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68 CHAPTER 4 Thermochemistry

FIGURE 4.1
Enthalpy changes are shown for
individual steps in the overall reaction N(g) 3H(g)
1>2 N 2 (g) + 3>2 H 2 (g) ¡ NH 3 (g).

3
314 10 J
NH(g) 2H(g)

3
390 10 J
3
1124 10 J
(g) H(g)
NH 2

3
466 10 J

3
45.9 10 J
1 2 N (g) 3 2 H (g)
2
2
NH 3 (g)

reaction is carried out in an electrochemical cell. In Chapters 6 and 11, the extraction
of nonexpansion work from chemical reactions will be discussed. In this chapter, the
focus is on using measurements of heat flow to determine changes in U and H due to
chemical reactions.

Internal Energy and Enthalpy Changes
4.2 Associated with Chemical Reactions

In the previous chapters, we discussed how ¢U and ¢H are calculated from work and
heat flow between the system and the surroundings for processes that do not involve
phase changes or chemical reactions. In this section, this discussion is extended to
reaction systems.
Imagine that a reaction involving a stoichiometric mixture of reactants (the system)
is carried out in a constant pressure reaction vessel with diathermal walls immersed in a
water bath (the surroundings). If the temperature of the water bath increases, heat has
flowed from the system (the contents of the reaction vessel) to the surroundings (the
water bath and the vessel). In this case, we say that the reaction is exothermic. If the
temperature of the water bath decreases, the heat has flowed from the surroundings to
the system, and we say that the reaction is endothermic.
Consider the reaction in Equation (4.1):

Fe O (s) + 4 H (g) ¡ 3 Fe(s) + 4 H O(l) (4.1)
2
2
3 4
Note that the phase (solid, liquid, or gas) for each reactant and product has been speci-
fied because U and H are different for each phase. This reaction will only proceed at a
measurable rate at elevated temperatures. However, as we show later, it is useful to tab-
ulate values for ¢H for reactions at a pressure of 1 bar and a specified temperature,
generally 298.15 K. The pressure value of 1 bar defines a standard state, and changes
in H and U at the standard pressure of 1 bar are indicated by a superscript ° as in
¢H° and ¢U° . The standard state for gases is a hypothetical state in which the gas
behaves ideally at a pressure of 1 bar. For most gases, deviations from ideal behavior