18.1 The Laws of Thermodynamics
PRE-HEALTH PREP
The Laws of Thermodynamics
1st Law of Thermodynamics - Energy cannot be created or destroyed.
2nd Law of Thermodynamics - For a spontaneous process, the entropy of the universe increases.
3rd Law of Thermodynamics - A perfect crystal at zero Kelvin has zero entropy.
The First Law of Thermodynamics
The First Law of Thermodynamics states that energy cannot be created or destroyed. Alternatively, some sum it up as "the conservation of energy." Ultimately, the First Law of Thermodynamics is a statement that energy can be transferred between the system and the surroundings through the transfer of heat (q) or by the performance of mechanical work (w).
ΔE = q + w
Energy doesn't simply materialize or disappear. Any gain in energy by the system will correspond to a loss in energy by the surroundings, or any loss in energy by the system will correspond to a gain in energy by the surroundings.
The First Law is covered in more detail in the following lesson in Chapter 5: 5.1 The First Law of Thermodynamics, Enthalpy, and Phase Changes
The Second Law of Thermodynamics
The Second Law of Thermodynamics can be stated in any of three synonymous ways:
For a spontaneous process, the entropy of the universe increases.
For a spontaneous process, ΔSuniverse > 0.
For a spontaneous process, ΔSsystem + ΔSsurroundings > 0
Ultimately, for any spontaneous process the entropy, which is related to randomness or disorder, of the universe increases. Entropy is covered in much greater detail in the next lesson (18.2 Entropy) but that it correlates with disorder will suffice for now.
The last statement of the Second Law of Thermodynamics divides the universe into two parts: the system (what you're investigating) and the surroundings (everything in the universe besides the system). In chemistry the system is often a chemical reaction under investigation. To be clear the Second Law does NOT mean that ΔSreaction must be positive as ΔSreaction is just the ΔSsystem which can be either positive or negative. But if ΔSreaction for a spontaneous reaction is negative, then the Second Law does mean that ΔSsurroundings must be positive and of greater magnitude in this example so that ΔSsystem + ΔSsurroundings > 0.
In one sense you can view the Second Law of Thermodynamics as being the most important of the three; in the sense that it is the one most commonly tested upon in this chapter and students should recognize it in any of the three forms presented above.
The Third Law of Thermodynamics
The Third Law of Thermodynamics states that a perfect crystal at zero Kelvin (absolute zero) has zero entropy. First, a perfect crystal means that there are no impurities, has achieved thermodynamic equilibrium, and that it is in a crystalline state where all the atoms/ion/molecules are in well-defined positions in a highly-ordered crystalline lattice. This would exclude amorphous solids like glass that don't have an ordered, crystalline structure and have not achieved thermodynamic equilibrium.
The Third Law is not intuitive but was derived empirically as a system's entropy always approached the same minimum value as the absolute temperature was lowered and approached zero. From the standpoint of statistical mechanics the Third Law implies that a perfect crystal at absolute zero must exist in a single microstate as we'll see in the next lesson on Entropy.
The Third Law also provides us with a reference point, which combined with the fact that entropy is a state function allows us to determine the absolute entropy of a substance at any temperature which is very useful for calculating ΔSreaction as we'll see later in the chapter (18.4 Delta G, Delta H, Delta S and Formation Reactions). We'll also learn that such a reference point is not available for Gibbs Free Energy or Enthalpy leading to the use of formation reactions in calculating ΔGreaction and ΔHreaction.