Celebrating the birth of De Donder's chemical affinity (1922-2022): from the uncompensated heat to his Ave Maria

Théophile De Donder, a Belgian mathematician born in Brussels, elaborated two important ideas that created a bridge between thermodynamics and chemical kinetics. He invented the concept of the degree of advancement of a reaction, and, in 1922, he provided a precise mathematical form to the already known chemical affinity by translating Clausius's uncompensated heat into formal language. These concepts merge in an important inequality that was the starting point for the formalization of out-of-equilibrium thermodynamics. The present article aims to reconstruct how De Donder elaborated his ideas and how he developed them by exploring his teaching activity and its connection with his scientific production. Furthermore, it emphasizes the role played by the discussions with his disciples who became his collaborators. The paper analyzes De Donder's efforts in participating in the second Solvay Chemistry Council in 1925 to call the attention of the international community of chemists. Even if his mathematical approach did not receive much attention at the time, his work on chemical affinity was the basis for the birth of the so-called Brussels school of thermodynamics.