A good source of information to predict a chemical combination is the heat of
formation table. A heat of formation table gives the number of kilojoules
evolved or absorbed when a mole (gram-formula mass) of the compound in
question is formed by the direct union of its elements. For chemists, a positive
number indicates that heat is absorbed and a negative number that heat is evolved.
It makes some difference whether the compounds formed are in the solid, liquid,
or gaseous state. Unless otherwise indicated (g = gas, ℓ, = liquid), the compounds
are in the solid state. The values given are in kilojoules, 4.18 joules is the amount
of heat needed to raise the temperature of 1 gram of water 1 unit on the Kelvin
scale. The symbol ΔHf is used to indicate the heat of formation.
If the heat of formation is a large number preceded by a minus sign, the
combination is likely to occur spontaneously and the reaction is exothermic. If, on
the other hand, the number is small and negative or is positive, heat will be
needed to get the reaction to proceed at any noticeable rate. Some examples are:
Example 1
This means that 1 mole of zinc (65 grams) reacts with 1 mole of sulfur (32 grams)
to form 1 mole of zinc sulfide (97 grams) and releases 202.7 kilojoules of heat.
Example 2
indicates that the formation of 1 mole of magnesium oxide requires 1 mole of
magnesium and mole of oxygen with the release of 601.6 kJ of heat. Notice the
use of the fractional coefficient for oxygen. If the equation had been written with
the usual whole-number coefficients, 2 moles of magnesium oxide would have
been released.
2Mg(s) + O 2 (g) → 2MgO(s) + 2(+601.6) kJSince, by definition, the heat of formation is given for the formation of 1 mole,
this latter thermal equation shows 2 × (−601.6) kJ released.
2. Decomposition (Also Known as Analysis)
The prediction of decomposition reactions uses the same source of information,
the heat of formation table. If the heat of formation is a high exothermic (ΔH is
negative) value, the compound will be difficult to decompose since this same
quantity of energy must be returned to the compound. A relatively low and