Entropy: Physical and Chemical Changes in GOB Chemistry

Entropy in Chemistry

Definition and Importance

Entropy (S) is a measure of disorder or randomness in a system, its surroundings, and the universe. It is a key concept in thermodynamics and helps explain the direction of physical and chemical changes.

• The Second Law of Thermodynamics: States that the entropy of the universe is always increasing.

• All spontaneous reactions involve an increase in the entropy of the universe.

Key Points

• Entropy increases when systems become more disordered (e.g., solid to liquid to gas).

• Entropy decreases when systems become more ordered (e.g., gas to liquid to solid).

Physical and Chemical Changes Affecting Entropy

Physical Changes

Physical changes can increase or decrease entropy depending on the change in the state of matter.

• Entropy Increasing (ΔS > 0): Solid → Liquid → Gas

• Entropy Decreasing (ΔS < 0): Gas → Liquid → Solid

Chemical Changes

Chemical reactions can also affect entropy, depending on the number and type of particles involved.

Reaction Type	Entropy Change	Example
More moles of gas produced	Entropy increases (ΔS > 0)	CaCO3 (s) → CaO (s) + CO2 (g)
Fewer moles of gas produced	Entropy decreases (ΔS < 0)	N2 (g) + 3 H2 (g) → 2 NH3 (g)

Practice Questions and Examples

Understanding the Second Law of Thermodynamics

• Example: The second law of thermodynamics leads us to conclude that the disorder of the universe is increasing with the passage of time.

Identifying Entropy Changes

• Example: Which process shows a decrease in entropy?

  • a) Boiling water to form steam

  • b) Melting ice to form water

  • c) Mixing of two gases into one container

  • d) Freezing water to form ice (Correct: entropy decreases as water becomes more ordered)

  • e) Dissolution of solid KCl in water

Predicting Entropy Changes in Reactions

• Practice: Predict how the entropy of the substance is affected in the following process:

  • CH4 (g, 125°C) → CH4 (g, 200°C)

    • Answer: Increases (higher temperature increases molecular motion and disorder)

• Practice: Which reaction is most likely to have a positive ΔS for the system?

  • a) SiO2 (s) + 3 C (s) → SiC (s) + 2 CO (g)

  • b) COCl2 (g) + H2 (g) → CH2 (g) + 2 Cl2 (g)

  • c) CO (g) + Cl2 (g) → COCl2 (g)

  • d) H2O (l) → H2O (g) (Correct: liquid to gas increases entropy)

  • e) 3 NO2 (g) → 2 HNO3 (l) + NO (g)

• Practice: Which of the following processes shows a decrease in entropy of the system?

  • a) NaCl (s) → Na+ (aq) + Cl- (aq)

  • b) H2O (l) → H2O (g)

  • c) 2 NO (g) → 2 NO2 (g)

  • d) H2O (g) → H2O (l) (Correct: gas to liquid decreases entropy)

  • e) COCl2 (g) → CO (g) + Cl2 (g)

Summary Table: Entropy Changes in Physical and Chemical Processes

Process	Entropy Change	Reason
Melting, boiling, sublimation	Increase (ΔS > 0)	More disorder, particles move more freely
Freezing, condensation, deposition	Decrease (ΔS < 0)	Less disorder, particles become more ordered
Mixing substances	Increase (ΔS > 0)	Greater randomness
Separating substances	Decrease (ΔS < 0)	Less randomness

Key Equations

• Change in Entropy: $\Delta S = S_{\text{final}} - S_{\text{initial}}$

• Second Law of Thermodynamics: $\Delta S_{\text{universe}} > 0$

Additional info:

• Entropy is a central concept in predicting the spontaneity of chemical reactions and physical changes.

• In general, processes that increase the number of particles or the freedom of movement (such as phase changes to gas or mixing) increase entropy.