The Zeroth Law of Thermodynamics states that if two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with one another.
The First Law of Thermodynamics states that energy cannot be created or destroyed, but only transformed from one form to another. Stated another way, the first law of thermodynamics says that the energy of the universe is constant. Mathematically, the first law is expressed as follows:
\(\ce{\Delta E = q + w}\)
Where ΔE is the change in internal energy of a system, q is the amount of heat transferred into or out of the system, and w is the amount of work that is done on or by the system. In chemistry, a positive sign indcates a process that is being performed on the system, while a negative sign indicates a process that is performed by the system. For example, +w indicates that work is being performed on the system, while -w indicates that the system is performing work on the surroundings.
The Second Law of Thermodynamics states that heat naturally flows from hotter to colder regions. Additionally, it states that the entropy of the universe increases over time.
A state function is a property that does not depend on the path taken, only the inital and final states. An example of a state fuction is displacement, ΔX. Displacement only depends on the initial and final position. Distance, on the other hand, is a non-state function and depends on the path taken to get from the initial to final point. In other words, it is path dependent.
The spontaneity of a reaction depends both on enthalpy H and entropy S. The state function defined by Gibbs, Gibbs free energy, combines these as follows:
\(\Delta G = \Delta H - T \Delta S\)
Where ΔG is the change in free energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy. The spontaneity of a reaction is based on the sign of ΔG as follows:
\(\Delta G < 0\), spontaneous in the forward direction
\(\Delta G = 0\), reaction is at equilibrium
\(\Delta G > 0\), reaction is nonspontaneous in the forward direction, reverse reaction is spontaneous
A spontaneous reaction is also known thermodynamically favorable.