Construct a standard free-energy comparison with explicit states

Assemble a consistent standard-state ledger

For the fictional written gas reaction A+B→C, supplied standard molar entropies are100,150 and150 J/(mol K). Products minus reactants gives ΔrS°=150−100−150=−100 J/(mol reaction K). Coefficients and phase labels matter; changing either changes the property combination.

Standard reaction enthalpy can similarly be computed from formation enthalpies. Stable elemental reference formation enthalpies are assigned zero by convention, but their standard molar entropies at ordinary temperature are not generally zero. The third-law zero refers to an ideal perfectly ordered crystal at absolute zero, with its stated conditions.

Compare temperature conditionally

Using ΔrH°−30 kJ/mol and constant ΔrS°−.100 kJ/(mol K), ΔrG°=−30+.100T. It is−5 kJ/mol at250 K, zero at300 K and+10 kJ/mol at400 K. This is a standard-state tendency under the fixed-property approximation, not a total energy yield for any finite starting mixture.

Real reaction enthalpy and entropy can vary with temperature through heat capacities and phase changes. The model bounds its comparison and does not extrapolate that straight line universally.

A separate formation-free-energy check

For a different explicitly supplied standard-state packet A+B→C, let formation Gibbs energies be −50, −20 and −90 kJ/mol respectively. Predict the reaction value before checking: −90−(−50)−(−20)=−20 kJ/mol of the written reaction. This separate packet is not the same thermodynamic data set as the temperature slider. Reversing the written reaction reverses that value, while doubling its coefficients doubles it.

Transfer and check

If ΔrH° is−25 kJ/mol with the same entropy, the conditional zero occurs at250 K. At350 K the standard free energy is+10 kJ/mol. Explain why a positive standard value does not by itself prove every actual mixture must react backward: its current activity quotient remains to be specified.

Source and evidence boundary

Conceptual reference checked 30 October 2026: OpenStax Chemistry 2e. All prose, cases, graphs and questions are original. These supplied thermodynamic, electrical, nuclear or orbital models are not actual measurements, apparatus instructions, synthesis protocols or practical certification. Retain conventions, independent constraints, changed-case reasoning and limits. No heating, wiring, electrolysis, source handling or chemical manipulation is requested.