Physical Chemistry , 1st ed.

has an unusually large heat of vaporization for such a small molecule. This

is caused by the strong hydrogen bonding between water molecules. It takes

a lot of energy to separate the individual water molecules (which is about

what happens during the vaporization process), and the high heat of vapor-

ization reflects that fact.

2.10 Chemical Changes

When a chemical reaction occurs, the chemical identities of the system are

changing. Although most of the equations and definitions we have considered

so far are still directly applicable, we need to expand the applicability ofU

and H.

It should be understood that all chemical substances have a total internal

energy and enthalpy. When a chemical change occurs, the change in the inter-

nal energy or enthalpy that accompanies the chemical change is equal to the

total enthalpy of the final conditions, the products, minus the total enthalpy of

the initial conditions, the reactants. That is,

rxnHHfHi

rxnHHproductsHreactants

where we are using rxnHto indicate the change in enthalpy for the chemical

reaction.rxnUis the equivalent for internal energy. Figure 2.11 illustrates this

idea. In each graph, one line represents the total enthalpy of the products; the

other is the total enthalpy of the reactants. The difference between the lines

represents the change in enthalpy for the reaction,rxnH. In one case, Figure

2.11a, the amount of enthalpy in the system is going down. That is, the system

is giving off energy into the surroundings. This is an example of an exother-

mic process. In the other case, Figure 2.11b, the amount of enthalpy in the sys-

tem is going up. This means that energy is going into the system, so this is an

example of an endothermic process.

The change in energy of a chemical process depends on the conditions of

the process, like temperature and pressure. The standard condition of pressure

is 1 bar (which is almost equal to 1 atm, so use of 1 atm as the standard con-

dition of pressure does not impart too much error). There is no defined stan-

dard temperature, although many thermodynamic measurements are reported

2.10 Chemical Changes 53

H

(a)

Total enthalpy of reactants

rxnH

Total enthalpy of products

H

(b)

Total enthalpy of reactants

rxnH

Total enthalpy of products

Figure 2.11 A graphical interpretation of the statement that rxnHfor a chemical process is

the difference between the total enthalpies of the products minus the total enthalpies of the re-

actants. (a) An exothermic reaction, since the total energy of the system is going down (meaning

that energy is given off ). (b) An endothermic reaction, since the total energy of the system is go-

ing up (meaning that the energy is entering the system).