**Entropy**

➾The random dispersal of molecules and of energy quanta between molecules.The greater the disorder the greater the entropy.

#### The entropy change of the system can be calculated from the formula:

**∆S**_{system}** = Σ∆S**_{ (Products)}** - Σ∆S**_{(Reactants)}

the units of ΔS in the formula is in JK^{-1}mol^{-1}

🤥remember ENTHAPLY in AS/A-Level Chemistry? Enthalpy isn't the only thing that affect the reaction!

😲Suppose we know that energy is going into a system (or coming out of it), and yet we do not observe any change in temperature. What is going on in such a situation? Changes in internal energy, that are not accompanied by a temperature change, might reflect changes in the entropy of the system.

The second law of thermodynamics states that __spontaneous changes result in an increase in entropy__. Entropy increases when:

- Solids melt
- Liquids boil
- Solids dissolve in water
- The number of gasmolecules increases
- The temperature increase

💡The entropy of a substance has an absolute value of 0 entropy at 0 K.

- Particles are not moving ->stationary
- No disorder, maximum order
- when temperature is increased, particles start moving, vibrating ->More disorder

💡Unlike *enthalpies of formation*, standard molar entropies of __elements__ are not 0.

💡When comparing standard molar entropies for a substance that is either a solid, liquid or gas at 298 K and 1 atm pressure, the gas will have more entropy than the liquid, and the liquid will have more entropy than the solid

The standard entropy of a substance depends on:

- State – As the

temperature increases, and the substance changes state, it becomes more disordered and entropy increase.

- Complexity – As the

complexity of a substance increases, entropy increases. E.g. the entropy of ethane is greater than that of methane.

The total entropy change for reaction and can be expressed by the equation:

**∆S**_{total}** = ∆S**_{system }**+ ∆S**_{surr}

- If a system becomes more disorder ∆S becomes positive.
- ∆S

_{surr} is positive for all exothermic reactions.

- ∆S

_{surr }is negative for all endothermic reactions.

- When an exothermic reaction takes place, heat energy is transferred to the surrounding air, causing an increase in the disorder of the air molecules. As the temperature increases, the molecules have a greater range of energy, so they are more disordered.

The entropy change in the surroundings, caused by transfer of heat, depends on the value of the heat change, i.e. if the surroundings are hot, the entropy increase is smaller than if its cold because the molecules have high entropy and are already in chaotic motion. Therefore ∆S_{surr }is also inversely proportional to the temperature of the surroundings:

**∆S**_{surr}**= - ∆H/T**

Changes are thermodynamically feasible if the total entropy change is positive:

If ∆S_{total} is positive, the change is said to be thermodynamically spontaneous, sothe change will occur, providing the kinetics are favourable. If ∆S_{total} is negative, the reverse reaction is thermodynamically spontaneous. The value of ∆S_{total} can be altered by changing the temperature.

Drafted by Eunice (Chemistry)

References:

https://digital-health.blog/2020/04/06/lesson-learned-from-the-coronavirus-entropy-is-a-problem-not-the-solution/

https://byjus.com/physics/differences-between-enthalpy-and-entropy/