Question

Which of the following changes will always be true for a spontaneous reaction?

+deltaG

+deltaS

-deltaG

-deltaH

Answer 1

When ΔG is negative, a process or chemical reaction proceeds spontaneously in the forward direction.
When ΔG is positive, the process proceeds spontaneously in reverse.
When ΔG is zero, the process is already in equilibrium, with no net change taking place over time.

We can further distinguish four cases within the above rule just by examining the signs of the two terms on the right side of the equation.
When ΔS is positive and ΔH is negative, a process is always spontaneous
When ΔS is positive and ΔH is positive, a process is spontaneous at high temperatures, where exothermicity plays a small role in the balance.
When ΔS is negative and ΔH is negative, a process is spontaneous at low temperatures, where exothermicity is important.
When ΔS is negative and ΔH is positive, a process is not spontaneous at any temperature, but the reverse process is spontaneous.

from here

http://en.wikipedia.org/wiki/Spontaneous_process