Chemistry · Unit 9: Applications of Thermodynamics · 14 min read · Updated 2026-05-11
Thermodynamic favorability versus rate — AP Chemistry
AP Chemistry · Unit 9: Applications of Thermodynamics · 14 min read
1. Core Distinction: Favorability vs Rate★★☆☆☆⏱ 4 min
Thermodynamic favorability (or spontaneity) describes whether a reaction can proceed without continuous external energy input, determined solely by the overall difference in Gibbs free energy between reactants and products. Reaction rate describes how fast a reaction proceeds to products, determined by the height of the activation energy barrier. Favorability tells you only if a reaction *can* occur, not how fast it will occur.
Reaction rate, by contrast, depends on the reaction pathway, not just the initial and final states. Rate is governed by activation energy, the minimum energy colliding reactants need to reach the transition state. From the Arrhenius relationship, rate decreases exponentially as $E_a$ increases: even a very negative $\Delta G$ can result in an immeasurably slow reaction if $E_a$ is very large.
\Delta G = G_{\text{products}} - G_{\text{reactants}}
Exam tip: On any AP question asking about rate vs favorability, always check ΔG first for favorability, then Ea for rate. Never assume a negative ΔG means a fast reaction—this is the most common exam trap.
2. Kinetic vs Thermodynamic Control of Competing Reactions★★★☆☆⏱ 5 min
When the same starting materials can form two different products via two competing reaction pathways, the final product mixture depends on whether the reaction is under kinetic or thermodynamic control, determined by reaction conditions and reversibility.
Exam tip: On FRQ, always explicitly connect control type to conditions: kinetic control = low temperature, irreversibility, only low Ea barriers accessible. Do not just state "low T gives kinetic product" without this reasoning to earn full points.
3. Catalyst Effects on Favorability vs Rate★★☆☆☆⏱ 3 min
A common misconception repeatedly tested on the AP exam is what properties catalysts change versus what they do not. A catalyst works by providing an alternative reaction mechanism (pathway) from reactants to products, with a lower activation energy than the uncatalyzed pathway. Lower $E_a$ increases the rate constant $k$, so the reaction proceeds faster for both the forward and reverse reactions equally.
Because a catalyst does not change the chemical identity or free energy of the starting reactants or final products, it does not change the overall $\Delta G$ for the reaction. This means a catalyst cannot change the thermodynamic favorability of a reaction: it cannot make a non-spontaneous ($\Delta G > 0$) reaction become spontaneous, and it does not change the equilibrium constant or the final product yield at equilibrium. It only makes the reaction reach equilibrium faster.
Exam tip: Any multiple-choice option that claims a catalyst changes ΔG, spontaneity, or the equilibrium constant K is automatically wrong. Only rate and activation energy are changed.
Common Pitfalls
Why: Students associate the everyday definition of 'spontaneous' (happens immediately) with the chemical definition, confusing thermodynamic possibility with kinetic feasibility
Why: Students know catalysts speed up reactions, so they incorrectly extrapolate that catalysts can make a non-spontaneous reaction spontaneous
Why: Students forget competing reactions can be under kinetic control if equilibrium cannot be reached
Why: Students confuse 'will not occur on its own' with 'can never occur'
Why: Students reverse the relationship between temperature and control type
Why: Students mix up kinetic and thermodynamic properties, using Ea (kinetic) to infer favorability