Kaplan MCAT General Chemistry Review

where R is the gas constant 8.314 J/(K•mol), T is the absolute temperature in K, and K_eq is the equilibrium constant for the reaction.

Combining the two equations for standard free energy change (above), we see that

G° = -nFE°cell = -RT ln K_eq

or simply

nFE°_cell = RT ln K_eq

Key Concept

If E°_cell is positive, ln K is positive. This means that K must be greater than one and that the equilibrium lies toward the right (i.e., products are favored).

If the values for n, T, and K_eq are known, then the E° cell for the redox reaction can be readily calculated. On the MCAT, you will not be expected to calculate (or even approximate) natural log values in your head. That being said, these equations will still be tested but in a more conceptual way. Analysis of the equations shows us that for redox reactions that have equilibrium constants less than 1 (which tells us that the equilibrium state favors the reactants), the E°_cell will be negative, because the natural log of any number less than 1 is negative. These properties are characteristic of electrolytic cells, which house nonspontaneous redox reactions. If the K_eq for the redox reaction is greater than 1 (which tells us that the equilibrium state favors the products), the E°_cell will be positive, because the natural log of any number greater than 1 is positive. These properties are characteristic of galvanic cells, which house spontaneous redox reactions. If the K_eq is equal to 1 (which tells us that the concentrations of the reactants and products are equal at equilibrium), the E°_cell will be equal to zero, because by definition of standard conditions (all ionic species at the same concentration, 1 M), the reaction is already at equilibrium. An easy way to remember this is E°_cell = 0 V for any concentration cell, because, by definition, the equilibrium state of a concentration cell is when the concentrations of the ion in the two half-cells are equal (K_eq = 1 for concentration cell).

Conclusion

In this chapter, we covered the essential MCAT topics of redox reactions and electrochemistry. We reviewed the rules for assigning oxidation numbers to help us keep track of the movement of electrons from the species that are oxidized (reducing agents) to the species that are reduced (oxidizing agents). We also covered the sequence of steps involved in balancing redox reactions through the half-reaction method and the properties and behavior of the different types of electrochemical cells. Galvanic cells rely on spontaneous redox reactions to produce current and supply energy. The concentration cell is a special type of galvanic cell for which the current is dependent upon an ion concentration gradient rather than a difference in reduction potential between two chemically distinct electrodes. Electrolytic cells rely on external voltage sources to drive a nonspontaneous redox reaction called electrolysis. Finally, we considered the thermodynamics of the different cell types. Galvanic and concentration cells have positive emf and negative free energy changes. Electrolytic cells have negative emf and positive free changes.

Without further delay, we want to offer you our heartiest congratulations for completing this review of general chemistry for the MCAT. The hard work, time, and energy you have invested in a careful, thorough, and thoughtful review of the topics covered within the pages of this book will pay off in points on Test Day—you can be sure of that. We hope that we have been successful in meeting our goals in writing these review notes: to assess the general concepts and principles essential to answering correctly the general chemistry questions on the MCAT; to guide you in the development of critical thinking skills necessary for analyzing passages, question stems, and answer choices; and to provide holistic preparation for your Test Day experience. In addition to all of these, we had in mind the particular goals of helping you relate the science to your everyday life experiences and future experiences as a physician, demystify it, and have some fun in the process. We are grateful for the opportunity to partner with you in your journey to success on the MCAT and, beyond that, success in your medical education and future practice as a great physician!

CONCEPTS TO REMEMBER

Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG). Oxidation and reduction always occur as paired of reactions, in accordance with the law of conservation of charge.

The oxidizing agent is the chemical species that causes another species to be oxidized and is itself reduced. The reducing agent is the chemical species that causes another species to be reduced and is itself oxidized. Assign oxidation numbers to identify the oxidizing and reducing agents.

Redox reactions can be balanced by the half-reaction method:

—Separate the half-reactions.

—Balance the atoms in each half-reaction.