Reversible heat effect

We first assume that the Peltier effects are the only reversible heat effects in the circuit. Then if 7Ti, 7t2 are the Peltier effects at the hot and cold junctions ... 

Assuming thermodynamic reversibility of the cell reaction and with the help of eqs 1 and 3, we can obtain the reversible heat effect. 

A process is thermodynamically reversible when an infinitesimal reversal in a driving force causes the process to reverse its direction. Since all actual processes occur at finite rates, they cannot proceed with strict thermodynamic reversibility and thus additional nonrevers-ible effects have to be regarded. In this case, under practical operation conditions, voltage losses at internal resistances in the cell (these kinetic effects are discussed below) lead to the irreversible heat production (so-called Joule heat) in addition to the thermodynamic reversible heat effect. 

For an isothermal-isobaric addition of a small amount of component i to a mixture, the reversible heat effect is given by the change in partial molar entropy on mixing. 

Consider an isothermal-isobaric process in which a small amount of component i is removed from an ideal-gas mixture at T, P, x and injected into a real mixture at the same T, P, and x. (a) Show that the required minimum work is given by RT In q)j . (b) Show that the reversible heat effect is given by TS . 

The difference between AH and AG, the product T AS, is called reversible heat effect. It represents the heat exchange with the surroundings when the process occurs reversibly , which means that all equilibria are balanced. T AS can be positive or negative. In the first case additional energy is generated by cooling of the environment (Peltier or heat pump effect). Otherwise, T AS contributes additional heat (cf. also Section 1.4.1). 

Electrochemical reactions, like chemical reactions, are always connected with heat effects, determined by the (positive of negative) reversible heat effect, already mentioned in Eq. (4). When current flows through the cell, additional heat is generated by ohmic resistances in the electrodes and the electrolyte, but also by polarization effects, which together cause Joule heating . 

The reversible heat effect per time unit can be related to current flow, because each multiple of the cell reaction requires the current amount n F ... 

Ucal is a hctive equilibrium voltage that includes the reversible heat effect and is convenient with heat calculations (cf., e.g. Eq. (41)). 

Summation of the Joule effect and the reversible heat effect gives the total heat generated in the cell or the battery, which means... 

The reversible heat effect, which amounts to about 20% of the converted energy and means cooling of the cell during electrolysis (Column 5 in Table 1.3), and a corresponding increase of the energy content of the gas. 

At the bottom of Fig. 1.11, the heat generation is drawn as blocks that represent average values for the corresponding sections of the charging/discharging process. The distribution between reversible heat effect, charging, and water decomposition is marked by different patterns of the areas concerned. The value above each block is the total heat generation in Wh. 

When 2.4 V is reached, the current is reduced and, as a consequence. Joule heating and the reversible heat effect caused by the charging reaction are reduced too. But now the approximately constant gas evolution causes most of the generated heat ((U- 1.48)-i). 

During discharge, due to the small overvoltage, heat generation is also small, and further reduced by the reversible heat effect that now causes cooling. 

The reversible heat effect of the discharge reaction is Q,ev = T-AS -26kJ... 

The free enthalpy of reaction AG and the reversible heat effect T-AS are... 

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