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So O2 has zero of heat of formation (or enthalpy of formation). The enthalpy of a reaction is defined like this: H = Hproducts-H reactants. The enthalpy of reaction in this case, is the same as the heat of combustion. Hproducts is obtained summing the heats of formation.
Thus we can use a combination of heat capacity measurements (Equation 18.20 or Equation 18.21) and experimentally measured values of enthalpies of fusion or vaporization if a phase change is involved (Equation 18.18) to calculate the entropy change corresponding to a change in the temperature of a sample.ne way of calculating Î´ S for a reaction is to use tabulated values of the standard molar entropy (SÂ°)The entropy of 1 mol of a substance at a standard temperature of 298 K. , which is the entropy of 1 mol of a substance at a standard temperature of 298 K; the units of S Â° are J/(molÂ·K).