Question

Find a case where the sign convention in thermochemistry is not true. A case where it would not work?

Answer 1

This cannot be answered, because in all cases, the sign convention (though arbitrary!) works and is chosen the same way each time. Chemists are consistent in choosing the perspective of the system (e.g. `w ~~ -PDeltaV` is `PV` work so that `DeltaU = q + w`). Physicists, however, generally choose the perspective of the surroundings (e.g. `w ~~ PDeltaV` is `PV` work so that `DeltaU = q - w`).