Element compound, fonnation

In particular, one can combine reactions to yield the aG. , AH and ASj for fonnation of compounds from their elements, quantities rarely measurable directly. (Many AH s.for fonnation of substances are easily... 

This compound is unlikely to exist owing to a large positive value for the heat of fonnation for CsCU that is mainly due to the large second ionization energy for Cs (/, = 375 kJ mor vs. 72=2420 kJ mol the alone for Cs is higher than tbe sum of 7, and 72 for Mg see Self-Test 3.13.). The compound is predicted to be unstable with respect to its elements mainly because the large ionization enthalpy to form Cs is not compensated by the lattice enfoalpy. 

As defined in 4 8, the standard Gibbs free energy of formation of a compound at 25 is the increase in G when 1 mol (1 g formula weight) of the substance at 1 atm pressure is formed from its elements, each at 1 atm pressure. The question arises whether any modification needs to be adopted when the substance in question, or the elements from which it is formed, are not in a stable gaseous state at 25 and 1 atm. This can best be discussed by use of an example, and we shall take the case of the free energy of fonnation of water. 

The composition MCu2 has been chosen for comparing the behavior of the rare earths and actinides when alloyed with copper. It appears that the enthalpies of formation of the MCu2 intermetallic compounds are quite similar for all the elements of the rare-earth series, with the enthalpy of formation of the ThCu2 compound being more negative (fig. 73). To compare the enthalpies of fonnation with composition, we have plotted in fig. 74 the enthalpies of formation of the intermetallic compounds of La-Cu obtained by... 

The fonnation of an ionic compound from its constituent elements is usually quite exothermic. For example, when sodium chloride (table salt) forms from elemental sodium and chlorine, 411 kJ of heat is evolved in the following violent reaction ... 

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