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MCAT general chemistry essentials: acid-base, thermodynamics, and kinetics
The three general chemistry clusters that drive Chem/Phys section scores: acid-base equilibria, thermodynamics, and kinetics, with the conceptual depth the MCAT actually tests.
Content Review · 10 min read · Published 2026-05-06
Acid-base equilibria: the math is the easy part
Most acid-base questions on the MCAT do not test whether you can compute a pH from scratch. They test whether you understand what is happening at the molecular level when an acid donates a proton, how that proton transfer shifts an equilibrium, and how buffers resist changes when small amounts of acid or base are added.
Start with the equilibrium constant intuition. Ka is the acid dissociation constant; a higher Ka means more dissociation, which means a stronger acid. pKa is just -log(Ka), so a lower pKa means a stronger acid. When pH equals pKa, the acid is exactly half-dissociated — this is the most useful single fact in acid-base.
The Henderson-Hasselbalch equation (pH = pKa + log([A-]/[HA])) tells you the ratio of conjugate base to acid at any pH. Buffer questions almost always come down to applying this equation. A buffer works best within about one pH unit of its pKa, which is why physiological buffers like bicarbonate (pKa around 6.1) and phosphate (pKa around 7.2) are biologically chosen.
- Higher Ka, lower pKa = stronger acid.
- pH = pKa when [A-] = [HA].
- Buffers work within ±1 pH unit of their pKa.
- Strong acids fully dissociate; weak acids partially dissociate.
The most useful acid-base intuition is what happens at the pKa, not how to compute exact pH values.
Thermodynamics: spontaneity, energy, and what actually drives reactions
Thermodynamics on the MCAT centers on Gibbs free energy: ΔG = ΔH - TΔS. A negative ΔG means the reaction is spontaneous. A positive ΔG means it is not spontaneous in the forward direction. ΔG of zero means the system is at equilibrium.
Enthalpy (ΔH) reflects whether the reaction releases heat (negative ΔH, exothermic) or absorbs heat (positive ΔH, endothermic). Entropy (ΔS) reflects whether the system becomes more disordered (positive ΔS) or more ordered (negative ΔS). Temperature matters because TΔS scales with T — at high temperatures, the entropy term dominates; at low temperatures, the enthalpy term dominates.
MCAT thermodynamics questions often combine these concepts: 'A reaction has negative ΔH and negative ΔS. At what temperatures is it spontaneous?' The answer requires you to recognize that ΔG = ΔH - TΔS becomes positive when TΔS exceeds ΔH, which happens at high T. So the reaction is spontaneous only at low temperatures.
- Spontaneous: ΔG < 0.
- Always spontaneous: ΔH < 0, ΔS > 0.
- Never spontaneous: ΔH > 0, ΔS < 0.
- Temperature-dependent: same signs for ΔH and ΔS.
Reaction kinetics: how fast vs how far
Thermodynamics tells you whether a reaction will go and how far it will go. Kinetics tells you how fast it will go. The two are independent — a thermodynamically favorable reaction can be kinetically slow, and that gap is where most enzyme questions live in the Bio/Biochem section.
Rate laws describe how reaction rate depends on reactant concentrations. A rate law looks like rate = k[A]^m[B]^n, where m and n are the orders with respect to A and B. The overall order is m + n. Rate constants depend on temperature via the Arrhenius equation; higher temperature generally means faster reaction.
Catalysts lower the activation energy of a reaction without being consumed. They do not change the equilibrium position or the overall ΔG; they only change how fast equilibrium is reached. Enzymes are biological catalysts, which connects this material directly to the Bio/Biochem section.
Kinetics and thermodynamics answer different questions. Do not let one mask the other.
Connecting clusters: where the section actually tests integration
The MCAT Chem/Phys section excels at building questions that span topic clusters. A passage might describe a buffer system in the body, ask you to calculate a pH (acid-base), then ask whether a temperature change would shift the equilibrium (thermodynamics), then ask how an enzyme inhibitor would change the rate of the underlying biochemical reaction (kinetics).
When you review, deliberately link the clusters. After studying acid-base equilibrium, ask 'how would temperature shift this equilibrium?' (thermodynamics) and 'how does an enzyme catalyze acid-base reactions in the body?' (kinetics + biology). The integration is what the test rewards.