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General Chemistry Principles, Patterns, and Applications, 2011

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c. What is K?<br />

d. This reaction requires thermal ignition. Why?<br />

15. ♦ Cesium, a silvery-white metal used in the manufacture of vacuum tubes, is produced industrially by the<br />

reaction of CsCl with CaC 2 :<br />

2CsCl(l) + CaC 2 (s) → CaCl 2 (l) + 2C(s) + 2Cs(g)<br />

Compare the free energy produced from this reaction at 25°C <strong>and</strong> at 1227°C, the temperature at which it is<br />

normally run, given these values<br />

= 32.0 kJ/m<br />

3.6 J/(mol·K).<br />

: DH 298 K<br />

, DS 298 K = 8.0 J /( mol·K ); DH 1500 K = - 0.6 kJ / mol, DS 1500 K =<br />

a. If you wanted to minimize energy costs in your production facility by reducing the<br />

temperature of the reaction, what is the lowest temperature at which products are<br />

favored over reactants (assuming the reaction is kinetically favorable at the lower<br />

temperature)? Assume ΔH° <strong>and</strong> ΔS° vary linearly with temperature.<br />

b. What is the ratio K 1500 K /K 298 K ?<br />

16. Dessicants (drying agents) can often be regenerated by heating, although it is generally not economically<br />

worthwhile to do so. A dessicant that is commonly regenerated is CaSO 4·2H 2 O:<br />

CaSO4 ×2H 2O s<br />

( ) ® CaSO4 ( s) + 2H 2O g<br />

= 25.1 kJ / mol, DS 298 = 69.3 J / (mol × K)<br />

Regeneration is carried out at 250°C.<br />

( )DH 298<br />

a. What is ΔG° for this reaction?<br />

b. What is the equilibrium constant at 25°C?<br />

c. What is the ratio K 250°C /K 25°C ?<br />

d. What is the equilibrium constant at 250°C?<br />

e. Is regeneration of CaSO 4 (s)·2H 2 O an enthalpy- or entropy-driven process? Explain your<br />

answer.<br />

17. The nitrogen triiodide complex with ammonia (NI 3·NH 3 ) is a simple explosive that can be synthesized from<br />

common household products. When detonated, it produces N 2 <strong>and</strong> I 2 . It can be painted on surfaces when<br />

wet, but it is shock sensitive when dry (even touching it with a feather can cause an explosion). Do you<br />

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

Saylor.org<br />

1710

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