Entropy Change and Calculations

Entropy is described as a quantifiable measure of a system’s disorder or unpredictability. The idea stems from thermodynamics, which is concerned with the transport of heat energy inside a system. Physicists often describe the change in entropy that occurs in a given thermodynamic process, rather than any type of “absolute entropy.”

Where np and nr are the stoichiometric coefficients of the products and reactants in the balanced equation, respectively.

To give you an instance, ΔS°rxn will be the reaction at room temperature.

Entropy is related to spontaneity, which means that the more the spontaneity in a thermodynamic process, the greater its entropy or degree of disorder. In other words, entropy describes the percentage of energy that does not transform into work done and instead contributes to the system’s disorder. Because energy provides the potential to accomplish labour, it is nearly impossible for all of the energy to be utilised in doing work. Entropy is a measure of this.

Because energy cannot be generated or destroyed, but can only be changed from one form to another, it is impossible to symbolise entropy at a single location, and so it can only be measured as a change, and we will have to calculate the entropy change. 

Calculation of the entropy change in the environment 

So far, you’ve learned how to calculate the entropy change of a system for a particular reaction if you know the entropies of all the chemicals involved. The entropy change of the surroundings may be calculated using a simple equation.

ΔH is the enthalpy change for the reaction. T is the temperature.

The units of enthalpy change and entropy change are mismatched. When you give values for enthalpy change, the energy units will be kJ. However, the entropy change is measured in energy units of J.

That is if you want to calculate entropy change, multiply the enthalpy change amount by 1000.

So, if you have an enthalpy change of -92.2 kJ mol-1, you must enter -92200 J mol-1 into the calculation.

If the temperature was 298 degrees Celsius, then:

The present sign of negative in the given equation will transform the negative exothermic enthalpy that will enable it to change into a positive entropy change. Moreover, an exothermic change will end up warming the environment and increasing the entropy in the particular environment and area.

The Second Law of Thermodynamics and Entropy

The second law of thermodynamics may be stated as follows: In every closed system, the entropy will either remain constant or grow.

Increasing heat to a system causes the molecules and atoms to move faster. In a closed system, it may be conceivable (albeit difficult) to reverse the process without taking or releasing energy from someplace else in order to return to the beginning condition. You can never make the entire system “less energetic” than it was when it began. There is nowhere for the energy to go. The total entropy of the system and its surroundings constantly grows in irreversible processes.

Here are some examples:

Example 1: Ammonium nitrate dissolved in water

It is a basic example of an endothermic transition that occurs despite the fact that there is a hike in disorder during the time when the crystal will break, and it will divide itself into individual ions and will gel up with the water. 

The entropy change to the surroundings will be negative due to the cooling induced by the ammonium nitrate dissolving, but this will be more than offset by the significant rise in the system’s entropy. So the overall entropy change is positive, and the shift is conceivable – and in this case, truly spontaneous.

Example 2: The interaction of concentrated ethanoic acid and crystalline ammonium carbonate

2CH3COOH + (NH4)2CO3→ 2CH3 COONH4  + H2O + CO2

This is another endothermic transition that is possible because the increase in entropy due to the formation of gaseous carbon dioxide surpasses the decrease in entropy of the surroundings. As the overall entropy rises, the reaction becomes viable (and spontaneous).

Conclusion 

When we talk about entropy in thermodynamics, we focus on its behaviour rather than its other properties. It is linked to other thermodynamic variables such as pressure, temperature, and heat. All other elements are taken into account while determining the system’s equilibrium state. Entropy depicts the molecular motions that take place inside a system. As a result, it serves as a statistical indicator of molecular malfunction.