Question

Spontaneity of Solid Phase Change Two crystalline phases of white phosphorus are known. Both contain P4 molecules, but the molecules are packed together in different ways. The a phase of the solid (P4; densitya 1.82 g/cm) is always obtained when molten phosphorus crystallizes below the melting point (44.1 °C). However, when cooled below -76.9 °C, the a phase spontaneously converts to the ß phase (P4; density=1.89 g/cm3). P4(s, 0) – P4(s, 8) Indicate which of the following statements are true or false, with regard to the above process. - The sign of AH for this process is positive. At -76.9°C, AG for this process is zero. At -76.9°C, the two solid phases cannot coexist. The B phase has the less ordered crystalline structure, - The sign of AS for this process is positive. Below -76.9°C, the sign of AG for this process is positive. cf Section 10.7 "Free Energy" pp 368-71 in Zumdahl Chemical Principles 8th ed.

Answer 1

From the above passage, we can understand some points

1) As the temperature decreases, P converts from alpha form to beta form such that some quantity of heat is liberated during cooling process so this reaction is exothermic reaction.

Therefore delta H is -ve

So first statement is wrong

2)A spontaneous process is carried out when delta G value is in -ve value so that at -76.9 °c delta G is not zero.

This is also wrong

3) Below -76.9°c only alpha P concverts into beta P so that at -76.9° c both solid phases cannot exist

It is true

4) Beta- phase of P is higher ordered strucure ( Hexagonal cubic packing )

It is wrong

5) As it is spontaneous process, Entropy (disorder) increases and delta S is ' + ve'

It is true

6) For a spontaneous process delta G is ' -ve '

Below -76.9°c spontaneous process is carried out

so that delta G value must be ' -ve '

It is wrong