Question

Will you explain the terms "enthalpy" and "entropy" and its significance?

Answer 1

Enthalpy is easier to understand than entropy. I'll do my best to explain them each. My explanation will be from a thermodynamics point of view.

Enthalpy is an important quantity that defines the energy of a system. It is, simply put, a combination of the internal energy of the system and amount of energy required to displace the environment to accommodate the system. This is\[\Delta H = \Delta U + pV\]Enthalpy is important in studying the energy exchange involved in open systems. This is because the pV terms is captured in the boundary work expression of the first law with a closed system but no boundary work is ever done by an open system. The pV term is therefore captured in the first law of open systems as enthalpy. The derivation is highly conceptual so I won't go into it now.

Entropy is awesome and is defined by the Second Law of Thermodynamics. I had a professor once exclaim that the second law was the most beautiful equation in all of thermodynamics. \[\Delta S = \int\limits {\delta Q \over T} + S_{gen}\]Entropy is highly conceptual and hard to define to first time students of thermodynamics. Entropy is often described as the disorder of energy. It is a measure of how much energy involved in a process is lost to the environment as heat. When heat is lost to the environment it no longer has the potential to do work on that system. That heat loss is measured by entropy. Entropy (and the Second Law) has so many valuable and powerful implications. First, it states that a perpetual motion machine cannot exist. The US Patent Office will not review patents claiming to be of perpetual motion machines. Second, it is central to the Carnot Efficiency. The Carnot heat engine describes a perfect heat engine. It represents an efficiency that all engineers strive to accomplish. However, the Carnot Efficiency is never 100% because as you know, heat engines requires two temperature reservoirs to operate between. When heat is rejected from the heat engine to the cold reservoir, entropy increases and efficiency is lost.

Answer 2

entropy is also a measure of the tendency of a process, such as a chemical reaction, to be entropically favored, or to proceed in a particular direction.

Thermodynamic entropy has the dimension of energy divided by temperature, which has a unit of joules per kelvin (J/K) in the International System of Units.
entropy is a measure of disorder, and that nature tends toward maximum entropy for any isolated system.

Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.
The unit of measurement for enthalpy in the International System of Units (SI) is the joule,

Enthalpy (H) - The sum of the internal energy of the system plus the product of the pressure of the gas in the system and its volume:
Hsys=Esys+PV
After a series of rearrangements, and if pressure if kept constant, we can arrive at the following equation:
Delta Hsys= q ( at constant pressure)

where H is the Hfinal minus Hinitial and q is heat

Answer 3

I'm doing some brush up on thermodynamics right now. In addition to the answers above, if you want to dig a little deeper without getting lost in a massive text book, this little book is excellent:
http://www.amazon.com/The-Laws-Thermodynamics-Short-Introduction/dp/0199572194/ref=sr_1_1?ie=UTF8&qid=1340720563&sr=8-1&keywords=oxford+vsi+thermodynamics
I downloaded it last night onto my Kindle.