Physical Chemistry Third Edition

86 2 Work, Heat, and Energy: The First Law of Thermodynamics

Assume that the heat capacities are constant and equal to
their values in Table A.6 of the appendix.
2.47 Find the value ofqand the value of∆Hif 2.000 mol of
solid water (ice) at− 10. 00 ◦C is turned into liquid water at

80.00◦C, with the process at a constant pressure of

1.000 atm. Assume that the heat capacities are

constant and equal to their values in Table A.6 of the

appendix.

2.7 Calculation of Enthalpy Changes of a Class of

Chemical Reactions

A chemical reaction involves the breaking of some chemical bonds and/or the formation

of other chemical bonds. The breaking of bonds requires an input of energy and the

formation of bonds gives off energy, so that nearly every constant-temperature chemical

reaction is accompanied by energy and enthalpy changes. If the system gives off heat

when a reaction takes place at constant temperature, the reaction is calledexothermic.

If the system absorbs heat at constant temperature, the reaction is calledendothermic.

In this chapter we will consider only chemical reactions in which every reactant or

product is either a gas or a pure liquid or solid, and we will assume that all gases are

ideal gases. Three reactions in this class are:

2H 2 (g)+O 2 (g)−→2H 2 O(l) (2.7-1)

CaCO 3 (s)−→CaO(s)+CO 2 (g) (2.7-2)

N 2 O 4 (g)−→2NO 2 (g) (2.7-3)

The label “s” refers to solid, the label “l” refers to liquid, and the label “g” refers to

gas. We will consider solution reactions in a later chapter.

Physical chemists like to write a single equation that can apply to every possible

case. In order to do this, we write chemical reaction equations with the symbols for all

substances on the right-hand side and replace the−→symbol by an equals sign. The

three reactions of Eqs. (2.7-1) through (2.7-3) become

0 2H 2 O(l)−2H 2 (g)−O 2 (g) (2.7-4)

0 CaO(s)+CO 2 (g)−CaCO 3 (s) (2.7-5)

0 2NO 2 (g)−N 2 O 4 (g) (2.7-6)

We denote the stoichiometric coefficient of substance numberibyνior byν(Fi),

whereFirepresents the chemical formula of the substance. If substance numberi

is a product,νiis positive, and if substance numberjis a reactant,νjis negative.

If NO 2 is called substance number 1 and N 2 O 4 is called substance number 2, then

ν 1 ν(NO 2 )2 andν 2 ν(N 2 O 4 )−1 in Eq. (2.7-6). If the number of substances

involved in the reaction is represented bys, we number the substances from 1 tos, and

represent any chemical reaction by the equation

0 

∑s

i 1

νiFi (2.7-7)