[sundrops]

Two crystalline forms of white phosphorus are known. Both forms contain P4 molecules, but the molecules are packed together in different ways. The "a" form is always obtained when the liquid freezes. However, below -76.9 °C, the "a" form spontaneously converts to the "b" form.

P4(s, a)  P4(s, b)


Indicate which of the following statements are true or false, with regard to the above process.


1. Above -76.9°C, the sign of ?G for this process is negative.

2. At -76.9°C, ?G for this process is less than zero.

3. The sign of ?H for this process is positive.

4. The alpha phase has the less ordered crystalline structure.

5. The sign of ?S for this process is negative.

6. At -76.9°C, the two solid phases cannot coexist.

 
Alright I'm having a little bit of trouble with this question, any help would be AWESOME!

#1 not sure
#2 = not sure
#3 = True - entropy increases
#4 = False, "a" has a more ordered structure - its frozen solid
#5 = False, its going from a frozen state to a non frozen one, increasing entropy ans spontaniety
#6 = True, you can either be in one phase or the other

can anyone help me out?
thx

[jdurg]

Okay the first thing you'll need to do is figure out what ?G, ?H, and ?S are and what each of the signs mean.  

A positive ?G means that the reaction, as written, requires something to get it going.  I.E. it is NOT spontaneous.

A positive ?H means that the reaction absorbs energy as it goes to completion so the final temperature is lower than the initial temperature.

A positive ?S means that the disorder (Entropy) of the system increases when the reaction goes to completion.

All three values can be arranged into one equation as follows:  ?G = ?H - T?S where T is the temperature in Kelvin.

As you can infer from the equation, if you have a positive ?S and a negative ?H, your ?G value will be negative as well.  A negative ?G is indicitive of a spontaneous reaction.

If we use the freezing of water as an example, you'll notice that at 273 Kelvin (the freezing point of water), H2O can exist without any trouble at all as water and ice.  The two phases would be in complete equillibrium as the water has too much energy to form an ordered ice structure, and the ice required energy to break down the ordered crystal structure into liquid water.  As a result, if there is no energy input the ice and water will coexist beautifully.  Neither the forward or backward reaction is spontaneous, so ?G would equal zero.  If you add energy, then the ice will break down and form water.  If you take away energy, you make it more favorable for the unordered water to form ice.  

So with all that information given, you should be able to easily figure out your questions.