The Second Law of Thermodynamics

Thermodynamics is a branch of science that addresses relationships between heat and various energy forms under different conditions. Three different laws of thermodynamics explain the behaviour of heat energy, its conversion into another form, and the flow of energy between a system and its surroundings. The following article mainly focuses on the second law of thermodynamics, its statements and related facts.

What led to the introduction of the second law of thermodynamics?

Although the first law of thermodynamics introduced the concepts of enthalpy and energy transfer, it had certain limitations.

  1. The first law does not explain the direction of energy flow and its reason. For example, it states that the amount of heat lost by a system is equivalent to the heat gained by the surrounding; however, it does not explain whether the heat will always flow from the system to the surrounding or vice versa.
  2. The first thermodynamics law established the relationship between heat loss and work done, but it does not mention the source of heat energy and its flow.
  3. It states that the energy would be conserved in an isolated system when a change occurs; however, it does not specify the nature of the change.

Due to these limitations, the second law of thermodynamics was introduced.

Spontaneity and randomness

Randomness and spontaneity are essential for understanding the second law of thermodynamics.

Let us consider an example of two isolated systems wherein gases A and B behave like ideal gases at extremely low pressure. These systems are connected via a closed valve. Thus, in one system, all the gas molecules belong to A, whereas in the other system, the molecules belong to gas B, thereby depicting a perfect state of order.

Once the valve is opened, these gases will exchange their positions easily and mix with no energy change. The ease with which gas A and B combine is known as spontaneity. The number of molecules of each gas in both systems will lose their order after mixing. Thus randomness occurs when gases can easily mix at low pressure, creating a remarkable disorder.

Entropy and its introduction

It is essential to know entropy to understand the 2nd law of thermodynamics. The letter ‘S’ represents the extent of randomness in a system. Whether entropy is an object of the state depends on the values of the initial and final system states. We can write the expression for entropy as:

ΔS = Sf– Si 

where ΔS is the change in entropy, Sf is the entropy of the system’s final state, and Si is the system’s initial state.

What does the second law of thermodynamics state?

The second law of thermodynamics states that

In a reversible process, the entropy of the system and surroundings collectively remains constant, whereas the net entropy of the system and the surroundings increases in an irreversible reaction.

All reactions or changes occurring in nature are irreversible. Therefore, the ΔS of the universe cannot be zero. Thus, the second law of thermodynamics states that the universe’s total energy remains constant, but the entropy gradually increases.

Gibbs free energy equation

For describing the spontaneity of a system, the two most essential functions that need to be considered are:

  1. Helmholtz free energy function/Work function/A
  2. Gibbs free energy/thermodynamic potential/G

Based on these two functions, we can form the following relations.

A = U – TS ….. (i) 

G = H – TS …. (ii)

where,

U = internal energy (SI unit: joule)

T = temperature (SI unit: kelvin)

S = entropy (SI unit: joule/kelvin)

By combining these two equations and considering the first and second laws of thermodynamics, we can write Gibbs free energy equation as:

ΔG = ΔH – TΔS ….. (iii) 

or, -ΔG = Wrev – PΔV.…. (iv)

where,

P = pressure (SI unit: pascal)

V = volume (SI unit: m3)

-ΔG represents the network where G is the Gibbs free energy function in this equation. In an isochoric process, ΔG = ΔA provides constant volume. 

Here, ΔA represents the product of pressure and constant volume as expressed in equation (ii).

Classification of chemical reactions based on Gibb’s free energy

Gibb’s free energy is mostly considered for classifying the chemical reactions into three parts that have been discussed below.

  1. When ΔG > 0, Gibb’s free energy increases during the chemical reaction. Therefore, it is considered a non-spontaneous or non-favourable reaction.
  2. If ΔG = 0, it means that the entropy and enthalpy of the system have attained a state of thermal equilibrium. Therefore, the rate of heat absorption is equal to the rate of heat loss.
  3. When ΔG < 0, it means that the system’s total free energy is negative, which relates to the fact that a spontaneous reaction will have negative enthalpy.

Conclusion

After gaining knowledge on the second law of thermodynamics, one can describe spontaneity and randomness and use this theory for real-life examples. The second law holds utmost importance over the first and third law of thermodynamics as it defines the randomness and feasibility of a chemical or physical process in a system. Furthermore, it describes the flow of energy or its source.