Electrochemistry drives the batteries in your car, runs your cell phone, and even powers cells within your body! Thus, electrochemistry has high biological and medical importance.

In this article, we’ll go over everything you need to know about electrochemistry for the MCAT. We’ll discuss how these concepts are applied using different electrochemical cells as well as their application to human biology.

Throughout this guide, several important terms are highlighted in bold. At the end of this guide, there are also several AAMC-style practice questions for you to test your knowledge with.

Let’s get started!

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Part 2: Cell potentials

a) Oxidation and reduction potentials

Recall that reduction and oxidation reactions are types of reactions in which atoms exchange electrons. Oxidation refers to the loss of electrons from an atom, whereas reduction refers to the gain of electrons by an atom. The mnemonic “OIL RIG,” Oxidation Is Loss and Reduction Is Gain, may be helpful in remembering this distinction. (For more information on these topics, be sure to refer to our guide on oxidation and reduction reactions.)

The standard oxidation potential tells us how likely a species is to be oxidized, or lose electrons, under standard conditions (1M concentrations, 1 atm pressure, 298K). Similarly, the standard reduction potential tells us how likely it is for a species to be reduced, or gain electrons, under standard conditions. Both of these are measured in volts (V).

Reduction and oxidation potentials are measured relative to a reference electrode, such as the standard hydrogen electrode (SHE) or the saturated calomel electrode (SCE). These reference electrodes allow us to calculate the emf of half-reactions in relation to a benchmark value. Thus, a reference electrode is assumed to have zero potential (0 V) in relation to itself.

Oxidation and reduction potentials are critical considerations in analyzing electrochemical cells. Electrochemical cells contain two sets of electrodes: an anode and a cathode. Electrons flow from the anode and toward the cathode. Thus, oxidation occurs at the anode and reduction occurs at the cathode. (Another useful mnemonic is “AN OX, RED CAT”: the first few syllables of the words anode—oxidation, and reduction—cathode.)

b) Electromotive force

The standard electromotive force (E° _cell), or emf, is the difference between the reduction potentials of the cathode and anode of an oxidation-reduction reaction. A spontaneous reaction will have a positive E° _cell, while a nonspontaneous reaction will have a negative E° _cell. Therefore, the free energy change (ΔG) of a reaction and the standard electromotive force of a reaction always has opposite signs.

E° _cell = E° _{reduction of cathode} – E° _{reduction of anode}

A half-reaction shows the chemical oxidation or reduction that takes place at a single electrode. Thus, the net overall equation for an electrochemical cell requires two half-reactions.

When presented with a problem where you have to calculate the standard electromotive force, you will most likely be given a set of half-reactions and their standard reduction potentials. For example, you might see something like this:

A⁺ + e^- →A (E°_reduction = 1.2 V)
B⁺ + e^- →B (E°_reduction = 0.6 V)

How can these half-reactions be put together to represent the overall electrochemical cell?

These are both reduction reactions. However, when two half-reactions are found in a single electrochemical cell, the one with a higher E°_reduction is the one that can be spontaneously reduced in the overall reaction. The one with a lower E°_reduction is the one that can be spontaneously oxidized in the overall reaction. In our example reactions above, A will be spontaneously reduced by B, and B will be spontaneously oxidized by A.

Thus, the half-reaction involving B would actually occur in the reverse direction. As a result, a better representation of the oxidation of species B is as follows:

B →B⁺ + e^- (E°_oxidation = -0.6 V)
Note that the E°_oxidation of a half-reaction is equal to -1 x E°_reduction, and vice-versa.
Thus, the net reaction for our example electrochemical cell is as follows:

A⁺ + B →A + B⁺

The table below provides examples of reduction potentials for several different half-reactions. You do not need to memorize these for test day, as any required values will be provided when needed.

Reaction	E° (V)
Al³⁺ + 3e^- →Al	-1.66
Zn²⁺ + 2e^- →Zn	-0.76
Cu²⁺ + 2e^- →Cu	0.34
Fe³⁺ + e^- →Fe²⁺	0.77
Ag+ + e^- →Ag	0.80

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Electrochemistry for the MCAT: Everything You Need to Know

Learn key MCAT concepts about electrochemistry, plus practice questions and answers

Part 1: Introduction to electrochemistry

Part 2: Cell potentials

Part 3: Concentration cells

Part 4: Electrochemical cells

Part 5: High-yield terms

Part 6: Passage-based questions and answers

Part 7: Standalone questions and answers

Part 1: Introduction to electrochemistry

Part 2: Cell potentials

a) Oxidation and reduction potentials

b) Electromotive force