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Chemical Thermodynamics

We have already looked at two of the most important questions that chemists ask when designing and understanding chemical reactions: First, how rapidly does the reaction progress? Second, how far toward completion does the reaction proceed? The first question is addressed by a study of the reaction rate, which we discussed in Chapter 14. The second question involves the equilibrium constant, which was the focus of Chapter 15. Let’s briefly review how these concepts are related.

In Chapter 14 we learned that the rates of reactions are largely controlled by a factor related to energy; namely, the activation energy of the reaction. (Section 14.5) The lower the activation energy, the faster a reaction proceeds. In Chapter 15 we saw that equilibrium depends on the rates of the forward and reverse reactions: Equilibrium is reached when the opposing reactions occur at equal rates. (Section 15.1) Because reaction rates are closely tied to energy, it is logical that equilibrium should also depend in some way on energy. In this chapter we will see how chemical equilibrium is related to the energies of the reactants and products. To do so, we will take a deeper look at chemical thermodynamics, the area of chemistry that explores energy relationships. We first encountered thermodynamics in Chapter 5, where we discussed the nature of energy, the first law of thermodynamics, and the concept of enthalpy. Recall that, for a reaction that occurs at constant pressure, the enthalpy change equals the heat transferred between the system and its surroundings. (Section 5.3) As we discussed in the "Strategies in Chemistry" box in Section 5.4, the enthalpy change of a reaction is an important guide as to whether the reaction is likely to proceed. However, we also pointed out that enthalpy is not the only factor that governs whether the reactants or products of a reaction are more favored. For example, ice melts at room temperature even though the process is endothermic. Similarly, NH4NO3(s) readily dissolves in water even though ΔHsoln is positive. (Section 13.1) Clearly, the sign of the enthalpy change alone is not enough to tell us whether a reaction will proceed.

In this chapter we will address several aspects of chemical thermodynamics. We will see that in addition to enthalpy, we must consider the change in the randomness or disorder that accompanies a chemical reaction. We alluded to this notion in Section 13.1. Finally, we will learn how to combine the enthalpy change of a reaction with the change in randomness to define a new type of energy that relates directly to equilibrium. We begin by introducing a new aspect to our discussion of thermodynamics, namely the idea of spontaneous processes.

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