Understanding Redox Reactions and Oxidation States

Oxidation-Reduction (Redox) Reactions

• Redox reactions are chemical reactions involving the transfer of electrons between two species.
• Oxidation number of a molecule, atom, or ion changes through gaining or losing electrons.
• These reactions are vital for life, including photosynthesis, respiration, combustion, and corrosion/rusting.

Rules for Assigning Oxidation States

• Oxidation state (OS) of an element corresponds to the number of electrons an atom loses, gains, or appears to use when joining with other atoms in compounds.
• The OS of an individual atom is 0.
• The total OS of all atoms in a neutral species is 0 and equals the ion charge in an ion.
• Group 1 metals have an OS of +1 and Group 2 have an OS of +2.
• Fluorine's OS is -1 in compounds.
• Hydrogen's OS is usually +1 in compounds.
• Oxygen's OS is generally -2 in compounds, but -1 in hydrogen peroxide (H2O2) and peroxide ion.
• In binary metal compounds: Group 17 elements have an OS of -1, Group 16 have an OS of -2, and Group 15 have an OS of -3.
• In neutral molecules, the sum of the oxidation numbers of all atoms is zero, while in polyatomic ions, it equals the net charge of the ion.
• Oxidation numbers can be non-integers; for example, the oxidation number of O in the superoxide ion, O2-, is 1/2.

Oxidation-Reduction Reactions

• Redox reactions consist of a reduced half (gains electrons, oxidation number decreases) and an oxidized half (loses electrons, oxidation number increases).
• The ion or molecule accepting electrons is called the oxidizing agent.
• The species donating electrons is the reducing agent.
• Oxidized substances are reducing agents, while reduced substances are oxidizing agents; both agents can be the same in disproportionation reactions.
• The extent of oxidation must be equal to the extent of reduction; the number of electrons lost by a reducing agent must equal the number gained by an oxidizing agent.

Intermolecular vs. Intramolecular Redox Reactions

• Intermolecular redox reactions occur between molecules of two different substances.
• Intramolecular redox reactions occur when a molecule of a single substance undergoes oxidation-reduction via decomposition.

Disproportionation Reactions

• A single substance is both oxidized and reduced.

Redox Reactions in Biology

• Many significant biological processes involve redox reactions.
• Cellular respiration: Glucose (C6H12O6) is oxidized to CO2, and oxygen is reduced to water; C6H12O6 + 6 O2 → 6 CO2 + 6 H2O.
• Cellular respiration relies on the reduction of NAD (Nicotinamide adenine dinucleotide) to NADH.
• Photosynthesis and cellular respiration are complementary; photosynthesis is not the reverse of the redox reaction in cell respiration: 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2.
• Biological energy is stored and released frequently through redox reactions; photosynthesis reduces carbon dioxide into sugars and oxidizes water into molecular oxygen.

Significance of Redox Reactions

• Redox reactions are important in chemistry, geology, and biology.
• Earth's surface is a redox boundary between the planet's reduced metallic core and oxidizing atmosphere.
• Earth's crust is mainly metal oxides, and oceans are filled with water (hydrogen oxide).
• Photosynthesis reverses the atmosphere's oxidization of surface materials and renews carbon dioxide, allowing Earth's complex compounds to exist.
• Chemical technology relies on reducing materials to lower oxidation states than naturally occurring.
• Examples of this include ammonia, hydrogen, and nearly all metals created by industrial processes
• Solar radiation is turned into energy on a global scale due to a redox cycle in which photosynthesis converts radiant energy into stored chemical energy; carbon compounds are reduced to low oxidation states, while oxidation reactions at ambient or heated conditions releases chemical energy.