Require a mechanism before adding favorable reactions

An energy budget is necessary, not a mechanism

Adding a favorable reaction to an unfavorable one can give a favorable total on paper. That does not make the desired product appear. The reactions must be physically coupled through a suitable pathway, shared intermediate or external work-transfer arrangement. Burning fuel in an unrelated container is not automatically molecular coupling to a nearby synthesis.

Consider original hypothetical steps A+X→I with ΔG=+40 kJ per mole of target reaction, followed by I+Y→B+X with ΔG=−50. Add the equations and cancel I and X to obtain A+Y→B with total ΔG=−10. The intermediate I must be produced and consumed through compatible steps, and the assigned energies must refer to the same temperature and chemical-state conventions.

Match coefficients before cancellation

In the lab each mole of driving reaction supplies −25 kJ, while a mole of target reaction costs 10–60 kJ. With target cost 40, one driving event gives +15; two give −10; three give −35 kJ per mole of target reaction. These numbers screen a proposed coupling ratio. They do not establish that a mechanism with that stoichiometry exists or that it runs quickly.

If one step produces two intermediates but the other consumes only one, they do not cancel as written. Multiply the required step and its energy consistently. Reversing a step reverses its free-energy sign. A favorable total obtained by canceling unequal coefficients is not a valid thermodynamic argument.

External energy belongs in the larger system

Electrical input can drive electrolysis or recharge a battery. Light can support photochemical transformations. The chemical subsystem may then undergo an unfavorable change because another part of the overall system supplies energy. Report that input rather than claiming a catalyst alone changes the equilibrium.

Useful work available from a favorable process is bounded by its Gibbs-energy decrease under the appropriate constraints; real operation includes dissipation. A favorable combined standard free energy also does not guarantee favorable operation at every actual concentration. Apply Q-dependent reasoning to the same steps when their composition changes.

Changed-case challenge

A proposed pathway needs +60 kJ per mole of target reaction. Two moles of driving reaction each contribute −25 kJ/mol. Predict whether the sum passes the thermodynamic screen and what changes with three events. The totals are +10 and −15 kJ per mole of target reaction respectively. Then list the missing evidence: a feasible coupled mechanism, compatible material balance and rate, and actual-state free energies. The screen is one necessary piece of a full explanation, not a proof of yield.

Source scope and continuation

Original teaching explanation, calculations and practice. Framework alignment: AP Chemistry topic 9.7 in the College Board course framework, effective Fall 2024, checked 30 October 2026. These pages do not reproduce restricted examination questions or establish AP authorization. The investigation is a bounded computational model, not a conducted laboratory.

Return to the preceding Unit 9 lesson when its reasoning is unfamiliar.

Continue to Diagnose electrode roles and broken current paths.

Identify the light input and the shared chemical intermediate

Oxygenic photosynthesis uses light-driven electron transfer and chemical energy carriers to support reduction of carbon dioxide into carbohydrate. The familiar net account 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂ summarizes carbohydrate production; it is not one elementary step or a claim that free glucose is the immediate carbon-fixation product. Light input and a functioning coupled pathway belong in the energy account. A catalyst alone cannot make the same uphill closed chemical conversion favorable without an energy source.

ATP conversion to ADP plus inorganic phosphate can support cellular work through coupled reactions. Breaking a bond alone costs energy; the favorable hydrolysis difference belongs to the whole reactant-to-product process, including new interactions. Simply hydrolyzing ATP somewhere nearby does not establish coupling to a desired synthesis.

Consider an original schematic fixed-pH biochemical network: A + ATP → I + ADP, followed by I + H₂O → B + Pi. I is a shared phosphorylated intermediate; A and B are hypothetical isomeric substrate/product forms. These are grouped biochemical equations, not complete protonation/charge ledgers. Assigned actual-state step differences are +8 and −20 kJ per mole at compatible conditions. Predict: cancel I, calculate the net difference, then compare a failed pathway that produces a different intermediate J which the second step cannot use.

Check the coupling mechanism

The compatible pathway gives A + ATP + H₂O → B + ADP + Pi and −12 kJ per mole of target conversion. The incompatible J/I steps do not cancel: a favorable arithmetic sum cannot establish production of B. These synthetic step energies are not measured ATP hydrolysis values and do not guarantee a useful rate. Removing light input or chemical coupling is not repaired merely by adding more catalyst.

Mechanism sources checked 31 October 2026: OpenStax ATP coupling and photosynthesis overview. No biological or chemical procedure is requested.