Not considering the phase change

Oct 13, 2025 by pynkocean.com 112 views

Entropy Change: Understanding the Sign of Entropy Change in Chemical Reactions

Entropy is a fundamental concept in chemistry that describes the disorder or randomness of a system. In the context of chemical reactions, entropy change is a crucial factor in determining the spontaneity of a reaction. In this article, we will explore the concept of entropy change and predict the sign of entropy change for two given examples.

What is Entropy Change?

Entropy change, denoted by ΔS, is a measure of the change in disorder or randomness of a system. It is an extensive property, meaning that it depends on the size of the system. Entropy change can be calculated using the following equation:

ΔS = ΔSsys + ΔSsurr

where ΔSsys is the change in entropy of the system and ΔSsurr is the change in entropy of the surroundings.

Predicting the Sign of Entropy Change

To predict the sign of entropy change, we need to consider the following factors:

Phase change: A change from solid to liquid or liquid to gas increases the entropy of the system.
Number of moles: An increase in the number of moles of a substance increases the entropy of the system.
Temperature: An increase in temperature increases the entropy of the system.
Order/disorder: A decrease in order or an increase in disorder increases the entropy of the system.

Example a: $AgCl (s) \rightarrow Ag^{+}(aq) + Cl^{-}(aq)$

In this example, we have a solid salt (AgCl) dissolving in water to form ions (Ag+ and Cl-). Let's analyze the factors that affect the entropy change:

Phase change: The solid salt dissolves in water, which is a liquid. This is a phase change from solid to liquid, which increases the entropy of the system.
Number of moles: The number of moles of the substance increases from 1 mole of solid salt to 2 moles of ions (1 mole of Ag+ and 1 mole of Cl-).
Temperature: The temperature is not explicitly mentioned, but we can assume that the temperature is constant.
Order/disorder: The ions (Ag+ and Cl-) are more disordered than the solid salt, which increases the entropy of the system.

Based on these factors, we can conclude that the entropy change for this reaction is positive (ΔS > 0). The dissolution of the solid salt in water increases the entropy of the system due to the phase change, increase in the number of moles, and increase in disorder.

Example b: One mole of $H_2O$ at $15^{\circ} C \rightarrow$ One mole of $H_2O$ at $25^{\circ} C$

In this example, we have one mole of water at a temperature of $15^{\circ} C$ changing to one mole of water at a temperature of $25^{\circ} C$ . Let's analyze the factors that affect the entropy change:

Phase change: There is no phase change in this example.
Number of moles: The number of moles of the substance remains constant at 1 mole.
Temperature: The temperature increases from $15^{\circ} C$ to $25^{\circ} C$ , which increases the entropy of the system.
Order/disorder: The temperature increase does not affect the order or disorder of the system.

Based on these factors, we can conclude that the entropy change for this reaction is positive (ΔS > 0). The increase in temperature increases the entropy of the system.

Conclusion

In conclusion, entropy change is a crucial factor in determining the spontaneity of a reaction. By analyzing the factors that affect the entropy change, we can predict the sign of entropy change for a given reaction. In the two examples discussed in this article, we found that the entropy change is positive for both reactions. The dissolution of the solid salt in water increases the entropy of the system due to the phase change, increase in the number of moles, and increase in disorder. The increase in temperature also increases the entropy of the system.

References

Atkins, P. W., & de Paula, J. (2010). Physical chemistry. Oxford University Press.
Chang, R. (2010). Chemistry. McGraw-Hill.
Levine, I. N. (2012). Physical chemistry. McGraw-Hill.

Further Reading

Entropy change: A comprehensive review of the concept and its applications in chemistry.
Spontaneity of reactions: A discussion of the factors that affect the spontaneity of a reaction.
Thermodynamics: A comprehensive review of the laws of thermodynamics and their applications in chemistry.
Entropy Change: Frequently Asked Questions

In this article, we will address some of the most frequently asked questions about entropy change.

Q: What is entropy change?

A: Entropy change, denoted by ΔS, is a measure of the change in disorder or randomness of a system. It is an extensive property, meaning that it depends on the size of the system.

Q: How is entropy change calculated?

A: Entropy change can be calculated using the following equation:

ΔS = ΔSsys + ΔSsurr

where ΔSsys is the change in entropy of the system and ΔSsurr is the change in entropy of the surroundings.

Q: What are the factors that affect entropy change?

A: The factors that affect entropy change are:

Phase change: A change from solid to liquid or liquid to gas increases the entropy of the system.
Number of moles: An increase in the number of moles of a substance increases the entropy of the system.
Temperature: An increase in temperature increases the entropy of the system.
Order/disorder: A decrease in order or an increase in disorder increases the entropy of the system.

Q: How do I predict the sign of entropy change?

A: To predict the sign of entropy change, you need to consider the factors that affect entropy change. If the entropy of the system increases, the entropy change is positive (ΔS > 0). If the entropy of the system decreases, the entropy change is negative (ΔS < 0).

Q: What is the relationship between entropy change and spontaneity?

A: Entropy change is a crucial factor in determining the spontaneity of a reaction. A positive entropy change (ΔS > 0) indicates that the reaction is spontaneous, while a negative entropy change (ΔS < 0) indicates that the reaction is non-spontaneous.

Q: Can entropy change be negative?

A: Yes, entropy change can be negative. This occurs when the entropy of the system decreases, such as in a phase change from liquid to solid.

Q: What is the difference between entropy change and enthalpy change?

A: Entropy change (ΔS) and enthalpy change (ΔH) are two different thermodynamic properties. Entropy change measures the change in disorder or randomness of a system, while enthalpy change measures the change in energy of a system.

Q: How do I apply entropy change in real-world scenarios?

A: Entropy change has many practical applications in fields such as chemistry, physics, and engineering. For example, entropy change is used to predict the spontaneity of chemical reactions, to design more efficient engines, and to understand the behavior of complex systems.

Q: What are some common mistakes to avoid when calculating entropy change?

A: Some common mistakes to avoid when calculating entropy change include:

Not considering the phase change: Failing to consider the phase change of a substance can lead to incorrect calculations of entropy change.
Not accounting for the number of moles: Failing to account for the number of moles of a substance can lead to incorrect calculations of entropy change.
Not considering the temperature: Failing to consider the temperature of a system can lead to incorrect calculations of entropy change.

Conclusion

In conclusion, entropy change is a fundamental concept in thermodynamics that has many practical applications in fields such as chemistry, physics, and engineering. By understanding the factors that affect entropy change and how to calculate it, you can apply entropy change in real-world scenarios and make more informed decisions.

References

Atkins, P. W., & de Paula, J. (2010). Physical chemistry. Oxford University Press.
Chang, R. (2010). Chemistry. McGraw-Hill.
Levine, I. N. (2012). Physical chemistry. McGraw-Hill.

Further Reading

Entropy change: A comprehensive review of the concept and its applications in chemistry.
Spontaneity of reactions: A discussion of the factors that affect the spontaneity of a reaction.
Thermodynamics: A comprehensive review of the laws of thermodynamics and their applications in chemistry.