ΔG
Summary
ΔG is the change in Gibbs free energy, the energy available to do useful work at constant temperature and pressure. A negative ΔG means a process is spontaneous (thermodynamically favorable) as written, ΔG = 0 means equilibrium, and ΔG says nothing about speed.
Detail
Meaning: ΔG balances the enthalpy change (ΔH) against the entropy change (ΔS) at constant temperature and pressure. If ΔG < 0, the process is spontaneous (exergonic) as written. If ΔG > 0, it is nonspontaneous (endergonic). If ΔG = 0, the system is at equilibrium. Reversing a reaction flips the sign.
Equation: ΔG = ΔH − TΔS, where ΔG and ΔH are in J (often J/mol), T is absolute temperature in K, and ΔS is in J/K (often J/(mol·K)). Related forms are ΔG = ΔG° + RT ln Q and ΔG° = −RT ln Keq, where R = 8.314 J/(mol·K), Q is the reaction quotient and Keq is the equilibrium constant (both unitless).
On the MCAT: Expect sign-analysis questions. ΔH < 0 with ΔS > 0 is spontaneous at any temperature. ΔH > 0 with ΔS > 0 is favored at high T. ΔH < 0 with ΔS < 0 is favored at low T. If Q < Keq, ΔG is negative and the reaction shifts forward. Biochemistry passages couple ATP hydrolysis to an unfavorable step. Electrochemistry links ΔG° = −nFE°. Watch units: ΔH is usually in kJ and ΔS in J/K.
Common confusion: ΔG° and ΔG are different. ΔG° applies at standard conditions, while ΔG depends on current concentrations through Q. A reaction can have ΔG° > 0 and still run forward if Q is small enough. Also, a negative ΔG describes direction, not rate. Rate depends on activation energy.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- 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.