![]() ![]() We write \(S^o_A\left(T\right)\) to indicate the absolute entropy of substance \(A\) in its standard state at temperature \(T\). It is usually included in compilations of thermodynamic data for chemical substances. To evaluate the enthalpy change, we can use the virial equation for the volume of a real gas, set \(P0\), and evaluate the resulting integral. The standard entropy is usually given the symbol \(S^o\). These equations relate the enthalpy and entropy of the hypothetical ideal gas standard state to the enthalpy and entropy of the real gas at pressure \(P\), at the same temperature. When the entropy value is calculated for one mole of the substance in its standard state, the resulting absolute entropy is called the standard entropy. Where the substance undergoes phase changes, the contribution that the phase change makes to the entropy of the substance is equal to the enthalpy change for the phase change divided by the temperature at which it occurs.Īt any given temperature, the entropy value that is obtained in this way is called the substance’s absolute entropy or its third-law entropy. The Lewis and Randall statement asserts that the entropy goes to a constant at absolute zero, irrespective of the values of any other thermodynamic functions. ![]() Phase changes are isothermal and reversible. In temperature ranges where experimental heat capacity data are available, the entropy change is obtained by integration using these data. ![]() Much as the ideal gas temperature scale has a natural zero at the temperature at which the volume extrapolates to zero, a perfect crystalline substance has a natural. \), using Debye’s theoretical relationship, \(C_P=AT^3\) \(A\) is obtained from the value of \(C_P\) at the lowest temperature for which an experimental value of \(C_P\) is available. The third law of thermodynamics has two important consequences: it defines the sign of the entropy of any substance at temperatures above absolute zero as positive, and it provides a fixed reference point that allows us to measure the absolute entropy of any substance at any temperature. By the Lewis and Randall statement of the third law, the entropy of a substance that forms a perfect crystal is identically equal to zero at absolute zero. ![]()
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