MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020
Equilibrium
Introduction
Chapter Profile
The content in this chapter should be relevant to about 3% of all questions about general chemistry on the MCAT.
This chapter covers material from the following AAMC content categories:
1D: Principles of bioenergetics and fuel molecule metabolism
5A: Unique nature of water and its solutions
5E: Principles of chemical thermodynamics and kinetics
MCAT Expertise
The AAMC has shown that it will only rarely directly test the details of equilibrium in this chapter. However, you’ll need an excellent conceptual understanding of equilibrium to master many other high-yield topics, such as acid-base chemistry (Chapter 10 of MCAT General Chemistry Review) and enzymes (Chapter 2 of MCAT Biochemistry Review).
Introduction
You’re on the first call of your pediatrics rotation. You get a page from the resident: Come to the emergency room, now, she says. They just brought in a kid with DKA. DKA, as you know, stands for diabetic ketoacidosis and is a fairly common way for undiagnosed type I diabetes mellitus to present. You remember from your second-year classes about endocrine pathophysiology that ketoacidosis can arise as a result of the body’s metabolism of fatty acids when insulin production shuts down. Fatty acids are metabolized into ketone bodies as an alternative energy source to glucose. Some of the ketones produced are ketoacids, and as the diabetic crisis continues and worsens, the concentration of these ketoacids increases (termed metabolic acidosis), resulting in a plasma pH below 7.35.
As you enter the child’s room, the examination is already under way; he’s young, about ten years old, conscious but agitated, and the most obvious sign—which you notice immediately—is his rapid, shallow breathing. You ask the resident why the boy is hyperventilating, and she takes a piece of paper and writes the following:
CO2 (g) + H2O (l) ⇌ H2CO3 (aq) ⇌ H+ (aq) + HCO3− (aq)
It’s Le Châtelier’s principle! you realize. The respiratory system is trying to compensate for the metabolic acidosis; the increased breathing rate allows him to blow off more CO2, which causes the equilibrium to shift to the left. Hydrogen ions combine with bicarbonate ions to produce carbonic acid, which decomposes into CO2 gas that’s expelled from the lungs. The result is a decrease in the plasma hydrogen ion concentration, which stabilizes the pH and keeps it from getting too low. Wow, chemistry really is essential for medical school!
Chemical equilibrium is the dynamic state of a chemical reaction in which the concentrations of reactants and products stabilize over time in a low-energy configuration. Pay particular attention to the concepts of chemical equilibrium because we will return to these topics during our review of solutions, acid—base chemistry, and oxidation—reduction reactions.