Collections:Texts:Chemistry:Chapter 16 - Entropy and Spontaneous Reactions
From ChemEd Collaborative
ENTROPY AND SPONTANEOUS REACTIONS
16.1 Spontaneous Processes and Molecular Probability
16.2 Thermodynamic Probability and Entropy
16.3 Getting Acquainted with Entropy
16.4 Including the Surroundings
16.6 Equilibrium Constants Revisited
The experiences you have had in the chemical laboratory have probably already taught you that there is an uphill character to some reactions and a downhill character to others. A simple example is the combination of mercury with bromine which we considered in detail in Sec. 2.4 and which was shown in Plate 3:
Hg(l) + Br2(l) → HgBr2(s)
This reaction occurs of its own accord, much as a ball rolls downhill. On the other hand, decomposition of HgBr2 to the elements is an uphill process, and we must “push” this reaction to force it to occur. One way to do this is to dissolve HgBr2 in water and pass an electrical current through the solution. Mercury will appear at one electrode and bromine at the other.
This chapter is designed to show you why some chemical reactions are downhill while others are uphill. In particular we will indicate what kinds of atomic and molecular processes will occur of their own accord. We will also develop criteria based on macroscopic measurements which will permit you to predict which reactions will occur and which must be pushed. This should help to sharpen the intuition you have already developed about what is likely to happen when you mix certain chemicals together.
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