(s) + O2 (g) (oxidizing agent) → 2 MgO(s)
2Al(s) + 3 Cl2 (g) (oxidizing agent) → 2 AlCl3(s)
Cu(s) + Br2 (g) (oxidizing agent) → CuBr2(s)

Electron Transfer Theory:

* A reaction is a combination of two parts, called half-reactions. A half-reaction represents what is happening to only one reactant in an overall reaction. Another half-reaction is required to complete the description of the reaction. * When Zinc is place in hydrochloric acid, gas bubbles form (hydrogen) as it dissolves. Hydrogen and zinc ions are present during this reaction. The Zinc metal is oxidized to zinc chloride.
Zn(s) + 2 HC l (aq) → ZnCl2 (aq) + H2 (g)

* The zinc atoms in the solid, Zn(s), are converted to zinc ions in solution, Zn2+ (aq). Atomic theory requires that the zinc atoms lose two electrons, as shown by the following half-reaction equation:
Zn(s) → Zn2+ (aq) 2 e-

* Simultaneously, hydrogen ions in the solution gain electrons and are converted into hydrogen gas, as shown below:

2 H+ (aq) + 2 e- → H2 (g) * Both of the half-reaction equations, or half-reactions, are balanced by mass (same number of element symbols on both sides) and by charge (same total charge on both sides). * Single displacements in aq solutions are easier to study than metallurgy or corrosion. They share a common feature, ions are converted to atoms and atoms are converted to ions.
Example: reduction of aqueous silver nitrate to silver metal in the presence of solid copper. According to atomic theory, silver atoms are electrically neutral particles (47p+, 47e-) and silver ions are charged particles (47p+, 46e-). In this reaction, an electron is required to convert a silver ion into a silver atom.

Ag+ (aq) + e- → Ag0 (s) (reduction)

Reduction is the gain of electrons. This does not explain where the electrons came from.

According to atomic theory, copper atoms (29p+, 29e-) must be losing electrons as they form copper (II) ions (29p+, 27e-).

Cu0 (s) → Cu2+ (aq) + 2 e- (oxidation)

Oxidation is the loss of electrons

Theory suggests that the total number of electrons gained in a reaction must equal the total number of electrons lost. Oxidation and reduction reactions are separate processes, they occur simultaneously, and are simply called "redox" reactions.

Oxidation states:

An oxidation number is a positive or negative number corresponding to the oxidation state assigned to an atom.

Atom or ion | Oxidation number | Examples | All atoms in elements | 0 | Na is 0, Cl in Cl2 is 0 | Hydrogen in all compounds | +1 | H in HCl is +1 | Except hydrogen in hydrides | -1 | H in LiH is -1 | Oxygen in all compounds | -2 | O in H2O is -2 | Except oxygen in peroxides | -1 | O in H2O2 is -1 | All monatomic ions | Charge on ion | Na+ is +1, S2- is -2 |

Page 656 #7, 8, 9

7. Redox reaction: A chemical reaction involving both reduction and oxidation, which results in changes in oxidation numbers of atoms included in the reaction.
Reduction: The process of gaining electrons.
Oxidation: The process of loosing electrons

8.
a) Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)

Zn(s) → Zn2+ (aq)
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