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My limited understanding is that with regards to Hess' Law, there are 2 ways to do it.

  1. The main Hess' Law, where we algebraically manipulate several equations and take the sum to find the intended enthalpy of reaction
  2. Hess' Law 2, where we take enthalpy of formation of products minus enthalpy of formation of reactants

My question is, why do we use 1? It seems like a lot more work, and for any reaction why can't we just always use 2, as the enthalpies of formation for most molecules are known?

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  • $\begingroup$ It is just an exercise to demonstrate how the heats of formation come into play, and that doing 2 is a valid approach. $\endgroup$
    – Chet Miller
    Commented Oct 20 at 10:48

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The Hess law is just little cryptic and particular application of more general principle that changes of state variables like enthalpy depend on process endpoints, but not on the path between them.

We do not always know enthalpies of formation, just reaction enthalpies. Both procedures are nothing else but matching different paths between system state endpoints.

Both cases calculate the enthalpy change of a single step between the given endpoints from a parallel multistep path between the same endpoints.

The first case formally calculates it from enthalpy changes of several subsequent steps of formal serial reactions between the same endpoints.

The second case does the same, but going via steps of formal decomposing reactants to elements and then synthesizing products from these elements.

Which way is suitable for the task depends on available data.

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