Question

Please help me with whats wrong?

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; density=1.82 g/cm3) 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 B phase (P4; density=1.88 g/cm3). P4(s, a) → P4(S, B) Indicate which of the following statements are true or false, with regard to the above process. true false true false true The sign of AS for this process is negative. Below -76.9°C, the sign of AG for this process is positive. The a phase has the less ordered crystalline structure. The sign of AH for this process is positive. At -76.9°C, AG for this process is greater than zero. At -76.9°C, both solid phases can coexist indefinitely.

Answer 1

(a) TRUE

$\alpha$ form of the phosphorus gets spontaneously converted to $\beta$ form below -76. 9^oC. As the temperature is lowered, the movement of atoms or the randomness will also be lowered. That is, the entropy is decreasing for the process. Therefore the sign of $\Delta$ S will be negative for the process.

(b) FALSE

Given that the above reaction is spontaneous, therefore $\Delta$ G will be negative.

(c) TRUE

$\alpha$ form has a higher entropy than $\beta$ form . As mentioned above, disorder decreases as temperature decreases. Hence $\alpha$ form has a less ordered crystalline structure.

(d) FALSE

$\Delta$H for the process is negative.

(e) FALSE

At -76.9^oC the system is in equilibrium, therefore $\Delta$ G will be zero.

(f) TRUE

At  -76.9^oC the two phases coexist and the system is in equilibrium.