State function entropy

1.
Chaos and the number of micro-states

Study the endothermic reaction that cannot proceed spontaneously at room temperature but can proceed spontaneously at high temperature

Look at the exothermic reaction that occurs spontaneously at room temperature and is more likely to occur spontaneously at high temperature.

To summarize this kind of reaction, it is not difficult to find that heating is conducive to the generation of gaseous substances from solid or liquid substances, from less gaseous substances to more gaseous substances
.

In other words, heating is beneficial to the direction in which the movement range of the product molecules is large or the number of molecules with a large movement range increases
.

The larger the range of activities of molecules or the increase in the number of molecules with a large range of activities is that the degree of chaos in the system becomes greater

The number of microscopic states (Ω) is often used to quantitatively describe the disorder of the system
.

It is not difficult to understand that with the same range of activity (volume), the more particles in the system, the more the number of microscopic states and the more chaotic the system; the same number of particles, the larger the range (volume) of particles, the more microscopic states and the more chaotic the system

2.

The concept of disease

When the state of the system is constant, the number of microscopic states has a definite value
.

On the macro level, a physical quantity can be found to be related to the number of microscopic states to express the chaos of the system

If the number of microscopic states of the system is Ω, then the entropy S is

S=klnΩ

In the formula, k=1.
38×108 ^-23 J·K ^-1 , which is called Boltzmann constant; the unit of entropy is J·K ^-1
.

Entropy is an additive state function
.

In fact, it is generally not necessary to calculate the entropy change △S of a process by using the relationship between S and the number of microscopic states of the system Ω.
The reason is that the number of microscopic states of the system is difficult to accurately calculate
.

The entropy change △S of the constant temperature reversible process can be calculated by the above formula, if it is a non-constant temperature reversible process, the entropy change △S can be calculated by the integral method
.

At the temperature of the phase transition point, the phase transition of matter can be regarded as a reversible pathway

Increasing chaos in the system is a tendency for chemical reactions to proceed spontaneously
.

Therefore, chemical reactions tend to increase in entropy, that is

△ _r S>0

Like the melting change of the reaction, the entropy change is less affected by the temperature change.

The entropy change within a certain temperature range can be replaced by the entropy change data at 298K

3.
The third law of thermodynamics

At 0K, the pick value of the complete crystal is 0
.
This is the third law of thermodynamics
.

There is only one arrangement of the particles in a complete crystal, that is, Ω=1, so S=0
.

Starting from the state where the entropy value is 0, the system is changed to the final state (standard atmospheric pressure and temperature T).
The value of the entropy change in this process is the absolute entropy value of the system in the final state of the process
.
The entropy value of 1 mol substance in the standard state is called standard molar entropy, referred to as standard entropy, represented by the symbol Sm ^Θ , and its unit is J·mol ^-l ·K ^-1
.

The standard entropy Sm ^Θ is an absolute value, and the standard heat generation ΔHR is a relative value
.

4.
Qualitative judgment of the sign of entropy change

The process of increasing entropy is a process in which the degree of chaos (liquidity) changes from small to large
.
Therefore, the reaction of solids to liquids, solids or liquids to gas, and fewer gas molecules to generate more gas molecules generally increases entropy, while the reverse process decreases entropy
.

E.
g

The qualitative judgment of the sign of entropy change is very meaningful
.
For example, ZnSO ₄ is stable at room temperature, indicating that its decomposition reaction is endothermic; but its decomposition product contains gaseous substance SO ₃ , the decomposition reaction is a process of entropy increase (△ _r S>0), heating is conducive to the increase in entropy Therefore, the decomposition reaction of ZnSO ₄ will definitely occur at high temperature
.

Related link: The direction of the chemical reaction