Phosphorus oxychloride (POCl3) is a chlorinating agent that is frequently used i

Question

Phosphorus oxychloride (POCl3) is a chlorinating agent that is frequently used in organic chemistry to replace oxygen with chlorine. Given Svap = 93.08 J/(mol·K) and Hvap = 35.2 kJ/mol, does POCl3 spontaneously convert from a liquid to a gas at 110°C? Does it spontaneously crystallize at 10.0°C if Hfus = 34.3 kJ/mol and Sfus = 125 J/(mol·K)? Using the information provided, what is the melting point of POCl3?

A) yes; yes; 274 K

B) yes; no; not applicable

C) yes; yes; 3.6 K

D) yes; no; 274 K

E) yes; yes; 383 K

Explanation / Answer

Svap = 93.08, Hvap = 35.2 kJ/mol

hence G = Hvap -T Svap

= 35.2 X 103 -(383 X 93.08)

= -449.64 J/mol

hence reaction is spontaneous

during crystallization

G = Hfus -T S

or,

G = 34.3 X 103 -(283 X 125)

-1.075 kJ/mol

hence spontaneous

during phase change delta G = 0

so during melting

0 = Hvap -T Svap

or,

Hvap /Svap = T

T = 378.16 K

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