∆H°rxn = (sum of ∆H°f of products) – (sum of ∆H°f of reactants)Earlier we gave a general definition ∆Hrxn = Hproducts − Hreactants. This expression, however, is not very
useful. The actual values of the enthalpies of the species cannot be measured (and are in fact more a
matter of definition); what we can measure is only enthalpy changes, and those often only with
difficulty. But from our discussion on Hess’s law, we know we can concoct a scheme consisting of a
series of steps that yield the same net reaction, and the sums of enthalpy changes would be the
same as the net enthalpy change of the reaction. The equation above, ∆H°rxn = (sum of ∆H°f of
products) – (sum of ∆H°f of reactants), establishes a common scheme for all reactions: We first break
the reactants down to give elements in their standard states, then we combine these rudimentary
“building blocks” in new ways to give the products of the reaction. For example, when we express
the enthalpy change for the reaction
CaCO 3 (s) → CaO (s) + CO 2 (g)as ∆H°rxn = ∆H°f of CaO + ∆H°f of CO 2 – ∆H°f of CaCO 3 , we are essentially reporting the reaction as
follows: First we break down each mole of CaCO 3 into 1 mole of Ca, 1 of C, and 3/2 of O 2 , then we
combine the Ca with half a mole of O 2 to form a mole of CaO, and use the remaining oxygen to form
CO 2 with carbon. The enthalpy changes for the last two steps are the standard enthalpies of
formation of CaO and CO 2 , while the first step is the reverse of the formation of CaCO 3 , from which
we get the minus sign in front of the enthalpy of formation of CaCO 3 , and which explains why we
subtract the enthalpies of formation of the reactants in the general equation.
It should be emphasized again that this scheme is not actually carried out, but merely used for
“accounting conveniences”: Instead of tabulating an enthalpy change for every reaction imaginable,
we need only a list of enthalpies of formation of different compounds which, while still numerous,
do not possess this almost infinite possible number of combinations.
Bond Dissociation Energy
Enthalpies or heats of reaction are related to changes in energy associated with the breakdown and
formation of chemical bonds. The reason why bonds are formed in the first place is because it is
energetically favorable for the atoms to come together—it corresponds to a state of lower energy.
This implies, therefore, that energy needs to be supplied to break a bond and separate the atoms