Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | страница 2

Before You Begin, Review These Skills
• System and surroundings [9 Section 10.1]
• Hess’s law [9 Section 10.5]
• Standard Enthalpies of Formation [9 Section 10.6]

14.1 ? Spontaneous Processes
An understanding of thermodynamics enables us to predict whether or not a reaction will occur
when reactants are combined. This is important in the synthesis of new compounds in the laboratory, the manufacturing of chemicals on an industrial scale, and the understanding of natural
processes such as cell function. A process that does occur under a specific set of conditions is
called a spontaneous process. One that does not occur under a specific set of conditions is called
nonspontaneous process. Table 14.1 lists examples of familiar spontaneous processes and their
nonspontaneous counterparts. These examples illustrate what we know intuitively: Under a given
set of conditions, a process that occurs spontaneously in one direction does not also occur spontaneously in the opposite direction.
Processes that result in a decrease in the energy of a system often are spontaneous. For
example, the combustion of methane is exothermic:
CH4(g) + 2O2(g)

Student Annotation: The conditions that
most often are specified are temperature,
pressure, and in the case of a solution,
concentration.

CO2(g) + 2H2O(l)?????H° = –890.4 kJ/mol

Thus, the energy of the system is lowered because heat is given off during the course of the reaction. Likewise, in the acid-base neutralization reaction,
H+(aq) + OH–(aq)

H2O(l)?????H° = –56.2 kJ/mol

heat is given off, lowering the energy of the system. Each of these processes is spontaneous, and
each results in a lowering of the system’s energy.
Now consider the melting of ice:
H2O(s)

H2O(l)?????H° = 6.01 kJ/mol

In this case, the process is endothermic and yet it is also spontaneous at temperatures above 0°C.
Conversely, the freezing of water is an exothermic process:
H2O(l)

H2O(s)?????H° = –?6.01 kJ/mol

Yet it is not spontaneous at temperatures above 0°C.

Ta b le 1 4 .1

Familiar Spontaneous and Nonspontaneous Processes

Spontaneous

Nonspontaneous

Ice melting at room temperature
Sodium metal reacting violently with water
to produce sodium hydroxide and hydrogen
gas [9 Section 8.5]
A ball rolling downhill
The rusting of iron at room temperature

Water freezing at room temperature
Sodium hydroxide reacting with hydrogen gas
to produce sodium metal and water

Water freezing at –10°C

Ice melting at –10°C

A ball rolling uphill
The conversion of rust back to iron metal at
room temperature

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9/10/13 12:01 PM