confused about 2nd law !!! [ENDORSED]

[Maeve Gallagher 1J]

I know that the second law of thermodynamics states that the entropy of an isolated system has a tendency to increase, but how is this law related to the equation deltaS = qrev/T ?

[Chem_Mod]

There are many, many formulations (statements) of the 2nd Law of Thermodynamics. They are all related to each other but sometimes in relatively difficult to derive ways that are sometimes not apparent at first glance. However, the definition ΔS = q_rev/T can be used to write:

ΔS_tot = ΔS_sys + ΔS_surr > 0 for all spontaneous processes
ΔS_sys = q_rev/T and ΔS_surr = -q_rev/T

[Clarissa Molina 1D]

How do we know when to use the 2nd law or 3rd law equations? What is the main difference between these 2 laws?

[Diego Zavala 2I]

The third law of thermodynamics is used to analyze the probability of finding one compound in a certain direction (think Schrodinger's cat) given the different orientations that it can have. The second law of thermodynamics will involve examining what will occur to the entropy of the system/surroundings when reacted/heated/cooled