Oxidation and Reduction in terms of Electron Transfer

Most reactions are accompanied by the loss and gain of electrons. Consider the reaction represented by the following ionic equation:

Zn + Cu²⁺ ————————> Zn²⁺ + Cu

Zinc metal and copper react in aqueous solution to produce copper metal and aqueous zinc. There is loss or transfer of two electrons from the zinc metal, which becomes oxidized, to the copper ion. The copper ion gains the electrons lost by the zinc metal to become reduced to copper metal. The net change is the transfer of two electrons lost by zinc to be gained or acquired by copper.

Redox Reactions

When oxidation occurs, invariably reduction also takes place. In other words, they are complementary processes. The oxidation-reduction reactions together are referred to redox reactions (that is red from reduction and ox from oxidation). The reactant that brings about the oxidation, in this case the electron acceptor (Cu2+) becomes the oxidizing agent. The zinc that loses the electrons is oxidized. In the example, the electron donor, which is the zinc metal is the reducing agent and copper ion that gains the electrons is reduced.

From this, it is obvious to see that oxidation involves loss of electrons or electron transfer while reduction involves the gain of electrons. OIL RIG (Oxidation is Loss; Reduction is Gain) is a good mnemonic or memory aid.

Oxidation number & State

Every atom consists of a positive nucleus, surrounded by negative electrons that determine the way and manner in which it loses, shares and gains electrons. This enables us to assign to every atom an oxidation number. Oxidation number is that which specifies the number of electrons that can be involved in the formation of bonds with other atoms.

From the particular atoms in a molecule and their known bonding capacities, the bonding pattern within a molecule is determined and each atom is regarded as being in a specific oxidation state, expressed by an oxidation number.

Rules to be followed in determining oxidation numbers

· The oxidation state of an uncombined element is taken as zero. Thus, the oxidation state or number of aluminum, sodium, iron or chlorine molecule (Cl₂) is zero. This is because they are in uncombined state.

· The algebraic sum of the oxidation states of the elements in a compound is zero. In other words, the collective oxidation number of a compound is zero. Thus, the oxidation numbers of CaCl₂ and HNO₃ are equal to zero because when the oxidation state of each of the components of the compound are added together, zero is the result.

· The oxidation number of an ion is equal to the size and the charge on the ion. Thus, the oxidation number of Na⁺, Mg²⁺ and Al³⁺ are +1, +2 and +3 respectively;

Oxidation and Reduction in terms of Oxidation number

Oxidation and reduction can be recognized by changes in oxidation state (oxidation number). An increase in oxidation state corresponds to oxidation and a decrease in oxidation state corresponds to reduction. Manganese, for example, can exist in oxidation states 0 (in the metal), +1, +2, +3, +4, +5, +6, and +7. In the reaction of manganese (IV) oxide (MnO₂) with concentrated hydrochloric acid (HCl), the oxidation state of manganese changes from +4 (represented by the Roman numerals IV) to +2:

MnO₂ + 4HCl'------------>MnCl₂ + 2H₂O + Cl₂

Manganese(IV) oxide is reduced to manganese chloride (MnCl₂) and hydrochloric acid is oxidized to chlorine (Cl₂).

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