non-spontaneous
Summary
A non-spontaneous process has ΔG > 0 under the given conditions, so it will not proceed on its own and needs an energy input or coupling to a favorable process. Its reverse is spontaneous, and ΔG says nothing about how fast either direction occurs.
Detail
Meaning: A process is spontaneous when it can proceed without continuous outside work at constant temperature and pressure, which means the system's free energy decreases. A non-spontaneous process would raise the free energy (ΔG > 0). Examples include electrolysis, charging a battery, and endergonic cell reactions driven by coupling to ATP hydrolysis. At equilibrium ΔG = 0. If ΔH > 0 and ΔS < 0, the process is non-spontaneous at any temperature. If ΔH > 0 and ΔS > 0, it is non-spontaneous only at low temperature.
Equation: ΔG = ΔH − TΔS, where ΔG is the change in Gibbs free energy (J/mol), ΔH is the change in enthalpy (J/mol), T is absolute temperature (K), and ΔS is the change in entropy (J/(mol·K)).
On the MCAT: You may be given ΔH and ΔS and asked for the sign of ΔG, or for the temperature at which a process switches, which is T = ΔH/ΔS when ΔG = 0. In electrochemistry, a negative E°cell (ΔG = −nFE) means a galvanic cell is non-spontaneous as written, so an electrolytic cell with an external voltage is needed. Biochemistry passages use coupling of an endergonic step to an exergonic one.
Common confusion: Non-spontaneous does not mean impossible or slow. Rate depends on activation energy, not ΔG. Also, ΔG° > 0 (K < 1) does not forbid the forward reaction in a real mixture. If Q is small enough that ΔG = ΔG° + RT ln Q is negative, it proceeds.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.