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Thermodynamics - Laws, Definitions & Equations

Thermodynamics is the science that defines the relationships between heat and the other types of energy especially mechanical energy or work. The classical thermodynamics deals with macroscopic properties such as pressure, temperature, mass density, etc.; which means it doesn’t study systems at the atomic or molecular level. On the other hand, statistical thermodynamics is the science that study the changes of such properties at the atomic level.

There are four laws of thermodynamics; the first, the second, the third, and the zeroth. The concepts of these laws were developed in the nineteenth century, but the laws were enunciated in different times; the first and the second in the nineteenth century, the third and the zeroth in the twentieth century. The zeroth law was named so because it was not possible to rearrange the numbering.

You may be wondering now why it is named the zeroth; why not the fourth law. Simply because this is the logical flow of things. The zeroth law came to establish temperature measurements and to put the term “temperature” in a concrete frame. In simple words, it came to define temperature based on measurements. The law states that “if a system A is in thermal equilibrium with a system B, and this system B is in thermal equilibrium with another system C, then, system A will be in thermal equilibrium with system C”.  It may occur to you that these observations are obvious; it is true and this is exactly the reason that this law was stated late. It is like when you prove a mathematical equation and miss a step because it is obvious, but the proof won’t be complete without stating it; obvious as it is.

The first law of thermodynamics is simply another version or an extension of the law of conservation of energy. So, you can’t win because you can’t get something that was not there in the first place. In other words, energy remains constant. This lead us to the expression of the first law which states that “the internal energy of a given system is equal to the heat added to the system minus the work done by the system”.

This expression can be written as follows:

∆U=Q-W

U: Internal energy

Q: Heat

W: Work

If you see the definition with the words “work done on the system” then the expression will change to have a plus instead of a minus.

∆U=Q+W

The second law is the one that put everything in perspective, and add the randomness of the real world to the equation; it is the one that introduces the expression of efficiency and entropy. You can’t even break even, you can’t get what you put into the system; there is always a price or a loss. The second law says that heat can’t flow from a colder body to a hotter body without compensation. In other words, heat flows from the hotter body to the colder body spontaneously. This means that the system goes always in the direction which increases randomness or entropy.

Entropy is a measure of the randomness or messiness of a system. All natural systems tend to go into the direction of increasing the entropy. The same applies to the relationships between different energy forms. If work could easily be transformed into heat which is a messier form of energy, but to transform heat into work, a greater part of the energy will be lost to the ambient environment and the energy will leave its source to be lost in a form that is unusable i.e. efficiency will decrease. Imagine a machine operates on a form of fuel that burns converting chemical energy to thermal energy then transform it to work to operate the machine, not in the same efficiency because a great part is lost in the form of heat to the ambient air, to reverse this process, more energy will be lost to the ambient air.

The second law of thermodynamics states that the total entropy can never decrease over time for an isolated system, that is, a system in which neither energy nor matter can enter nor leave.

The second law is the most important of all four. It is the most applicable in industrial uses e.g. car engines, air conditioners and refrigerators. It also gives the most disappointments because it declares that we can’t have all the energy we spent in the form we want, for example we can’t operate a machine without a continuous supply of fuel.

Spontaneity of reactions

The balance of entropy and internal energy of a system decides whether a reaction (or process) will be spontaneous or not. The resultant of these two terms is “Free Energy” whose overall change indicates the route of reaction.

∆G = ∆H - T∆S

∆G = change in free energy

∆H = change in enthalpy

∆S = change in entropy

If ∆G = -ve, the reaction will proceed, even if ∆H = +ve (though it may be very slow if the reactants have kinetic stability).

If ∆G = +ve, the reaction will not proceed, even if ∆H = -ve.

You can’t get out of the game; the third law. Unlike the other laws, the third law is interpreted on the molecular level. It states that all processes in a system will cease to exist at a temperature of absolute zero, or alternatively no system can reach absolute zero. The molecules will stop moving at absolute zero, there will be no energy; this is why no system can reach absolute zero.

 

Key Points

  1. There are four laws of thermodynamics; the first, the second, the third, and the zeroth.
  2. The 1st law is stated as “the internal energy of a given system is equal to the heat added to the system minus the work done by the system”. ∆U=Q-W
  3. Entropy is a measure of the randomness or messiness of a system.
  4. The second law of thermodynamics states that the total entropy can never decrease over time for an isolated system, that is, a system in which neither energy nor matter can enter nor leave.
  5. The "Free Energy" is the difference between the internal energy & entropy terms which determines the spontaneity of a reaction. ∆G = ∆H - T∆S
  6. The third law states that all processes in a system will cease to exist at a temperature of absolute zero, or alternatively no system can reach absolute zero. The molecules will stop moving at absolute zero, there will be no energy; this is why no system can reach absolute zero.

 

References

  1. https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Thermodynamics
    /Laws_of_Thermodynamics/First_Law_of_Thermodynamics/First_Law_of_Thermodynamics
  2. https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Thermodynamics/
    Laws_of_Thermodynamics/Second_Law_of_Thermodynamics
  3. https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Thermodynamics/
    Laws_of_Thermodynamics/Third_Law_of_Thermodynamics