Modern inorganic chemistry

72 ENERGETICS

but. since the overall heat change is. by Mess's law. independent of

the path of the reaction, this is still valid as an energy diagram.)

The enthalpy changes in the reaction are:

A/I, the dissociation or bond energy of hydrogen (it is also, by

definition, twice the enthalpy of atomisation—two gram atoms

being produced).

A/i 2 the dissociation or bond energy of chlorine, again twice the

enthalpy of atomisation.

A/i 3 twice the bond energy of hydrogen chloride (twice since two

moles of hydrogen chloride are produced).

A/i 4 the enthalpy of reaction, which is in this case twice the enthalpy

of formation of hydrogen chloride. Clearly A/i 4 is the difference

between the total bond energies of the products and the total

bond energies of the reactants. That is

A/f reaction = ^ bond energies of products

— Z bond energies of reactants.

For a reaction to be exothermic the sum of the bond energies

of the products must exceed those of the reactants.

For the formation of the hydrogen halides by the direct com-

bination of the elements, the enthalpies of formation are:

A/ff (kJmoP^1 )

HF HC1

-269 -92.3

HBr HI

-36.2 +26

These values indicate a rapid fall in thermal stability of the halide

from fluorine to iodine, and hydrogen iodide is an endothermic

compound. If we now examine the various enthalpy changes in-

volved, we find the following values (in kJ):

1.

2.

3.

4.

5.

6.

iX 2 (L s)

H(g)

H(g)

» iX 2 (g)

X(g)

iX 2 (l. s) -> H(g) + X(g)

X(g) -» HX(g)

HX(g):

AHf ;

HF

+ 218

0

+ 79

+ 297

-566

HC1

+ 218

0

+ 121

+ 339

-431

HBr

+ 218

+ 15

+ 96

+ 330

-366

HI

+ 218

-1-31

+ 76

+ 325

-299

iX 2 (s.Lg)

^269 -92 -36 +26

Note that the term 2 is included: it is the enthalpy required to
convert the element in its standard state at 298 K to a gas at 298 K—
and it does not apply to fluorine and chlorine which are both gases
at this temperature.