Page 90 - Physical chemistry eng

4 CHAPTER

Thermochemistry 4.1 Energy Stored in Chemical
Bonds Is Released or Taken
Up in Chemical Reactions
4.2 Internal Energy and Enthalpy
Changes Associated with
Chemical Reactions
4.3 Hess’s Law Is Based on
Thermochemistry is the branch of thermodynamics that investigates Enthalpy Being a State
Function
the heat flow into or out of a reaction system and deduces the energy
4.4 The Temperature
stored in chemical bonds. As reactants are converted into products, energy Dependence of Reaction
can either be taken up by the system or released to the surroundings. For a Enthalpies
4.5 The Experimental
reaction that takes place at constant volume, the heat that flows to or out
Determination of ¢U and ¢H
of the system is equal to ¢U for the reaction. For a reaction that takes for Chemical Reactions
place at constant pressure, the heat that flows to or out of the system is 4.6 Supplemental: Differential
Scanning Calorimetry
equal to ¢H for the reaction. The enthalpy of formation is defined as the
heat flow into or out of the system in a reaction between pure elements

that leads to the formation of 1 mol of product. Because H is a state
function, the reaction enthalpy can be written as the enthalpies of for-
mation of the products minus those of the reactants. This property
allows ¢H and ¢U for a reaction to be calculated for many reactions
without carrying out an experiment.

Energy Stored in Chemical Bonds
Is Released or Taken Up in Chemical
4.1 Reactions

A significant amount of the internal energy or enthalpy of a molecule is stored in the
form of chemical bonds. As reactants are transformed to products in a chemical reac-
tion, energy can be released or taken up as bonds are made or broken, respectively.
For example, consider a reaction in which N (g) and H (g) dissociate into atoms, and
2
2
the atoms recombine to form NH (g). The enthalpy changes associated with individ-
3
ual steps and with the overall reaction 1>2 N (g) + 3>2 H (g) ¡ NH (g) are
2
3
2
shown in Figure 4.1. Note that large enthalpy changes are associated with the individ-
ual steps but the enthalpy change in the overall reaction is much smaller.
The change in enthalpy or internal energy resulting from chemical reactions
appears in the surroundings in the form of a temperature increase or decrease resulting
from heat flow and/or in the form of expansion or nonexpansion work. For example,
the combustion of gasoline in an automobile engine can be used to do expansion
work on the surroundings. Nonexpansion electrical work is possible if the chemical
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