Can Entropy Ever Decrease?

Entropy is a thermodynamic property that explains the state of a system. A thermodynamic system is a part of the universe under observation, and the rest of the universe is the surroundings. The entropy of a system cannot be measured directly but we can find the difference in entropy of the initial and final states of a system. 

Entropy is an extensive property that depends on the mass or quantity of matter present in a system. It is a state function similar to Internal energy or Free energy. A state function is a physical quantity that can be measured by knowing the initial and final values of the system, irrespective of the path followed by it to reach the final state. Entropy is the measure of the randomness of the particles in a system. 

Often the term entropy is confused with energy. Though they are related, they are different terms. “S” of a system is the measure of thermal energy of the system per unit temperature which is not used (unavailable) to do work. This is because work can be done only by the ordered motion of particles whereas entropy is the measure of disordered motion of particles. So, to find the entropy of a system, only the difference in entropy between the initial and final states can be calculated. The symbol used for Entropy is S, and the change in Entropy is given as

Sfinal – S initial = ∆S ( ∆ indicates the finite change in the property)

What is Entropy?

Entropy is the calculation of the randomness or the disorder present in the molecules or particles of the thermodynamic system. Entropy mainly aims at predicting the direction of the chemical reaction. If the system is more ordered, which means that the particles are arranged properly, predicting the movement or behaviour of particles is a little easy. On the other hand, if the system is less ordered, the prediction of the particle behaviour is difficult, and hence the entropy is high. Entropy can vary with various factors like physical state, attractive forces, temperature, pressure, volume, molar mass, number of moles. Entropy can be variable (high, low or zero), based on the conditions or the thermodynamic system like temperature, pressure, volume etc. 

Factor Affecting Entropy

1. Physical state: There are mainly three states of matter – solids, liquids, and gases. Characteristics of solids, liquids, and gases are different. Solids have a proper arrangement of particles and have fixed shape and volume, and do not need a container to store them. Liquids have no fixed shape but have a fixed volume. The arrangement of particles is a little less ordered in liquids and it takes the shape of the container it is stored in. Gases have no fixed shape or fixed volume, and their particles are far from each other. They need a sealed container to store them. Thus, solids have the least entropy, liquids have moderate entropy, and gases have the maximum entropy. 
2. Attractive forces: The attractive intermolecular forces also affect entropy. Solids have fewer spaces between the particles, so the attractive forces are high. Particles are placed in a fixed position and are more ordered. This orderliness decreases from solids to liquids to gases. So, entropy decreases with an increase in attractive forces.
3. Temperature: With the increase in temperature, entropy increases, and with a decrease in temperature, entropy decreases. As temperature increases, particles gain more energy and move with great speed and the randomness increases. As temperature decreases, particles have less energy, and therefore, move with less speed and are more ordered.
4. Pressure: With the increase in pressure, particles come closer to each other, are arranged in a regular pattern, and the system is more ordered. In contrast, with the decrease in pressure, particles tend to move away from each other and are less ordered or more random. So, entropy increases with a decrease in pressure and decreases with an increase in pressure.
5. Volume: Since pressure and volume are inversely proportional to each other at a given temperature, the entropy relationship also holds inversely. When the volume increases, particles move far from each other and are more random. When the volume decreases, particles come closer and are less random. So, entropy increases with an increase in volume and decreases with a decrease in volume.
6. Molar mass: Entropy is dependent directly on molar mass. As the size of the particles increases, molar mass increases, and so does the entropy of the system.
7. The number of moles: Similar to molar mass, entropy increases with an increase in the number of moles of molecules. Therefore, entropy is directly proportional to the number of moles in a system.

Conclusion

The entropy of a system can either increase or decrease with the change in the physical quantities that define a thermodynamic system. An increase in the entropy means the system’s entropy is increased with the gain of energy by the particles. Based on the increase or decrease in temperature, pressure or volume, entropy can change for a thermodynamic system, but the change can only be in accordance with the initial state. It can either increase than the initial or decrease than the initial, but in an actual sense, entropy cannot decrease.

The entropy of the universe always remains constant. The entropy of the universe is always the sum of the entropy of the system and the entropy of the surroundings.