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 α 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 α phase spontaneously converts to the β phase (P4; density≅1.88 g/cm3).
P4(s, α) --> P4(s, β)
Indicate which of the following statements are true or false, with regard to the above process.
1. Below -76.9°C, the sign of ΔG for this process is negative.
2. The sign of ΔS for this process is negative.
3. At -76.9°C, ΔG for this process is less than zero.
4. At -76.9°C, the two solid phases cannot coexist.
5. The α phase has the more ordered crystalline structure.
6. The sign of ΔH for this process is negative.
4 answers
2. false I'm guessing false cuz if low-temperature dS is positive
3. true
4. I'm not sure about this I'm guessing false?
5. I don't understand this one. I'm guessing false
6. true
I agree with the second answer for 2. My thought is that the density is lower at very cold T. If density is lower it means d = m/v ; therefore, volume must be lower so the crystal is packed closer together so it is more ordered thus dS is - and statement is true.
I agree with 3.
I agree with your guess on 4. Remember that ice and liquid water co-exist at 0o C, the melting point of water. Same thing should apply at -76.9 in my opinion.
I think your guess is right for 5 since the answer to 2 was that the b crystal @ -79.4 C was more compact and more ordered.
I agree with 6. dG = dH - TdS. dS is - so -TdS is + so dH must be - for dG to be -.
1 answer