Kaplan MCAT General Chemistry Review

Next, we’ll find the pOH from [OH^-]. The pOH lies between 4 and 5. How did we find that? Well, 1.2 × 10^-5 is between 10^-5 and 10^-4. So its logarithm is between 4 and 5. Let’s estimate 4.8.

We’re ready to calculate pH from pOH.

pH = 14 - pOH = 14 - 4.8 = 9.2.

(C) is very close to 9.2, so it must be the right answer.

Chapter 11: Redox Reactions and Electrochemistry

The mitochondria are the power company of your body. No, this doesn’t mean that they send vaguely hostile letters threatening to cut off your electricity if you don’t pay your overdue bill (although cells certainly do send loud and clear messages when there is an oxygen debt). Rather, like actual power companies, the primary purpose of the mitochondria is to manufacture a deliverable and usable form of energy. Certainly by now you are generally well aware of the complex processes by which the potential energy of chemical bonds, which is really just electric potential energy, in food molecules (carbohydrates, amino acids, and lipids) is converted into the potential energy of the phosphate bond in adenosine triphosphate (ATP). ATP is then delivered to different regions of the cell, where it is used to energize all of the processes essential to the maintenance of life.

The mitochondria generate tremendous amounts of ATP. (In humans, the average daily turnover of ATP is more than 50 kilograms!) Without a continuous supply and replenishment of ATP, we couldn’t live for even a second in its absence: It powers the contraction of our heart muscle and maintains the membrane potential essential for neurological function ( just to name a couple of life-essential roles of ATP). How do the mitochondria manufacture these packets of life-sustaining energy? You have read of the double-membrane structure of the mitochondria and their electron transport chain and F₁F_o ATP synthase. You have learned about oxidative phosphorylation and the role of O₂. But has it ever occurred to you that the mitochondria in more or less literal ways act as the batteries of the cell? Have you ever wondered at the similarity of the phrase proton motive force to another term that you have learned and used in the context of electrochemistry and circuits? Are proton motive force (pmf) and electromotive force (emf) the same thing or, at the very least, similar in nature?

In fact, the mitochondria do function in ways similar to batteries: There is separation and buildup of a charge gradient; there is potential difference (voltage) between separated compartments; there is movement of charge and dissipation of energy. We could say that mitochondria function in ways most similar to a particular type of electrochemical cell called the concentration cell. In both concentration cells and mitochondria, a concentration gradient of ions between two separated compartments connected to each other by some means of charge conduction establishes an electrical potential difference (a voltage). This voltage, called electromotive force in a concentration cell and proton motive force in the mitochondria, provides the “pressure to move” charge (that is, creates current) from one compartment to the other. In the concentration cell, a redox reaction takes place, and electrons move in the direction that causes the concentration gradient to be dissipated. In the mitochondria, the charge buildup is in the form of a hydrogen ion (proton) gradient between the intermembrane space and the matrix. Embedded in the inner membrane is the F₁F_o ATP synthase protein, which serves the dual role of proton channel (the conductive pathway) and catalyst (the electric motor) for the formation of the high-energy phosphate bond of ATP. As the hydrogen ions flow down their chemical-electrical gradient, energy is dissipated (remember, the positively charged ions are moving from high potential to low potential), and this energy is harnessed by the ATP synthase for the formation of ATP.

In this, the final chapter of our review of general chemistry for the MCAT, we will focus our attention on the study of the movement of electrons in chemical reactions. Such reactions are called oxidations and reductions, and because they always occur in pairs, they are usually referred to, in shorthand, as redox reactions. Electrochemistry is the study of the relationships between chemical reactions and electrical energy. We will learn of the ways in which the principles of electrochemistry can be applied to create different types of electrochemical cells, including galvanic (voltaic), electrolytic, and concentration cells. Regarding the thermodynamics of electrochemistry, we will focus on the significance of reduction potentials and examine the relationship among electromotive force, the equilibrium constant, and Gibbs function.

Oxidation-Reduction Reactions