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General Chemistry Principles, Patterns, and Applications, 2011

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Compound ΔHfus (kJ/mol) ΔSfus [J/(mol·K)] Melting Point (°C)<br />

CH4 0.94 −182.5<br />

CH3OH 18.2 −97.7<br />

formic acid 12.7 45.1<br />

5. Based on this table, can you conclude that entropy is related to the nature of functional groups? Explain your<br />

reasoning.<br />

6. Calculate the missing data in the following table.<br />

Compound<br />

ΔHvap (kJ/mol) ΔSvap [J/(mol·K)] Boiling Point (°C)<br />

hexanoic acid 71.1 105.7<br />

hexane 28.9 85.5<br />

formic acid 60.7 100.8<br />

1-hexanol 44.5 157.5<br />

7. The text states that the magnitude of ΔS vap tends to be similar for a wide variety of compounds. Based on the<br />

values in the table, do you agree?<br />

alculate the amount of work done against a pressure of 1.0 atm when 4.0 mol of acetylene are allowed to react<br />

with 10 mol of O 2 at 1.0 atm at 20°C. What is the change in internal energy for the reaction?<br />

4. When iron dissolves in 1.00 M aqueous HCl, the products are FeCl 2 (aq) <strong>and</strong> hydrogen gas. Calculate the work<br />

done if 30 g of Fe react with excess hydrochloric acid in a closed vessel at 20°C. How much work is done if the<br />

reaction takes place in an open vessel with an external pressure of 1.0 atm?<br />

A N S W E R<br />

1. −350 J; 8.2 kJ<br />

18.4 Entropy Changes <strong>and</strong> the Third Law of Thermodynamics<br />

L E A R N I N G O B JE C T I V E<br />

1. To use thermodynamic cycles to calculate changes in entropy.<br />

The atoms, molecules, or ions that compose a chemical system can undergo several types of molecular<br />

motion, including translation, rotation, <strong>and</strong> vibration (Figure 18.13 "Molecular Motions"). The greater the<br />

molecular motion of a system, the greater the number of possible microstates <strong>and</strong> the higher the entropy.<br />

A perfectly ordered system with only a single microstate available to it would have an entropy of zero. The<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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