Enthalpy Changes for Different Types of Reactions

The heat, or enthalpy, change accompanying a chemical reaction is expressed differently depending on exactly what kind of reaction (or physical change) is involved. This section -- continued after the intervening Sections 7.6 and ~7.6.1-~7.6.3 -- works through the full catalogue of named enthalpy quantities a chemist uses; it opens with the most fundamental one.

Standard heat (enthalpy) of formation, $\Delta H_f^0$. Defined as "the change in enthalpy that takes place when one mole of a compound is formed from its constituent elements, present in their standard states" (298 K and 1 bar pressure). By convention, every ELEMENT in its standard state is assigned $\Delta H_f^0=0$ -- formation enthalpies are always measured relative to that zero baseline.

Worked examples from the text:

Table 7.4 tabulates $\Delta H_f^0$ for ten common compounds, transcribed exactly as printed in the textbook -- including its H$_2$O(l) row, which the book lists as $-242$ kJ mol$^{-1}$ (a value that, in most other sources, is closer to the enthalpy of formation of water VAPOUR rather than liquid water; it is reproduced here without alteration, matching the printed source). …