General Chemistry: Thermodynamics and Kinetics

Thermodynamics tells you whether a reaction is favorable and how far it goes; kinetics tells you how fast it gets there. The MCAT loves to separate these two ideas, so the single most important habit is to never confuse favorability (thermodynamics) with speed (kinetics). Master the sign conventions below and most Chem/Phys questions in this area become predictable.

Core Idea

• Thermodynamics = favorability and equilibrium position; kinetics = rate. A reaction can be highly favorable (very negative delta G) yet infinitely slow, and vice versa.
• Gibbs free energy is the master variable: delta G = delta H - T delta S. A negative delta G means spontaneous (thermodynamically favorable) in the forward direction.
• Catalysts change kinetics only. They lower activation energy and speed both directions equally, but never shift the equilibrium position or change delta G.

The Laws and State Functions

The first law is conservation of energy: delta E = q + w; energy is neither created nor destroyed. The second law says the entropy of the universe always increases for a spontaneous process. The third law sets the entropy of a perfect crystal at absolute zero to zero.

A state function depends only on the current state, not the path taken. Enthalpy (H), entropy (S), Gibbs free energy (G), internal energy (E), temperature, and pressure are all state functions. Heat (q) and work (w) are path functions — a classic trap. Because H, S, and G are state functions, you can use Hess's law to add reaction steps.

Enthalpy, Entropy, and Gibbs Free Energy

Enthalpy (delta H) is heat exchanged at constant pressure. Exothermic reactions release heat (delta H negative); endothermic reactions absorb heat (delta H positive). Entropy (delta S) measures disorder or the dispersal of energy; it increases going solid to liquid to gas, on dissolving, and when moles of gas increase.

Gibbs free energy combines both: delta G = delta H - T delta S.

• delta G < 0: spontaneous (favorable) forward.
• delta G > 0: nonspontaneous forward (spontaneous in reverse).
• delta G = 0: system is at equilibrium.

Sign analysis is high-yield: if delta H is negative and delta S is positive, the reaction is spontaneous at all temperatures. If delta H is positive and delta S is negative, it is never spontaneous. When the signs match (both negative or both positive), temperature decides — the T delta S term determines the outcome.

Reaction Kinetics

Kinetics describes rate. The rate law has the form rate = k[A]^m[B]^n, where the exponents (the orders) are found experimentally, not from stoichiometric coefficients. The rate-determining step is the slowest elementary step and governs the overall rate. Activation energy (Ea) is the energy barrier reactants must overcome to reach the transition state.

By collision theory, reactions occur when particles collide with sufficient energy and correct orientation. Raising temperature, concentration, or surface area increases collision frequency or energy and thus rate. A catalyst provides an alternate pathway with lower Ea, speeding both forward and reverse reactions equally without being consumed and without altering delta G or Keq.

Chemical Equilibrium

At equilibrium, forward and reverse rates are equal and concentrations stop changing. Keq = products over reactants, each raised to its coefficient (pure solids and liquids excluded). A large Keq favors products.

Le Chatelier's principle: a system at equilibrium shifts to counteract a stress. Adding reactant shifts right; removing product shifts right; increasing pressure shifts toward fewer moles of gas; for temperature, treat heat as a reactant (endothermic) or product (exothermic).

Conceptually, delta G and Keq are linked: a negative delta G corresponds to Keq greater than 1 (products favored), delta G = 0 gives Keq = 1, and a positive delta G gives Keq less than 1 (reactants favored).

High-Yield Exam Patterns

• If a question says "favorable" or "spontaneous," think delta G sign, not speed. If it says "fast/slow," think kinetics/Ea.
• Given signs of delta H and delta S, predict the temperature range for spontaneity using delta G = delta H - T delta S.
• A catalyst answer never changes delta G, delta H, Keq, or equilibrium position — only Ea and rate.
• Reaction orders come from experimental data, never from the balanced equation's coefficients.
• Use Le Chatelier for any "add/remove/pressure/heat" stress; predict the direction of shift.
• Remember heat and work are path functions; H, S, G, E, T, P are state functions.

Common Traps to Avoid

• Confusing thermodynamic favorability with reaction speed — a very negative delta G says nothing about rate.
• Assuming a catalyst shifts equilibrium or improves yield; it only speeds the approach to equilibrium.
• Reading rate-law exponents off the balanced equation instead of experimental data.
• Forgetting the temperature dependence when delta H and delta S have the same sign.
• Thinking exothermic always means spontaneous; entropy and temperature can override enthalpy.