Equilibrium Thermodynamics and Electrochemistry

Though we frequently use the word equilibrium in our life, equilibrium is not a common state for any real system. The common thermodynamic state is called steady state when a system s parameters are not changing because the output and input fluxes of energy and mass are balanced. For example, the temperature of a human body is remarkably constant due to a delicately balanced steady state, and this temperature, 36.6°C, is quite different from any common temperature in the surroundings. 

If thermodynamic variables are temperature and pressure, the main characteristic for understanding whether the system (process, reaction) is in equilibrium or not is Gibbs energy (AG). Electrochemical cells commonly work at well-defined temperature and pressure, and therefore, AG is the thermodynamic function showing how far the electrochemical cell is from equilibrium. When an electrochemical cell is at equilibrium, AG is simply zero. Two other important thermodynamic functions to understand an electrochemical cell behavior are enthalpy (AH) and entropy (AS), which are connected to AG via a weU-known fundamental equation  

It is usually not easy to illustrate Equation 4.1 in a thermodynamics course, so some abstract approaches with a heat engine are used. However, in electrochemical science and engineering, all values of Equation 4.1 are well defined, and two of them, AG and AS, can experimentally be estimated using only electrochemical measurements. If the potential difference of the Daniell cell in the equilibrium mode, is measured, this value can immediately be used to calculate, A,G, the Gibbs energy of the total reaction taking place in the Daniell cell. If the temperature dependence of AjG can experimentally be estimated, the entropy of the electrochemical reaction, A,S, can be calculated as follows  

Equations 4.1 and 4.2 have been widely used in science to estimate Afi, A,5, and AjH of chemical reactions employing a variety of electrochemical cells operating in the equilibrium mode. Note that only two of the quantities of Equation 4.1 are independent, and the third one depends on two others. Also, any other thermodynamic characteristic can be defined if Afi is measured as a function of temperature and pressure. For example, the isobaric heat capacity, A Cp, and volume (change) of a reaction, A,V, can be calculated as follows  

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