The Foundations of Chemistry

In effect this form of Hess’s Law supposes that the reaction occurs by converting reac-

tants to the elements in their standard states, then converting these to products (Figure

15-4). Few, if any, reactions actually occur by such a pathway. Nevertheless, the 
H^0 for

this hypotheticalpathway for reactants nproductswould be the same as that for any other

pathway — including the one by which the reaction actually occurs.

EXAMPLE 15-9 Using Hf^0 Values: Hess’s Law

Calculate 
H^0 rxnfor the following reaction at 298 K.

SiH 4 (g)2O 2 (g)88nSiO 2 (s)2H 2 O(
)

Plan

We apply Hess’s Law in the form 
H^0 rxnn
H^0 f productsn
H^0 f reactants, so we use the


Hf^0 values tabulated in Appendix K.

Solution

We can first list the 
H^0 fvalues we obtain from Appendix K:

SiH 4 (g) O 2 (g) SiO 2 (g) H 2 O(
)


H^0 f, kJ/mol: 34.3 0 910.9 285.8

H^0 rxnn
H^0 f productsn
H^0 f reactants

H^0 rxn[
H^0 fSiO 2 (s) 2 
H^0 fH 2 O(
)][
H^0 fSiH 4 (g) 2 
H^0 fO 2 (g)]

H^0 rxn 

 

H^0 rxn1515.7 kJ/mol rxn

You should now work Exercise 36.

Each term in the sums on the right-hand side of the solution in Example 15-9 has the

units

 or

For brevity, we shall omit units in the intermediate steps of calculations of this type,

and just assign the proper units to the answer. Be sure that you understand how

these units arise.

Suppose we measure 
H^0 rxnat 298 K and know all but one of the 
H^0 fvalues for reac-

tants and products. We can then calculate the unknown 
H^0 f value.

kJ



mol rxn

kJ



mol substance

mol substance



mol rxn

0 kJ



mol O 2 (g)

2 mol O 2 (g)



mol rxn

34.3 kJ



mol SiH 4 (g)

1 mol SiH 4 (g)



mol rxn

285.8 kJ



mol H 2 O(
)

2 mol H 2 O(
)



mol rxn

910.9 kJ



mol SiO 2 (s)

1 mol SiO 2 (s)



mol rxn

O 2 (g) is an element in its standard
state, so its
H^0 fis zero.

608 CHAPTER 15: Chemical Thermodynamics

Figure 15-4 A schematic
representation of Hess’s Law. The
red arrow represents the directpath
from reactants to products. The
series of blue arrows is a path
(hypothetical) in which reactants are
converted to elements, and they in
turn are converted to products — all
in their standard states.

n
H^0 f reactants

n
H^0

H^0

f products

rxn

Elements

Reactants

Products