Spontaneity criteria for processes with useful (especially electrical) work have been discussed based on total differentials of thermodynamic functions. Reaction Gibbs energy ([Delta][subscript r]G) and electrochemical reaction Gibbs energy ([Delta][subscript r]G~) have been juxtaposed. Three-dimensional graphs showing the dependencies of [Delta][subscript r]G~ on the extent of reaction ([xi]) and the cell voltage ([phi][superscript R] - [phi][superscript L]), in connection with their sections, enable both coherent and intuitive discussion of equilibrium in electrochemical systems. It was clearly indicated that important, commonly known dependencies can be justified and illustrated using these graphs.