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Unit 9: Equilibrium and Advanced Thermodynamics—Balance in Chemical Reactions

Section 1: Introduction

In Unit 7, we discussed thermochemistry (the study of energy in chemical reactions), the First Law of Thermodynamics (the conservation of energy), and exothermic and endothermic reactions (how chemical reactions release or absorb energy). Another goal of thermodynamics is to understand why some reactions occur and others do not. In other words, chemists strive to determine if a reaction will be spontaneous or nonspontaneous.

Spontaneous Reactions

Figure 9-1. Spontaneous Reactions

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Spontaneous Reactions

Figure 9-1. Spontaneous Reactions

Two examples of spontaneous processes: ice melting at room temperature and iron rusting.

A spontaneous process happens without any outside intervention or input of energy. For example, liquids freeze spontaneously at temperatures below their freezing points. A solid melting at a temperature below its freezing point is nonspontaneous; it will not happen, which is why we don't worry about the ice melting in a kitchen freezer. Salt spontaneously dissolves in warm water, and will not spontaneously recrystallize. Iron rusts spontaneously; the process of "unrusting" is nonspontaneous. (Figure 9-1)

Many spontaneous processes release heat; they are exothermic and their ΔH values are negative (see Unit 7, Section 6). A ball spontaneously rolls downhill and loses energy as it goes. A ball does not spontaneously roll uphill; pushing it back uphill requires an input of energy. Many chemical reactions follow the same pattern as that ball. Burning glucose is spontaneous and releases energy:

C6H12O6 + 6O2 → 6CO2 + 6H2O

The reverse reaction is nonspontaneous, and requires an input of energy. In photosynthesis, plants reverse this reaction to create glucose; the energy input comes from sunlight.

It may be tempting, then, to assume that exothermic processes are always spontaneous, and endothermic reactions are nonspontaneous. While this is often true, there are many examples that run counter to this trend. Table salt dissolves spontaneously in water, for instance, and yet this process is endothermic. Steam condensing to water is exothermic; yet at temperatures above 100°C, condensation is nonspontaneous. Clearly, there is more to spontaneity than energy.

Glossary

Nonspontaneous

Referring to the direction of a process that proceeds toward thermodynamic instability.

Spontaneous

A process that proceeds toward thermodynamic stability.

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