Enthalpy and the Second Law of Thermodynamics
Abstract
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, ΔH = qP + wadditional. After deriving that relationship, most general and physical chemistry textbooks set wadditional = 0 to arrive at the claim that ΔH = qP, and nearly all further discussion of enthalpy assumes that ΔH = qP. Setting wadditional = 0 is viable for spontaneous reactions, but for nonspontaneous reactions, wadditional ≠ 0 as a consequence of the second law of thermodynamics. Therefore, ΔH ≠ qP 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 wadditional into ΔH allows for a more accurate discussion of the energy flow in these reactions. In this paper, I show that ΔH ≠ qP for nonspontaneous reactions, and I discuss how ΔH must be partitioned between qP and wadditional 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.
Faculty Members
- David Keifer - Department of Chemistry, Salisbury University, Salisbury, Maryland 21801, United States
Themes
- Misconceptions in chemistry education
- Thermodynamics in biological and technological applications
- Revisions in chemistry textbooks to enhance understanding
- Importance of nonexpansion work in chemical reactions
- Enthalpy and its relationship with heat and nonexpansion work