negative heat of formation indicates decomposition would not be difficult, such as
the decomposition of mercuric oxide with ΔHf = −90.8 kJ/mole:
2HgO(s) → 2Hg(s) + O 2 (g) (Priestley’s method of preparation)A high positive heat of formation indicates extreme instability of a compound,
which can explosively decompose.
3. Single Replacement
A prediction of the feasibility of this type of reaction can be based on a
comparison of the heat of formation of the original compound and that of the
compound to be formed. For example, in a reaction of zinc with hydrochloric
acid, the 2 moles of HCl have ΔHf = 2 × −92.3 kJ.
Example 1
and the zinc chloride has ΔHf = −415.5 kJ. This comparison leaves an excess of
230.9 kJ of heat given off, so the reaction would likely occur.
Example 2
In this next example, −928.4 kJ − (−771.4) kJ = −157.0 kJ, which is the excess to
be given off as the reaction occurs:
Another simple way of predicting single replacement reactions is to check
the relative positions of the two elements in the activity series below. If the
element that is to replace the other in the compound is higher on the chart, the
reaction will occur. If it is below, there will be no reaction.
Some simple examples of this are the following reactions.
In predicting the replacement of hydrogen by zinc in hydrochloric acid,
reference to the activity series shows that zinc will replace hydrogen. This
reaction would occur:
Zn(s) + 2HCl(aq) → ZnCl 2 (aq) + H 2 (g)In fact, most metals in the activity series would replace hydrogen in an acid