MCAT thermodynamics governs whether a process can happen and which direction it favors. Enthalpy tracks heat, entropy tracks disorder, and Gibbs free energy combines them into a single verdict on spontaneity. These ideas anchor everything from bioenergetics and ATP hydrolysis to protein folding, making thermochemistry one of the most reused frameworks on the exam.
check_circleEntropy (ΔS) measures disorder; it increases for melting, boiling, and forming more gas molecules.
check_circleGibbs: ΔG = ΔH − TΔS; ΔG < 0 is spontaneous, ΔG = 0 is equilibrium.
check_circleSpontaneity can be temperature-dependent when ΔH and ΔS have the same sign.
check_circleHess’s law: ΔH is a state function, so it adds up over any pathway.
check_circleDuring a phase change, added heat changes phase at constant temperature (a plateau).
target What you'll be able to do · aligned to AAMC content category 5E
5EClassify processes as exothermic or endothermic and compute ΔH from bond energies or Hess’s law.
5EPredict the sign of entropy change (ΔS) for physical and chemical processes.
5EUse ΔG = ΔH − TΔS to determine spontaneity and its temperature dependence.
5EInterpret heating curves, distinguishing temperature change from phase-change plateaus.
How the MCAT tests thermodynamics
Thermodynamics is part of AAMC content category 5E and pairs constantly with biochemistry — ATP, metabolism, and protein folding are all thermodynamics problems in disguise. The MCAT emphasizes signs and reasoning (is ΔG negative? does entropy rise?) over heavy calculation.
workspace_premium The one equation
ΔG = ΔH − TΔS ties the whole topic together. Enthalpy and entropy each push on spontaneity, and temperature decides which wins when they disagree.
Enthalpy: heat of reaction
Enthalpy change (ΔH) is the heat absorbed or released at constant pressure. Breaking bonds costs energy (endothermic); forming bonds releases it (exothermic). The net of the two sets the sign of ΔH.
Exothermic vs endothermic. Exothermic reactions release heat and place products below reactants; endothermic reactions absorb heat and place products above.
ΔH°rxn = Σ ΔH°f(products) − Σ ΔH°f(reactants)Standard enthalpyCompute a reaction’s enthalpy from standard enthalpies of formation; elements in their standard state have ΔH°f = 0.
workspace_premium Hess’s law
Because enthalpy is a state function, you can add the ΔH of any sequence of steps that sums to the overall reaction — the path does not matter, only the start and end.
Entropy: the drive toward disorder
Entropy (S) measures the number of ways to arrange a system’s energy and matter — loosely, its disorder. Processes that spread things out increase entropy.
ΔS > 0 (increasing disorder): solid → liquid → gas, dissolving a solid, or producing more gas molecules.
ΔS < 0 (increasing order): gas → liquid → solid, or combining many molecules into fewer.
A quick check: count moles of gas. More gas on the product side usually means ΔS > 0.
Gibbs free energy and spontaneity
Gibbs free energy combines enthalpy and entropy into a single spontaneity criterion. A process is spontaneous (thermodynamically favorable) when ΔG is negative.
Spontaneity map. When ΔH and ΔS favor opposite outcomes, temperature is the tiebreaker: the TΔS term grows with T, so it dominates at high temperature.
ΔG = ΔH − TΔSGibbs free energyΔG < 0 spontaneous, ΔG = 0 equilibrium, ΔG > 0 nonspontaneous (spontaneous in reverse). Also ΔG° = −RT ln K.
functions Worked example — temperature and spontaneity
A reaction has ΔH = +50 kJ/mol and ΔS = +0.10 kJ/(mol·K). Is it spontaneous at 300 K? At 600 K?
Because ΔH and ΔS are both positive, raising T makes the −TΔS term dominate.
Answer. Nonspontaneous at 300 K, spontaneous at 600 K — this reaction is entropy-driven at high temperature.
Heating curves and phase changes
When you add heat to a substance, its temperature rises — except during a phase change, when the added energy goes into breaking intermolecular forces at constant temperature.
Heating curve. Sloped segments warm a single phase; flat plateaus are melting and boiling, where temperature holds constant while the phase changes.
Sloped region: q = mcΔT — heat raises temperature within one phase (c is specific heat).
Plateau: q = nΔH — heat drives the phase change (fusion or vaporization) at constant T.
The boiling plateau is longer than melting because vaporization takes more energy than fusion.
The laws in brief and biological links
The three thermodynamic laws frame everything: energy is conserved (first law), the entropy of the universe increases for any spontaneous process (second law), and a perfect crystal at absolute zero has zero entropy (third law).
Living systems stay ordered (local ΔS < 0) by increasing the entropy of their surroundings even faster — powered by ΔG-negative reactions like ATP hydrolysis. This is the bridge into biochemistry and metabolism.
How to study it — and the traps to avoid
Thermodynamics rewards sign-sense over arithmetic. Practice predicting the sign of each term and what it does to ΔG.
For every process, predict the signs of ΔH and ΔS first.
Use ΔG = ΔH − TΔS to find the temperature where spontaneity flips.
Separate q = mcΔT (temperature change) from q = nΔH (phase change).
Remember spontaneous says nothing about speed — that is kinetics.
error Highest-yield trap
A spontaneous (ΔG < 0) reaction can still be immeasurably slow. Thermodynamics predicts direction and favorability; it says nothing about rate. Speed is set by activation energy — see kinetics.
3 real questions on Thermodynamics & Thermochemistry
Straight from the Progress Note question bank, each with a cited explanation. No account needed.
123
According to the second law of thermodynamics, for any spontaneous (naturally occurring) process, what must happen to the TOTAL entropy of the universe — that is, the entropy of the system plus its surroundings?
Answer: A. The second law of thermodynamics states that the total entropy of the universe always increases for a spontaneous process, even if the entropy of the system itself happens to decrease (the surroundings then gain more than enough entropy to compensate). B describes a non-spontaneous direction, C would describe a reversible ideal process rather than a real spontaneous one, and D and E misstate the law. For any spontaneous process, the total entropy of the universe increases.
Source: Atkins, P., & de Paula, J. (2014). Physical chemistry (10th ed.). Oxford University Press.
In an endothermic dissolution process, a salt dissolves more readily as temperature increases. Which thermodynamic parameter most directly explains this temperature dependence?
Answer: B. A salt with both positive ΔH and positive ΔS becomes more soluble at higher temperature because TΔS grows more influential, leading to more negative ΔG. Negative entropy changes would not favor dissolution as temperature rises. ΔG for dissolution is temperature dependent when ΔS is nonzero.
Source: Atkins, P., & de Paula, J. (2010). Physical chemistry (9th ed.). Oxford University Press.
Which statement best predicts the major product in the Diels–Alder cycloaddition of cyclopentadiene with maleic anhydride under kinetic control at room temperature?
Answer: B. Under typical conditions, the Diels–Alder reaction of an electron-rich diene and an electron-poor dienophile favors the endo product via secondary orbital interactions between the dienophile’s π* and the diene’s π system. This endo preference is kinetic; exo can be favored at higher temperatures or longer times via equilibration.
Source: Houk, K. N., et al. (2005). Distortion/interaction analysis of the Diels-Alder reaction. Journal of the American Chemical Society, 127(8), 2974-2983
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What is the difference between exothermic and endothermic?expand_more
An exothermic process releases heat to the surroundings and has a negative ΔH; its products lie below the reactants in energy. An endothermic process absorbs heat, has a positive ΔH, and its products lie above the reactants.
How do I predict the sign of entropy change?expand_more
Entropy increases when a system becomes more disordered: solid → liquid → gas, dissolving, or producing more moles of gas all give ΔS > 0. Processes that create more order, like a gas condensing or many molecules combining into fewer, give ΔS < 0. A fast check is to compare moles of gas on each side.
When is a reaction spontaneous?expand_more
A reaction is spontaneous when ΔG = ΔH − TΔS is negative. If ΔH < 0 and ΔS > 0 it is spontaneous at all temperatures; if both have the same sign, temperature decides — the −TΔS term grows with temperature and can flip the outcome.
Why does temperature stay constant during a phase change?expand_more
During melting or boiling, the added heat goes entirely into overcoming intermolecular forces rather than increasing kinetic energy, so the temperature holds steady until the phase change is complete — the flat plateau on a heating curve.
Does a negative ΔG mean the reaction is fast?expand_more
No. A negative ΔG means the reaction is thermodynamically favorable, but it says nothing about rate. A reaction can be very spontaneous yet extremely slow if its activation energy is high; speed is governed by kinetics, not thermodynamics.
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