MCAT Chem-Phys General Chemistry Review

General chemistry forms the indispensable molecular language of the MCAT Chemical and Physical Foundations section. Mastering it is not about rote memorization but about building a flexible, interconnected framework that allows you to dissect complex experimental passages and solve problems in biological contexts, from enzyme kinetics to renal physiology. Your success hinges on moving from isolated facts to applied reasoning.

Atomic Architecture and the Predictive Power of the Periodic Table

Everything in chemistry begins with the atom. The Bohr model, while simplified, introduces the concept of discrete energy levels and electron transitions, which explain atomic emission spectra—a common passage topic. The modern quantum mechanical model describes electrons in orbitals (probability clouds), characterized by quantum numbers. This underpins the organization of the periodic table.

Periodic trends are predictable patterns in elemental properties. Crucially, you must explain why trends exist. Effective nuclear charge ($Z_{eff}$) is the net positive charge an electron feels, accounting for shielding by other electrons. As $Z_{eff}$ increases across a period (left to right), electrons are held more tightly. This drives the trends:

• Atomic radius decreases across a period (increased pull) and increases down a group (additional electron shells).
• Ionization energy (energy to remove an electron) increases across a period and decreases down a group.
• Electronegativity (ability to attract bonding electrons) follows the same pattern as ionization energy.

For the MCAT, you'll often apply these trends to compare elements within a biological molecule or predict reactivity in a described compound.

Bonding, Geometry, and the Forces Between Molecules

Atoms bond to achieve stability. Ionic bonding involves electron transfer, forming cations and anions held by electrostatic forces, common in salts. Covalent bonding involves electron sharing. Drawing Lewis structures is a fundamental skill: count valence electrons, connect atoms with single bonds, and distribute remaining electrons to satisfy the octet rule (except for H, Be, B, and period 3+ elements).

Molecular shape dictates function. Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron groups (bonds and lone pairs) arrange to minimize repulsion. From the parent geometries (linear, trigonal planar, tetrahedral, etc.), you can derive molecular shapes like bent, trigonal pyramidal, and see-saw. Memorize the bond angles. Hybridization ($sp$, $s{p}^2$, $s{p}^3$) explains how atomic orbitals mix to form these geometries and the nature of bonds: sigma ($\sigma$) bonds (head-on overlap) and pi ($\pi$) bonds (sideways overlap, found in double/triple bonds).

The real-world behavior of substances is governed by intermolecular forces (IMFs), which are weaker than bonds but critical for properties like boiling point and solubility. In order of increasing strength:

1. London dispersion forces: Temporary dipoles in all molecules, strength increases with molecular weight and surface area.
2. Dipole-dipole interactions: Between permanent dipoles in polar molecules.
3. Hydrogen bonding: A strong dipole-dipole interaction between H bonded to N, O, or F and a lone pair on another N, O, or F.

For biology, hydrogen bonding is paramount—it holds DNA strands together, stabilizes protein secondary structure, and gives water its unique properties.

Stoichiometry, Solutions, and Acid-Base Equilibrium

Stoichiometry uses balanced chemical equations to relate quantities of reactants and products. The mole is your conversion unit. Be comfortable with limiting reactant problems and calculating percent yield, often presented in a lab synthesis passage.

In solution chemistry, concentration is key. Molarity ($M$) is moles solute per liter solution. Molality ($m$) is moles solute per kilogram solvent, used for colligative properties. Colligative properties (vapor pressure lowering, boiling point elevation, freezing point depression, osmotic pressure) depend only on the number of solute particles, not identity. Osmotic pressure ($\Pi =iMRT$) is especially high-yield for understanding fluid balance across cell membranes.

Acid-base chemistry is a major MCAT theme. The Brønsted-Lowry definition (acids donate $H^{+}$, bases accept $H^{+}$) is most used. Know how to use $K_a$ (acid dissociation constant) and $p{K}_a$ ($p{K}_a=-\log K_a$); a lower $p{K}_a$ means a stronger acid. The Henderson-Hasselbalch equation is essential for buffers: $$pH=p{K}_a+\log \frac{[A^{-}]}{[HA]}$$ A buffer resists pH change upon addition of small amounts of acid or base. It consists of a weak acid and its conjugate base (or weak base/conjugate acid). At the half-equivalence point, $pH=p{K}_a$. Buffers are everywhere in physiology (e.g., bicarbonate buffer in blood).

Energy, Electrochemistry, and Biological Systems

Thermochemistry studies energy changes. The First Law of Thermodynamics states energy is conserved. Enthalpy ($\Delta H$) is heat transfer at constant pressure; negative $\Delta H$ is exothermic. Entropy ($\Delta S$) is disorder; spontaneous processes favor increased entropy. Gibbs Free Energy ($\Delta G$) determines spontaneity: $\Delta G=\Delta H-T\Delta S$. A negative $\Delta G$ is spontaneous. This directly connects to bioenergetics and ATP hydrolysis.

Electrochemistry deals with redox reactions ($\text{reduction}=\text{gain }{e}^{-}$, $\text{oxidation}=\text{loss }{e}^{-}$). In a galvanic (voltaic) cell, a spontaneous redox reaction generates electrical energy. Reduction potential ($E^{\circ }$) measures a species' tendency to be reduced. Cell potential is $E_{\text{cell}}^{\circ }=E_{\text{cathode}}^{\circ }-E_{\text{anode}}^{\circ }$. The relationship between free energy and cell potential is $\Delta G^{\circ }=-nF{E}_{\text{cell}}^{\circ }$. Think about the electron transport chain as a biological galvanic cell.

Common Pitfalls

1. Memorizing Trends Without Understanding Zeff: Simply recalling that electronegativity increases left-to-right will fail you on a question that asks why fluorine is more electronegative than oxygen. Always root your answer in effective nuclear charge and electron shielding.

1. Confusing Molecular Geometry with Electron Geometry: VSEPR requires you to count electron groups (geometry) first, then derive the molecular shape based on the positions of atoms. A molecule like $H_{2}O$ has a tetrahedral electron geometry but a bent molecular shape due to the two lone pairs.

1. Misapplying the Henderson-Hasselbalch Equation: A common trap is using initial concentrations instead of equilibrium concentrations after a reaction. If strong acid is added to a buffer, it reacts stoichiometrically with the conjugate base ($A^{-}$). You must calculate the new $[HA]$ and $[A^{-}]$ before plugging into the equation.

1. Sign Errors in Thermodynamics and Electrochemistry: Remember the conventions. $\Delta G<0$ is spontaneous. For a galvanic cell, $E_{\text{cell}}^{\circ }>0$ indicates a spontaneous process. Always double-check the sign based on the process described (formation vs. combustion, oxidation vs. reduction).

Summary

• Foundational Models: Atomic structure (quantum model) and periodic trends (driven by $Z_{eff}$) provide the framework for predicting chemical behavior.
• Structure-Function Relationship: Covalent bonding and VSEPR theory determine molecular geometry, which, combined with intermolecular forces (especially hydrogen bonding), dictates physical properties and biological interactions.
• Quantitative Problem-Solving: Proficiency in stoichiometry, solution concentration units, and the Henderson-Hasselbalch equation for buffers is non-negotiable for passage-based calculations.
• Energy is Central: Thermochemistry ($\Delta G=\Delta H-T\Delta S$) and electrochemistry ($\Delta G=-nF{E}_{\text{cell}}$) provide the energetic principles underlying all biological processes, from metabolism to nerve conduction.
• MCAT Context is Key: Always be prepared to link general chemistry concepts to biochemical systems, such as buffer action in blood, osmosis in cells, or redox reactions in metabolism.