Redox Reactions and Oxidation Numbers - SS2 Chemistry Lesson Note

Redox (reduction-oxidation) reactions are chemical reactions that involve the transfer of electrons between reactants. In a redox reaction, one species undergoes oxidation (loses electrons) while another species undergoes reduction (gains electrons). Oxidation involves an increase in oxidation number, while reduction involves a decrease in oxidation number.

The oxidation number is a concept used to keep track of the electron transfer in redox reactions. It represents the hypothetical charge that an atom would have if all shared electrons were assigned to the more electronegative atom.

Rules for Assigning Oxidation Numbers:

a.    The oxidation number of an uncombined element is zero.

b.    The oxidation number of a monatomic ion is equal to the charge on the ion.

c.     In compounds, the sum of the oxidation numbers of all atoms is equal to the charge of the compound.

d.    Hydrogen is typically assigned a +1 oxidation number, but it can be -1 when bonded to metals.

e.    Oxygen is typically assigned a -2 oxidation number, except in peroxides (where it is -1) and when bonded to fluorine (where it is positive).

f.      Fluorine always has an oxidation number of -1.

g.    Group 1 elements (alkali metals) have an oxidation number of +1, and group 2 elements (alkaline earth metals) have an oxidation number of +2.

Steps for Balancing Redox Reactions using Oxidation Numbers:

a.    Assign oxidation numbers to each atom in the reactants and products.

b.    Identify the atoms that change oxidation numbers (oxidation or reduction).

c.     Balance the atoms and charges by adding electrons to the appropriate side of the reaction.

d.    Multiply the half-reactions by appropriate factors to ensure that the number of electrons gained equals the number of electrons lost.

e.    Combine the half-reactions, cancelling out the electrons, and balance the remaining atoms and charges.

f.      Finally, balance the equation by adjusting the coefficients of the compounds.

Redox reactions have many practical applications, such as in energy production (e.g., combustion reactions), electrochemistry, corrosion, and biological processes (e.g., cellular respiration). Redox reactions can be classified as either spontaneous (occur without external assistance) or nonspontaneous (require an external energy source, such as electrolysis). Reducing agents are substances that undergo oxidation and cause the reduction of another substance by donating electrons. Examples include metals, hydrides, and reducing gases. Oxidising agents are substances that undergo reduction and cause the oxidation of another substance by accepting electrons. Examples include halogens, oxygen, and oxidising gases.