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General Chemistry Principles, Patterns, and Applications, 2011

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3. Nitrogen fixation is the process by which nitrogen in the atmosphere is reduced to NH 3 for use by organisms.<br />

Several reactions are associated with this process; three are listed in the following table. Which of these are<br />

spontaneous at 25°C? If a reaction is not spontaneous, at what temperature does it become spontaneous?<br />

Reaction DH 298 (kcal / mol )<br />

( a) 12N2 +O2 ® NO2<br />

( )<br />

DS 298 é ëcal / °·mol ù û<br />

8.0 −14.4<br />

( b) 12N2 +12O2 ® NO<br />

21.6 2.9<br />

( c) 12N2 + 32H2 ® NH 3<br />

−11.0 −23.7<br />

4. A student was asked to propose three reactions for the oxidation of carbon or a carbon compound to CO or<br />

CO 2 . The reactions are listed in the following table. Are any of these reactions spontaneous at 25°C? If a<br />

reaction does not occur spontaneously at 25°C, at what temperature does it become spontaneous?<br />

Reaction DH 298(kcal / mol ) DS 298[cal / (°·mol)]<br />

C(s) + H2O(g) → CO(g) + H2(g) 42 32<br />

CO(g) + H2O(g) → CO2(g) + H2(g) −9.8 −10.1<br />

CH4(g) + H2O(g) → CO(g) + 3H2(g) 49.3 51.3<br />

5. Tungsten trioxide (WO 3 ) is a dense yellow powder that, because of its bright color, is used as a pigment in oil<br />

paints <strong>and</strong> water colors (although cadmium yellow is more commonly used in artists’ paints). Tungsten metal<br />

can be isolated by the reaction of WO 3 with H 2 at 1100°C according to the equation WO 3 (s) + 3H 2 (g) → W(s) +<br />

3H 2 O(g). What is the lowest temperature at which the reaction occurs spontaneously? ΔH° = 27.4 kJ/mol <strong>and</strong><br />

ΔS° = 29.8 J/K.<br />

6. Sulfur trioxide (SO 3 ) is produced in large quantities in the industrial synthesis of sulfuric acid. Sulfur dioxide is<br />

converted to sulfur trioxide by reaction with oxygen gas.<br />

a. Write a balanced chemical equation for the reaction of SO 2 with O 2 (g) <strong>and</strong> determine its<br />

ΔG°.<br />

b. What is the value of the equilibrium constant at 600°C?<br />

c. If you had to rely on the equilibrium concentrations alone, would you obtain a higher yield<br />

of product at 400°C or at 600°C?<br />

7. Calculate ΔG° for the general reaction MCO 3 (s) → MO(s) + CO 2 (g) at 25°C, where M is Mg or Ba. At what<br />

temperature does each of these reactions become spontaneous?<br />

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

Saylor.org<br />

1676

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