Laws of Thermodynamics: First Law, Second Law, Third Law:

What are Laws of Thermodynamics

Laws of thermodynamic help us understand why energy flows in certain directions and in certain ways. A lot of the concepts described by thermodynamics seems like common sense but the layer of math beneath the intuitive level that makes them very powerful to describe systems and making predictions.

Laws of thermodynamics:

First Law of Thermodynamics:

The first law of thermodynamics is described in most basics way highlighting conservation of energy. Energy is not created or destroyed it only changes forms from potential energy to kinetic energy to heat energy and so on. On the quantum level it is found to be untrue and for chemistry it does prove and just fine. However there seems to be a preferred direction in which energy flows from one form to another in order to understand why we look at second law of thermodynamics.

The first law of thermodynamics is an extension of the law of conservation of energy. The change in the internal energy of a system is equal to the head added to the system minus the work done by the system.

First law of Thermodynamics Equation:

First law of thermodynamics equation is typically of chemistry texts and it is written as follows: ΔU=Q+W

Second Law of Thermodynamics :

Second law of thermodynamics states that the circumstances(state) of the entropy of the entire universe as an inaccessible system will increase and also states that the change in entropy in the universe will never be negative. Second Law of thermodynamics is very important as it deals with entropy.

Second law of thermodynamics introduces a new concept called entropy(disorder) and entropy is quite difficult to understand but we can most easily describe entropy as disorder. The 2nd Law of thermodynamics states that the sum of the entropies of a system and its surroundings must always increase. In other words the entropy of the universe is always increasing within a system there is also a tendency to go towards higher entropy. The classic analogy is that your bedroom will over time becomes messy but it won’t suddenly become neat.

Another way to look at the entropy is a measure of how dispersal the energy of the system is amongst the ways that system can contain energy. Yet another way is to analogize entropic states to computer code. Let’s take an example of an ionic solid compared to the same substance as a liquid. Clearly the solid state is more ordered and the liquid state is more disordered or higher in entropy. To describe the solid state using computer code you need to include terms that describe the geometry of the lattice. The intermolecular distance, precise configuration of every molecule and many other things, but to describe the liquid state you need to simply describe the volume of liquid and the shape of the vessel because the configuration and motion of the molecules are random, that’s far less information that needs to encoding which is a way of rationalization why increasing the entropy of a system is a way of rationalizing why the entropy of a system is thermodynamically favorable. We can look at all kind of processes to highlight entropic influences. Heat flow from a hot coffee cup into the table or your hand because the heat energy will be disordered, if more dispersed. This is why heat spontaneously flows from hot to cold and not the other way around.

Third Law of Thermodynamics :

Third law of thermodynamics apprehensive with the limiting or preventive actions of systems as the temperature approaches absolute zero and it also states that the entropy of a perfect crystal is zero when the temperature of crystal is absolute zero (0 k) in third law of thermodynamics.

The third law of thermodynamics states that a perfectly crystalline solid state at absolute zero has entropy of zero as this the most ordered state the substances can be. Entropy is measured in joules per Kelvin and note that entropy is not a measure of energy itself but how the energy is distributed with in a system. It is Enthalpy the thermodynamic quantity that is more accurately described the energy of a system. We see Entropy and enthalpy intricately relate tell us something about Gibbs free energy of a system and applications of third law of thermodynamics.

Gibbs free energy:

G or Gibbs free energy tells us whether a process will be spontaneous or not, meaning if it will simply happen on its own. Change in Gibbs free energy is given by this equation which includes change in enthalpy, change in entropy and temperature.


If Delta G is negative the process is spontaneous, if positive it is non-spontaneous. So we can use this equation to see how spontaneous process can be either enthalapcally or entropically favored or both but not neither for example if delta H is negative which means exothermic and energetically favorable and Delta S is positive which means an increase in entropy which is also favorable, a negative minus a positive will always be negative or spontaneous. If the opposite is true and both are unfavorable we have a positive minus a negative which will always be positive or non-spontaneous. If only one of the two is favorable we have to do some math. If Delta H is positive or endothermic that energetic unfavorable could be outweighed by the other term if the process is entropically favorable and since T is here this factor will increase with a larger T, so entropically favorable processes are more likely to be spontaneous at higher temperature. Conversely if its energetically favorable but entropically unfavorable the entropic unfavourability will be minimized at lower temperatures. This is very important equation to understand because it describes all of the spontaneous processes in the universe.

The entropically unfavorable processes can be spontaneous at lower temperature if they are energetically favorable. An example of this is soap. We need soap to wash non polar dirt from hands because they are immiscible with polar water molecules, but soap molecules have polar heads and long non-polar tails which allows them to spontaneously form structure called micelles. These are spheres where the soap molecules themselves with the polar heads.

Ramana Tula is a Google Product Expert - He is a Full stack Web and Android Developer.

- Advertisement -