In AP Chemistry, spontaneity (the CED calls it being "thermodynamically favored") means a process can occur without continuous outside intervention. A spontaneous reaction has ΔG° < 0, which means K > 1 and products are favored at equilibrium under standard conditions.
Spontaneity answers one question: can this process happen on its own, without something outside continuously pushing it? The AP exam mostly uses the phrase thermodynamically favored instead of "spontaneous," and they mean the same thing. A thermodynamically favored process has ΔG° < 0, and that single fact tells you the equilibrium constant K is greater than 1, so products dominate the mixture at equilibrium under standard conditions (EK 9.5.A.1).
Here's the part students miss. Spontaneity says nothing about speed. Diamond converting to graphite is spontaneous, but you'll wait a few million years. "Spontaneous" is a thermodynamics word about where a reaction is headed, not a kinetics word about how fast it gets there. You determine spontaneity from ΔG° = ΔH° - TΔS°, which means it depends on the enthalpy change, the entropy change, and the temperature. Some reactions are spontaneous at all temperatures, some at none, and some flip from non-spontaneous to spontaneous (or back) as T changes.
Spontaneity lives in Unit 9: Thermodynamics and Electrochemistry, specifically Topic 9.5 (Free Energy and Equilibrium). It directly supports learning objective 9.5.A, which asks you to explain whether a process is thermodynamically favored using the relationships between K, ΔG°, and T. The two equations you need are K = e^(-ΔG°/RT) and ΔG° = -RT ln K (EK 9.5.A.2). You also need the qualitative logic from EK 9.5.A.3: when ΔG° is near zero, K is close to 1; when ΔG° is much more negative or positive than RT, K is far from 1 in the matching direction. Spontaneity is the bridge concept that ties Unit 9 thermodynamics back to Unit 7 equilibrium, which makes it a favorite for multi-step exam questions.
Keep studying AP Chemistry Unit 9
Gibbs Free Energy (Unit 9)
ΔG° is literally the number that decides spontaneity. Negative ΔG° means thermodynamically favored, positive means not favored, and near zero means K ≈ 1 with a real mix of products and reactants. If you can read a ΔG° value, you can call spontaneity instantly.
Enthalpy (ΔH) (Units 6 & 9)
ΔH° is half of the spontaneity equation ΔG° = ΔH° - TΔS°. Exothermic reactions (ΔH° < 0) help spontaneity but don't guarantee it. An endothermic dissolution can still be spontaneous at high temperature if a positive ΔS° makes the -TΔS° term win.
Law of Mass Action (Unit 7)
K and spontaneity are two views of the same thing. ΔG° = -RT ln K converts an equilibrium constant from Unit 7 into a free energy value in Unit 9. K > 1 and ΔG° < 0 are the same statement in different languages.
Rate constant (k) (Unit 5)
This is the classic trap. Lowercase k (rate constant) tells you how fast a reaction goes, and it comes from kinetics. Spontaneity comes from thermodynamics and says nothing about rate. A reaction can be hugely spontaneous and still crawl because of a high activation energy.
Spontaneity shows up as multiple-choice questions that give you ΔH° and ΔS° (watch the units, ΔS° is usually in J/(mol·K) while ΔH° is in kJ/mol) and ask you to determine both the sign of ΔG° and what that implies about K. Another common stem describes equilibrium mixtures at two temperatures, like mostly products at 300 K but mostly reactants at 600 K, and asks you to reason backward to the signs of ΔH° and ΔS°. FRQs in Unit 9 frequently ask you to justify a claim such as "the equilibrium mixture is almost entirely products" by connecting a given ΔG° (say, -50 kJ/mol at 298 K) to K through ΔG° = -RT ln K. The move that earns points is explicit reasoning that links the sign and magnitude of ΔG° to the value of K, not just stating "it's spontaneous."
Spontaneous does not mean fast. Spontaneity is thermodynamics, set by ΔG°, and tells you which side of the reaction is favored at equilibrium. Speed is kinetics, set by activation energy and the rate constant k, and tells you how quickly equilibrium is reached. Rusting iron is spontaneous and slow; many explosive reactions sit unreacted for years until something supplies activation energy. The AP exam loves answer choices that smuggle a rate claim into a thermodynamics question, so keep the two ideas in separate boxes.
"Spontaneous" and "thermodynamically favored" mean the same thing on the AP exam, and both translate to ΔG° < 0 and K > 1.
ΔG° = ΔH° - TΔS° determines spontaneity, so temperature can flip a reaction from non-spontaneous to spontaneous when ΔH° and ΔS° have the same sign.
ΔG° and K are linked by ΔG° = -RT ln K, so a more negative ΔG° means a larger K and a more product-heavy equilibrium mixture.
When ΔG° is near zero, K is close to 1 and the equilibrium mixture contains meaningful amounts of both reactants and products.
Spontaneity tells you nothing about reaction rate; a spontaneous reaction can be extremely slow if its activation energy is high.
Always convert ΔS° from J/(mol·K) to kJ/(mol·K) before plugging into ΔG° = ΔH° - TΔS°, since mixing units is the most common calculation error.
Spontaneity means a process can occur without continuous outside intervention. On the AP exam this is called being "thermodynamically favored," defined as ΔG° < 0, which guarantees K > 1 and products favored at equilibrium under standard conditions.
No. Spontaneity is about thermodynamics (where equilibrium lies), not kinetics (how fast you get there). Diamond turning into graphite is spontaneous but takes geologic time because the activation energy is huge.
Yes. If ΔS° is positive and the temperature is high enough, the -TΔS° term outweighs a positive ΔH° and makes ΔG° negative. Endothermic salt dissolutions that are favored at high temperature are the textbook AP example.
Through ΔG° = -RT ln K. If ΔG° < 0, then K > 1 and products are favored; if ΔG° > 0, then K < 1 and reactants are favored; if ΔG° ≈ 0, then K ≈ 1. A ΔG° of -50 kJ/mol at 298 K gives a K large enough that the mixture is almost entirely products.
Spontaneity comes from ΔG° and tells you whether products are favored at equilibrium. The rate constant k comes from kinetics (Unit 5) and tells you how fast the reaction proceeds. They are calculated from completely different quantities and don't predict each other.