Enthalpy and the Second Law of Thermodynamics

摘要

The change in enthalpy of a chemical reaction conducted at constant pressure is equal to the heat of the reaction plus the nonexpansion work of the reaction, Delta H = qP + w(additional). After deriving that relationship, most general and physical chemistry textbooks set w(additional) = 0 to arrive at the claim that Delta H = q(p), and nearly all further discussion of enthalpy assumes that Delta H = q(p). Setting w(additional) = 0 is viable for spontaneous reactions, but for non-spontaneous reactions, w(additional) not equal 0 as a consequence of the second law of thermodynamics. Therefore, Delta H # q(p) for nonspontaneous reactions. Moreover, nonexpansion work is important for many interesting and important spontaneous reactions in biology (e.g., muscular movement, nerve signal transmission) and in modern society (e.g., batteries); incorporating w(additional) into Delta H allows for a more accurate discussion of the energy flow in these reactions. In this paper, I show that Delta H not equal qp for nonspontaneous reactions, and I discuss how Delta H must be partitioned between q(p), and w(additional) for several kinds of reactions according to the second law of thermodynamics. Finally, I suggest how the discussion of enthalpy could be corrected in general and physical chemistry textbooks.