Safety boundary
Use supplied graphs and calculations only. No new reactive chemistry is required.
Explicit chemistry instruction
At equilibrium, the system has no net thermodynamic driving force under the model, so ΔG for the reaction condition is zero.
A favorable process can be coupled to an unfavorable one if the combined ΔG is negative.
Coupling requires a real shared mechanism or pathway; simply adding unrelated equations does not make processes influence each other.
Vocabulary: equilibrium; coupling; driving force; combined process; pathway.
Worked example
A +10 kJ process coupled to a −25 kJ process has combined ΔG=−15 kJ if the coupling is mechanistically valid.
Guided question
What condition must be met before two ΔG values can be meaningfully combined?
Independent work
- Add three coupled ΔG pairs.
- Identify favorable combined processes.
- State one mechanism requirement.
Instructional depth extension · free energy, equilibrium, and coupling
Gibbs free energy combines enthalpy and entropy effects through ΔG=ΔH−TΔS. A negative ΔG indicates a thermodynamically favorable direction under the stated conditions; it does not predict how fast the process occurs.
Worked example: if ΔH=+25 kJ/mol and TΔS=+40 kJ/mol, then ΔG=25−40=−15 kJ/mol. The process is thermodynamically favorable under those conditions even though it is endothermic.
At equilibrium, the driving force for net change is zero at the macroscopic level. Coupled processes can make an otherwise unfavorable step proceed when the total ΔG for the combined system is negative. Keep system boundaries explicit when adding free-energy changes.
Misconception check
Unrelated favorable reactions do not automatically drive unfavorable ones.
Formative check
What matters for a coupled process overall?
Reveal response
The sum of ΔG values for a genuinely linked pathway.
Mentor evidence conference
Ask Archaemenes to audit signs, units, thermodynamic conditions, and whether rate/direction are being kept distinct.
Teacher guidance
Foundation: Use additive energy bars.
Core: Analyze coupled processes.
Extended: Connect free energy with equilibrium constants qualitatively.