Oxidation and Reduction Reactions for the MCAT: Everything You Need to Know

Learn key MCAT concepts about oxidation and reduction reactions, plus practice questions and answers

Table of Contents

Part 1: Introduction to Oxidation and Reduction Reactions

Part 2: Overview of Atomic Structure

Part 3: Oxidation and Reduction

a) Definitions

b) Assigning oxidation states

c) Hydrides

Part 4: Applications of Redox Reactions

a) Writing and balancing redox reactions

b) Electrochemical cells

c) Electron transport chain

Part 5: High-Yield Terms

Part 6: Passage-Based Questions and Answers

Part 7: Standalone practice questions

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Part 1: Introduction to Oxidation and Reduction Reactions

Redox reactions constitute fundamental processes in our daily lives. Oxidation and reduction reactions are key in creating energy from the food we eat. Additionally, these processes are key to the operations behind batteries, in which reduction and oxidation reactions generate the power we need to drive our cars.

Redox reactions refer to chemical reactions in which the exchange of electrons results in the oxidation of some atoms and the reduction of others. Success on this topic requires a detailed understanding of both oxidation and reduction and how oxidation states change over the course of a given reaction.

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Part 2: Overview of Atomic Structure

How are electrons spatially distributed in an atom? The electron cloud model provides us with an answer and proves to be useful in visualizing how electrons are lost (oxidation) and gained (reduction) in redox reactions.

The electron cloud model, developed by Erwin Schrodinger in the 1920s, describes electrons as being distributed within certain regions of probability around the central nucleus. The electron cloud model-which is the preferred model of modern scientists-is distinct from Bohr's atomic model in which electrons were imagined to move in discrete concentric orbits around the nucleus-much like satellites around the Earth or the cabins of a Ferris wheel.

The electron cloud model emphasizes the indistinct nature of electron distribution. The electron cloud model theorized that electrons do not move in static orbits around the central nucleus, such that electrons are always a specific and discrete distance away from the center of the atom. Instead, we can only guess where an electron might be-that guess is mathematically computed and described as a region of probability called the electron cloud. These regions of probability can be visualized and illustrated as certain shapes, which gives rise to the atomic theory of subshells and electron orbitals.

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Part 3: Oxidation and Reduction

a) Definitions

You may already know that oxidation refers to a “loss of electrons,” while reduction refers to a “gain of electrons.” A useful mnemonic to use is: “LEO says GER”—which stands for “Losing Electrons is Oxidation and Gaining Electrons is Reduction.” The mnemonic “OIL RIG” can be used to a similar effect: “Oxidation Is Loss, Reduction Is Gain.” The word redox encompasses both of these processes: it is a portmanteau of the words “reduction” and “oxidation.”

Neutral copper (Cu), for example, becomes oxidized when it loses or donates electrons to become a positive ion (Cu²⁺). A compound is reduced when it gains electrons, for instance, as elemental oxygen (O₂) becomes water (H₂O).

It’s important to note the distinction between an oxidizing agent and a compound that is oxidized. The adjectives “oxidizing” and “reducing” describe the change that the agents or compounds are provoking in a different compound. They do not mean that the compound itself is becoming oxidized or reduced, respectively. Consequently, an oxidizing agent (or oxidant) is a compound that is reduced while enabling the oxidation of a different compound). A reducing agent (or reductant) is a compound that is oxidized, as it enables the reduction of a different compound (by donating electrons). 

Due to the nature of exchanging electrons, oxidation and reduction reactions are always coupled together. This implies that as one element is oxidized (or loses electrons), another element must be reduced (receives those electrons). 

The exception is in disproportion reactions, in which one element can undergo both oxidation and reduction. In this case, atoms from the same element simultaneously act as the oxidizing agent and the reducing agent.