The Foundations of Chemistry

Thus, when the reaction occurs to the extent indicated by the balanced chemical equation,56.4 kJ
is released. Remembering that exothermic reactions have negative values of
Hrxn, we write

CuSO 4 (aq)2NaOH(aq)88nCu(OH) 2 (s)Na 2 SO 4 (aq) 
Hrxn56.4 kJ/mol rxn

You should now work Exercise 62(b).

EXAMPLE 15-5 Amount of Heat Versus Extent of Reaction

When aluminum metal is exposed to atmospheric oxygen (as in aluminum doors and windows),
it is oxidized to form aluminum oxide. How much heat is released by the complete oxidation
of 24.2 grams of aluminum at 25°C and 1 atm? The thermochemical equation is

4Al(s)3O 2 (g)88n2Al 2 O 3 (s) 
H3352 kJ/mol rxn

Plan

The thermochemical equation tells us that 3352 kJ of heat is released for every mole of reac-
tion, that is, for every 4 moles of Al that reacts. We convert 24.2 g of Al to moles, and then
calculate the number of kilojoules corresponding to that number of moles of Al, using the unit
factors





m

3

o

3

l

5

r

2

xn

kJ

 and 

1

4

m

m

o

o

l

l

r

A

xn

l



Solution

For 24.2 g Al,

__?kJ24.2 g Al

2

1

7

m

.0

o

g

lA

A

l

l



1

4

m

m

o

o

l

l

r

A

xn

l





m

3

o

3

l

5

r

2

xn

kJ

751 kJ

This tells us that 751 kJ of heat is released to the surroundings during the oxidation of 24.2
grams of aluminum.

You should now work Exercises 14 and 15.

Heat is released, so this is an

exothermic reaction.

15-6 Standard States and Standard Enthalpy Changes 601

STANDARD STATES AND STANDARD

ENTHALPY CHANGES

The thermodynamic standard stateof a substance is its most stable pure form under
standard pressure (one atmosphere)* and at some specific temperature (25°C or 298 K
unless otherwise specified). Examples of elements in their standard states at 25°C are
hydrogen, gaseous diatomic molecules, H 2 (g); mercury, a silver-colored liquid metal,

15-6

Problem-Solving Tip:Mole of Reaction

Remember that a thermochemical equation can imply differentnumbers of moles of

differentreactants or products. In Example 15-5 one mole of reaction also corresponds

to 3 moles of O 2 (g) and to 2 moles of Al 2 O 3 (s).

*IUPAC has changed the standard pressure from 1 atm to 1 bar. Because 1 bar is equal to 0.987 atm, the differ-
ences in thermodynamic calculations are negligible except in work of very high precision. Many tables of thermodynamic
data are still based on a standard pressure of 1 atm, so we will use it in this book.

A temperature of 25°C is 77°F.

This is slightly above typical room

temperature. Notice that these

thermodynamic “standard conditions”

are not the same as the “standard

temperature and pressure (STP)” that

we used in gas calculations involving

standard molar volume (Chapter 12).

The signtells us that heat was released,

but it would be grammatical nonsense

to say in words that “751 kJ of heat

was released.” As an analogy, suppose

you give your friend $5. Your 
$ is

$5, but in describing the transaction

you would not say “I gave her minus

five dollars,” but rather “I gave her five

dollars.”