The Foundations of Chemistry

Plan

The definition of 
H^0 fof a substance refers to a reaction in which one moleof the substance is

formed. We put one mole of C 2 H 5 OH(
) on the right side of the chemical equation and put

the appropriate elements in their standard states on the left. We balance the equation without

changing the coefficient of the product,even if we must use fractional coefficients on the left.

Solution

2C(graphite)3H 2 (g)^12 O 2 (g)88nC 2 H 5 OH(
) 
H277.7 kJ/mol rxn

You should now work Exercise 26.

As an analogy, consider traveling from
Kansas City (elevation 884 ft above sea
level) to Denver (elevation 5280 ft).
The change in elevation is (5280
884) ft4396 ft, regardless of the
route taken.

604 CHAPTER 15: Chemical Thermodynamics

HESS’S LAW

In 1840, G. H. Hess (1802–1850) published his law of heat summation,which he derived

on the basis of numerous thermochemical observations.

The enthalpy change for a reaction is the same whether it occurs by one step or by

any series of steps.

Enthalpy is a state function. Its changeis therefore independent of the pathway by which

a reaction occurs. We do not need to know whether the reaction does,or even can,occur

15-8

Problem-Solving Tip:How Do We Interpret Fractional Coefficients?

Remember that we alwaysinterpret the coefficients in thermochemical equations as

numbers of molesof reactants or products. The ^12 O 2 (g) in the answer to Example 15-6

refers to ^12 moleof O 2 molecules, or

^12 mol O 2 

3

m

2.0

ol

g

O

O

2

2

16.0 g O 2

It is important to realize that this is notthe same as one mole of O atoms (though that

would also weigh 16.0 g).

Similarly, the fractional coefficients in

^12 H 2 (g)^12 Br 2 (
)88nHBr(g)

refer to

^12 mol H 2 1.0 g H 2

and

^12 mol Br 2 79.9 g Br 2

respectively.

159.8 g Br 2



mol Br 2

2.0 g H 2



mol H 2

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 6.16, Hess’s Law.