We study heat effects during electrochemical surface reactions with minute conversions. The heat which is reversibly exchanged at an electrode upon electrochemical processes is proportional to the entropy change during the reaction, including also non-Faradaic side processes like coadsorption or solvent reordering. It therefore provides complementary information to the current-potential relations, obtained by conventional electrochemical methods like cyclic voltammetry.
After a general introduction we will give examples for the role of entropy during anion adsorption and for the identification of side processes during underpotential metal-deposition. The time-dependent entropy evolution during an electrochemical reaction, i.e., Cu-bulk deposition, will also be discussed..