ABSTRACT
The change in internal energy of a system can be defined as the sum of the changes in work, dw, and thermal energy, heat, dq. Typically, either there isn’t enough time for a compensating heat flow in or out of the system for work done on or by the system or the system is thermally insulated from the surroundings. As a consequence, an adiabatic process cannot be isothermal. This condition will impact the amount of work that is either lost or gained by a gaseous system for a given volume change. One method to determine the enthalpy of a reaction is to use Hess’s Law, comparing the final and initial states, using standard enthalpies of formation of substances. It is also possible to use Hess’s Law to determine the standard enthalpy of formation, from a measured value for a known reaction involving the substance, by rearranging Hess’s Law.
