Oxidation and Reduction Reactions
High-Yield Summary
- A redox reaction transfers electrons between species; conservation of charge means oxidation and reduction always happen together — one can't occur without the other.
- Oxidation = losing electrons; reduction = gaining electrons. "LEO the lion says GER" or "OIL RIG."
- The oxidizing agent gets reduced (gains electrons); the reducing agent gets oxidized (loses electrons) — naming describes the effect on the other species, not itself.
- Mg + 2HCl → MgCl₂ + H₂: Mg is the reducing agent (Mg → Mg²⁺ + 2e⁻), H⁺ is the oxidizing agent (2H⁺ + 2e⁻ → H₂).
- NAD⁺/NADH and FAD/FADH₂ are biochemical redox pairs that shuttle electrons through metabolism — reduced during catabolism, oxidized during anabolism.
Key Terms
- Redox reaction
- A reaction involving transfer of electrons from one chemical species to another.
- Oxidizing agent
- The substance that causes another species to lose electrons (be oxidized); it itself gains electrons and is reduced.
- Reducing agent
- The substance that causes another species to gain electrons (be reduced); it itself loses electrons and is oxidized.
- NAD⁺/NADH
- Nicotinamide Adenine Dinucleotide — oxidized (NAD⁺) and reduced (NADH) forms of a biological electron-carrier coenzyme.
- FAD/FADH₂
- Flavin Adenine Dinucleotide — oxidized (FAD) and reduced (FADH₂, gains 2 e⁻ and 2 H⁺) forms of a flavin-based electron carrier.
Common Oxidizing vs. Reducing Agents
| Oxidizing agents (gain e⁻, get reduced) | Reducing agents (lose e⁻, get oxidized) |
|---|---|
| O₂, H₂O₂ | CO, C |
| Halogens (F₂, Cl₂, Br₂, I₂) | B₂H₆ (diborane) |
| H₂SO₄, HNO₃, NaClO | Sn²⁺ and other metal ions, Zn(Hg) |
| KMnO₄, CrO₃, Na₂Cr₂O₇, PCC | NaBH₄, LiAlH₄, Lindlar's catalyst |
| NAD⁺, FAD | NADH, FADH₂ |
Worked Example: Mg + HCl
- 1Overall: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
- 2Oxidation half-reaction: Mg → Mg²⁺ + 2e⁻ — magnesium loses electrons.
- 3Reduction half-reaction: 2H⁺ + 2e⁻ → H₂ — hydrogen ions gain those electrons.
- 4Mg is the reducing agent (donates e⁻, is oxidized); H⁺ is the oxidizing agent (accepts e⁻, is reduced).
Must-Know Points
- Oxidizing agents are often characterized by oxygen or a strongly electronegative element (halogens); reducing agents typically contain metal ions or hydrides (H⁻).
- NAD⁺ and FAD play dual roles depending on pathway direction: reduced (to NADH/FADH₂) during catabolism to store energy, oxidized back during anabolism to release it for biosynthesis.
- Recognizing common oxidizing/reducing agents by sight speeds up problem-solving, especially for organic chemistry reactions on the MCAT.
Common MCAT Trap
- It's counterintuitive but correct: the oxidizing agent itself gets reduced, and the reducing agent itself gets oxidized — the name describes what the agent does to the OTHER species.
- The oxidized form of the flavin cofactor is FAD, not "FADH" — FAD gains 2 e⁻ and 2 H⁺ to become the reduced form, FADH₂.
Quick Recall
In Mg + 2HCl → MgCl₂ + H₂, which species is the reducing agent?
What does "LEO the lion says GER" stand for?
During catabolism, is NAD⁺ oxidized or reduced?