Entropy Change

Entropy is a state function that is incorrectly referred to as a system’s state of disorder.’ Entropy is a qualitative measure of how much the energy of atoms and molecules spreads out in a process, and it can be expressed in terms of a system’s statistical probability or other thermodynamic characteristics. The Second and Third laws of thermodynamics, which explain changes in entropy of the universe with regard to the system and surroundings, and entropy of substances, respectively, are both based on entropy.

Examples of Simple Entropy Changes

Several examples are provided to show how the statistical definition of entropy and the second law might be used. Gas expansions, dilution, colligative characteristics, and osmosis are all examples of phase change.

Changes in Entropy in Chemical Reactions

Entropy Change can be described as a shift in a thermodynamic system’s state of disorder caused by the conversion of heat or enthalpy into work. In a system with a high degree of disorderliness, entropy is higher.We’ve seen how the energy emitted (or absorbed) by a reaction, as measured by the change in ambient temperature, can be used to calculate the reaction’s enthalpy (e.g. by using a calorimeter). Regrettably, there is no convenient technique to test the change in entropy of a reaction empirically. Consider the case where we know energy is flowing into (or out of) a system but there is no change in temperature. What exactly is going on in this situation? Internal energy changes that aren’t followed by a temperature shift could indicate a change in the system’s entropy.

Consider water at 0°C and 1 atm pressure, for example.

• This is the temperature and pressure at which water’s liquid and solid phases are in balance (also known as the melting point of ice)

H2O(s)→H2O(l)

• At such temperatures and pressures, we have ice and liquid water (by definition).

• If a modest amount of energy is added to the system, the equilibrium shifts to the right significantly (i.e. in favour of the liquid state)

• Similarly, removing a small quantity of energy from the system causes the equilibrium to shift to the left (more ice)

Since the heat capacity is a quantifiable word that connects the amount of heat energy input vs. the rise in temperature, it would seem that knowing the heat capacity (and how it changes with temperature) may help us determine the entropy change in a system. In fact, measurements for a substance’s “standard molar entropy” are expressed in J/mol K, the same units as molar heat capacity.

State variables and state functions

Many thermodynamic quantities are determined by state variables, which are physical variables that define a condition of thermodynamic equilibrium. The equilibrium condition determines state variables, not the path evolution to that state. In the sense that one state variable is a mathematical function of other state variables, state variables can be functions of state, also known as state functions. When some attributes of a system are identified, they are frequently enough to determine the system’s state and consequently the values of other properties. The ideal gas law, for example, determines the state and consequently the volume of a given quantity of gas based on its temperature and pressure. A system consists of a pure substance of a single phase at a specified uniform temperature and pressure is determined, and is thus a particular state with a specific volume and entropy. Entropy is useful since it is a function of state. Because the working fluid in the Carnot cycle returns to its original condition at the end of the cycle, the change or line integral of any state function, such as entropy, across this reversible cycle is zero.

Formula for Entropy:

1. If the operation is taking place at a constant temperature, the entropy will also be constant.

ΔS system =qrevT ,where

ΔS = it is the change in entropy

qrevT= it is the reverse of heat

T = it is the temperature in Kelvin

2. Moreover, if the reaction of the process is known then we can find ∆Srxn by using a table of standard entropy values.

∆Srxn = Σ∆Sproducts–Σ∆Sreactants

∆Srxn  – refers to the standard entropy values

Σ∆Sproducts = refers to the sum of the ∆Sproducts

Σ∆Sreactants– refers to the sum of the ∆Sreactants

3. Is the Gibbs free energy (ΔG) and the enthalpy (ΔH) can also be used to find ΔS

ΔG = ΔH – TΔS

Conclusion: 

Because work is derived from organised molecular motion, entropy is also a measure of a system’s molecular disorder, or unpredictability. The concept of entropy sheds light on the direction of spontaneous change in a variety of contexts.