The Foundations of Chemistry

We can also write the thermochemical equation as

C 2 H 5 OH(
)3O 2 (g)88n2CO 2 (g)3H 2 O(
) 
H1367 kJ/mol rxn

The negative sign indicates that this is an exothermicreaction (i.e., it gives offheat).

We always interpret 
Has the enthalpy change for the reaction as written; that is,

as (enthalpy change)/(mole of reaction), where the denominator means “for the

number of moles of each substance shown in the balanced equation.”

We can then use several unit factors to interpret this thermochemical equation.





The reverse reaction would require the absorption of 1367 kJ under the same conditions;

1367 kJ2CO 2 (g)3H 2 O(
)88nC 2 H 5 OH(
)3O 2 (g)

That is, it is endothermic,with
H1367 kJ.

2CO 2 (g)3H 2 O(
)88nC 2 H 5 OH(
)3O 2 (g) 
H1367 kJ/mol rxn

It is important to remember the following conventions regarding thermochemical equa-
tions:

1.The coefficients in a balanced thermochemical equation refer to the numbers of

molesof reactants and products involved. In the thermodynamic interpretation

of equations we neverinterpret the coefficients as numbers of molecules.Thus, it

is acceptable to write coefficients as fractions rather than as integers, when neces-

sary.

2.The numerical value of 
H(or any other thermodynamic change) refers to the

number of molesof substances specified by the equation. This amount of change

of substances is called one mole of reaction,so we can express 
Hin units of

energy/mol rxn. For brevity, the units of 
Hare sometimes written kJ/mol or

even just kJ. No matter what units are used, be sure that you interpret the ther-

modynamic change per mole of reaction for the balanced chemical equation to which

it refers.If a different amount of material is involved in the reaction, then the


H(or other change) must be scaled accordingly.

3.The physical states of all species are important and must be specified. Heat is

given off or absorbed when phase changes occur, so different amounts of heat

could be involved in a reaction depending on the phases of reactants and prod-

ucts.

4.The value of 
Husually does not change significantly with moderate changes in

temperature.

1367 kJ given off



3 mol H 2 O(
) formed

1367 kJ given off



2 mol CO 2 (g) formed

1367 kJ given off



3 mol O 2 (g) consumed

1367 kJ given off



mol C 2 H 5 OH(
) consumed

1367 kJ given off



mol of reaction

15-5 Thermochemical Equations 599

C 2 H 5 OH(
)3O 2 (g)

H1367 kJ

2CO 2 (g)3H 2 O(
)

Before we know what a value of 
H

means, we must know the balanced

chemical equation to which it refers.

Increasing enthalpy

88888888888888888888888888n

8888888888888888888888n

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 6.15, Enthalpy Changes for

Chemical Reactions.