spontaneously
Summary
A spontaneous process proceeds in the forward direction on its own, without continuous outside energy input. At constant T and P it has ΔG < 0, equivalently the entropy of the universe increases. Spontaneous says nothing about speed, which is set by kinetics.
Detail
Meaning: A spontaneous process proceeds in the forward direction on its own, without continuous outside work. At constant temperature and pressure this means ΔG < 0, which is equivalent to an increase in the entropy of the universe (ΔSuniv > 0). Spontaneous describes thermodynamic favorability only, not speed; rate depends on activation energy. At ΔG = 0 the system is at equilibrium, and if ΔG > 0 the reverse reaction is spontaneous.
Equation: ΔG = ΔH − TΔS, where ΔG is the Gibbs free energy change (J/mol), ΔH is the enthalpy change (J/mol), T is the absolute temperature (K), and ΔS is the entropy change of the system (J/(mol·K)).
On the MCAT: You are given ΔH and ΔS and asked for the sign of ΔG or how temperature changes it. ΔH < 0 with ΔS > 0 is spontaneous at any temperature; ΔH > 0 with ΔS < 0 is nonspontaneous at any temperature. When the signs match, temperature decides: high T favors spontaneity if ΔS > 0, low T if ΔS < 0.
Common confusion: Spontaneous does not mean fast or exothermic. Diamond converting to graphite is spontaneous but extremely slow, and ice melting above 0 °C is endothermic yet spontaneous because TΔS outweighs ΔH.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.