The Foundations of Chemistry

ENTHALPY CHANGES

Most chemical reactions and physical changes occur at constant (usually atmospheric) pres-

sure.

The quantity of heat transferred into or out of a system as it undergoes a chemical

or physical change at constant pressure, qp, is defined as the enthalpy change, H,

of the process.

An enthalpy change is sometimes loosely referred to as a heat changeor a heat of reaction.

The enthalpy change is equal to the enthalpy or “heat content,” H,of the substances

produced minus the enthalpy of the substances consumed.

HHfinalHinitial or 
HHsubstances producedHsubstances consumed

It is impossible to know the absolute enthalpy (heat content) of a system. Enthalpy is a

state function,however, and it is the change in enthalpyin which we are interested; this can

be measured for many processes. In the next several sections we focus on chemical reac-

tions and the enthalpy changes that occur in these processes. We first discuss the

experimental determination of enthalpy changes.

CALORIMETRY

We can determine the energy change associated with a chemical or physical process by

using an experimental technique called calorimetry.This technique is based on observing

the temperature change when a system absorbs or releases energy in the form of heat.

The experiment is carried out in a device called a calorimeter,in which the temperature

change of a known amount of substance (often water) of known specific heat is measured.

The temperature change is caused by the absorption or release of heat by the chemical

or physical process under study. A review of calculations involved with heat transfer

(Sections 1-13, 13-9, and 13-11) may be helpful for understanding this section.

A “coffee-cup” calorimeter (Figure 15-3) is often used in laboratory classes to measure

“heats of reaction” at constant pressure, qp, in aqueous solutions. Reactions are chosen so

that there are no gaseous reactants or products. Thus, all reactants and products remain

in the vessel throughout the experiment. Such a calorimeter could be used to measure the

amount of heat absorbed or released when a reaction takes place in aqueous solution. We

can consider the reactants and products as the system and the calorimeter plus the solu-

tion (mostly water) as the surroundings. For an exothermic reaction, the amount of heat

evolved by the reaction can be calculated from the amount by which it causes the temper-

ature of the calorimeter and the solution to rise. The heat can be visualized as divided

into two parts.





The heat capacity of a calorimeter is determined by adding a known amount of heat

and measuring the rise in temperature of the calorimeter and of the solution it contains.

This heat capacity of a calorimeter is sometimes called its calorimeter constant.

amount of heat



gained by solution

amount of heat gained



by calorimeter

amount of heat



released by reaction

15-4

15-3

We use qto represent the amount of
heat absorbed by the system. The
subscript pindicates a constant-
pressure process.

596 CHAPTER 15: Chemical Thermodynamics

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 6.10, Heat Transfer Between
Substances, and Screen 6.18,
Measuring Heats of Reaction:
Calorimetry.

The amount of heat absorbed by a
calorimeter is sometimes expressed as
the heat capacityof the calorimeter, in
joules per degree.

The polystyrene insulation of the
coffee-cup calorimeter ensures that
little or no heat escapes from or enters
the container.