Chemistry: Thermodynamics Question HELP

Hi Yahoo-ers,

Well I'm studying Advanced Placement Chemistry and I have my exam coming up VERY soon (May 2nd) and I have a subject to clear up.

Can someone PLEASE explain to me the conditions that take place when using this formula:
Delta G = Delta H - T X Delta S

Here's an example question:

1.) 2Al (s) + 3Cl (g) ----------------> 2AlCl3 (s)

The reaction above is not spontaneous under standard conditions, but becomes spontaneous as the temperature decreases toward absolute zero. Which of the following is true at standard conditions?

(A) ΔS and ΔH are both positive.
(B) ΔS and ΔH are both negative.
(C) ΔS is negative and ΔH is positive.
(D) ΔS is positive and ΔH is negative.
(E) ΔS and ΔH are both equal to zero

Could you please thoroughly explain the relationships because I keep getting all these questions incorrect!

THANK YOU!

yeah, this equation is the key idea behind the thermodynamics section. However, it really is all math and some basic concepts in these questions.

So step by step, H is enthalpy. It is a measure truly of the energy that is locked in a substance. The delta H is the change in enthalpy over the reaction. As you may know, a + ΔH means it is endothermic while a negative ΔH means it is exothermic. Really, look at from the substances point of view, where if it loses H, the surroundings gain it aka exothermic, and vice versa.

S is entropy, or a measure of disorder. In AP chem as I have seen it, it really is an arbitrary concept of whether the reaction causes the substances to become more ordered or disordered. It kind of is sketchy in many questions, but many times, use common sense to tell whether ΔS is positive (more disorder) or if it is negative (less disorder).

ΔG is the Gibbs free energy. As I see it, the only thing that matters is that it is related to whether a reaction is spontaneous (occurs favorably, a negative ΔG) or unspontaneous (occurs unfavorable, a positive ΔG). It depends on S and H by the above relation you gave.