Phosphorus exists in multiple solid phases, including two known as red phosphorus and white phosphorus. 

(a) Based on their respective heats of formation, which form of phosphorus is defined as the standard state?

(b) Now consider the phase transition between white and red phosphorous: P₄(s, white) → 4 P(s, red). Use data from Appendix E to determine which form of phosphorous is actually more stable at 25°C. (Your result should reveal that phosphorus is an exception to the usual convention for defining the standard state.) 

(c) Is the same form of the solid more stable at all temperatures? If not, what temperatures are needed to make the other form more stable?