JEE mains tips and tricks for Second law of thermodynamics
In this post I will write a little intro about JEE mains tips and tricks for Second law of thermodynamics.
Second law of thermodynamics
As with other laws of thermodynamics, the second principle is empirical, we come to it through experimentation. Thermodynamics does not bother to show why things are like that, and not in any other way. The second law of thermodynamics is expressed in several equivalent formulations:
Kelvin statement – Planck
A process that converts all the heat absorbed into work is not possible.
No process is possible whose only result is the extraction of heat from a cold body to a hot one.
Note that this second law does not say that it is not possible to extract heat from a cold source to a hot one. It simply says that this process will never be spontaneous.
JEE mains tips and tricks : Second Law Of Thermodynamics
The second principle of thermodynamics is not limited exclusively to thermal machines but deals, in general, with all natural processes that occur spontaneously. We can say that it deals with the natural evolution of thermodynamic systems, that is, the direction in which they advance. This direction is associated with the internal molecular distribution of the molecules.
To study the spontaneity of processes, the Austrian Ludwig Edward Boltzmann introduced a new magnitude called entropy.
The entropy S is a state variable. It is associated with the probability that a certain state will occur in a system. Those most likely have a greater entropy.
A more exhaustive study of entropy requires mathematical tools that are outside the scope of this educational level, however it is important that you know what relation entropy has with the second law of thermodynamics.
Any spontaneous natural process evolves towards an increase in entropy.
JEE mains tips and tricks : Inverse Function
Let’s see some concrete examples to better understand this concept:
- If you take a bunch of pencils and throw them in the air, when they fall it is unlikely that they fall in line. Most likely they fall into complete disorder
- If you sugar the water, the particles are distributed randomly throughout the solution in a spontaneous way, and not in a single direction.
- In a gas that expands freely, the pressure in the walls of the enclosure in which it is located is the same at any point. The reason is that gas particles have expanded in all directions equally and not in a particular one. We see, then, that an increase in disorder is the natural direction in which natural processes evolve.
From the first and second laws of thermodynamics we can say that in every natural transformation the energy of the universe is conserved and its entropy increases. So:
ΔUuniverse = 0; ΔSuniverse = 0
This increase in entropy is associated with an increase in the thermal energy of the systems. Thermal energy is the most degraded form of energy, since, as we have pointed out, it is not possible to take full advantage of producing work. This phenomenon has come to be called an entropic crisis since it leads to the universe, with the passage of millions of years, to a thermal death: all forms of energy will end up becoming heat.
JEE mains tips and tricks : Self Confidence
Gibbs free energy
The Gibbs free energy (G) of a system is a measure of the amount of usable energy (energy that can perform a job) in that system. The change in Gibbs free energy during a reaction provides useful information about the energy and spontaneity of the reaction (if it can be carried out without adding energy). In other words, ΔG is the change in free energy of a system that goes from an initial state, such as reactants, to a final state, like all products. This value tells us the maximum usable energy released (or absorbed) by initial deletion to the final state. In addition, its sign (positive or negative) tells us whether a reaction will occur spontaneously, that is, without additional energy. Enthalpy and entropy seem to go hand in hand when predicting the spontaneity of processes and reactions. If this is so, will not there be some quantity that relates them? The answer is yes. The free energy of Gibbs, or simply the free energy, symbolized as G, is the amount that is defined to consider the relative contributions of entropy and enthalpy, in order to predict spontaneity under certain conditions.
Criteria of spontaneity
So, after all this talk, we should be able to easily define and calculate the spontaneity of every reaction that comes across our way. That is, if we had a formula to do it.
JEE mains tips and tricks : Buoyancy & Archimedes
For a process at constant temperature and pressure, the change in Gibbs free energy, ΔG, is:
ΔG = ΔH – TΔS
The sign of ΔG depends on the signs of the changes in enthalpy (ΔH) and entropy (ΔS), as well as the absolute temperature (T, in Kelvin). ΔG changes from positive to negative (or vice versa) in the value where T = ΔH / ΔS.
Let’s see a list of all possible scenarios for this equation, regarding the spontaneity of the reaction that we are studying, taking specific care in considering the quantitative value of each term in the equation relative to zero:
- When ΔG is negative, a chemical process or reaction occurs spontaneously in the given direction.
- When ΔG is positive, the chemical process or reaction occurs spontaneously in the reverse direction as given.
- When ΔG is zero, the process is in equilibrium, without a net change taking place over time.
- We can also distinguish four cases with the rule described above, by examining the signs of the two terms on the right side of the equation.
- When ΔS is positive and ΔH is negative, a process is spontaneous.
- When ΔS is positive and ΔH is positive, a process is spontaneous at high temperatures, where the endothermic character of the process (indicated by ΔH) plays a small role in the final balance.
- When ΔS is negative and ΔH is negative, a process is spontaneous at low temperatures, where the exothermic nature of the process is now important.
- When ΔS is negative and ΔH is positive, a process is not spontaneous at any temperature, but the reverse process is spontaneous.
If you want to get a deeper understanding you can read here.
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