What happens to entropy if a crystalline substance such as 𝑁𝑎𝐶𝑙 dissolves?
1. Increases
2. Decreases
3. Remains the same
4. First decreases and then Increases

Introduction to Entropy and Dissolution

Entropy is a fundamental concept in thermodynamics that measures the degree of disorder or randomness in a system. When a crystalline substance such as NaCl (sodium chloride) dissolves in water, the entropy of the system typically increases due to the breaking of the highly ordered crystalline structure into free ions.

Understanding how entropy changes during the dissolution process is essential for explaining why some substances dissolve spontaneously while others do not. In this article, we will explore the relationship between entropy and dissolution, focusing on the behavior of sodium chloride (NaCl).

Key Phrase: Entropy change during NaCl dissolution

Question and Answer

Question:
What happens to entropy if a crystalline substance such as NaCl dissolves?

1. Increases
2. Decreases
3. Remains the same
4. First decreases and then increases

Correct Answer: ✔️ Option 1 – Increases

Explanation of the Correct Answer

What is Entropy?

Entropy (S) is a measure of the degree of randomness or disorder in a system. According to the Second Law of Thermodynamics, the total entropy of an isolated system always increases over time or remains constant in a reversible process.

In the context of dissolution, entropy describes the extent to which the ions or molecules of a solute become more randomly distributed within the solvent.

How NaCl Dissolves in Water

Sodium chloride (NaCl) is an ionic compound composed of positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl⁻) arranged in a highly ordered cubic crystal lattice.

1. Breaking of Ionic Lattice:

   • When NaCl is added to water, the polar water molecules surround the Na⁺ and Cl⁻ ions.
   • The hydrogen side of water molecules is attracted to the Cl⁻ ions, while the oxygen side is attracted to the Na⁺ ions.
   • The ionic bonds holding the NaCl crystal lattice together are broken, and the individual Na⁺ and Cl⁻ ions are released into the solution.

2. Hydration of Ions:

   • Once the ions are released, they are stabilized by hydration (solvation) as water molecules form a hydration shell around them.
   • This hydration process releases energy (exothermic), helping to offset the energy required to break the ionic bonds.

3. Increase in Disorder:

   • In the crystalline state, Na⁺ and Cl⁻ ions are highly ordered.
   • When dissolved, the ions are free to move and mix with water molecules, increasing the randomness of the system.
   • This increase in randomness corresponds to an increase in entropy.

🔬 Thermodynamic Equation for Entropy Change

The total Gibbs free energy change (ΔG) for the dissolution process is given by:

                                               ΔG=ΔH−TΔS

where:

• ΔG = Gibbs free energy change
• ΔH = Enthalpy change (heat absorbed or released)
• T = Temperature in Kelvin
• ΔS = Entropy change

 If ΔS is positive (increase in disorder), the process is more likely to be thermodynamically favorable.
 Even if ΔH is slightly positive (endothermic), the dissolution can still proceed if the increase in entropy is large enough to make ΔG negative.

Why Entropy Increases During NaCl Dissolution

1. Loss of Crystal Structure:

   • In solid NaCl, ions are held in a rigid, ordered lattice.
   • Upon dissolution, this structured arrangement is destroyed, leading to increased randomness.

2. Increased Freedom of Movement:

   • Na⁺ and Cl⁻ ions become mobile in the solution.
   • Water molecules can freely interact with the ions, further increasing the degree of randomness.

3. Hydration Shell Formation:

   • The arrangement of water molecules around the ions increases local order.
   • However, the increase in overall disorder from lattice breakdown outweighs the local order created by hydration.

Why Does NaCl Dissolve Spontaneously Despite a Positive Enthalpy Change?

• The dissolution of NaCl is slightly endothermic (ΔH > 0).
• However, the increase in entropy (ΔS > 0) makes the overall ΔG negative, driving the dissolution process.

                                  ΔG=(+ΔH)−T(+ΔS)             

 Since TΔS > ΔH, the process is thermodynamically favorable, and NaCl dissolves spontaneously.

Side Topics Related to Entropy and Dissolution

1. Factors Affecting Entropy in Dissolution

Temperature: Higher temperature increases molecular motion and entropy.
Solvent Polarity: Water is highly polar, which enhances ion solubility and entropy.
Nature of Solute: Ionic compounds like NaCl tend to have higher entropy upon dissolution compared to covalent compounds.

2. Biological Importance of Entropy in Dissolution

• Ion Transport: Sodium and chloride ions play a vital role in nerve transmission and muscle contraction.
• Osmoregulation: Dissolved ions maintain osmotic balance in cells.
• Protein Folding: Entropy drives the formation of stable protein conformations in aqueous environments.

🏆 3. Examples of Entropy Increase in Nature

• Melting of Ice: Solid ice has a structured lattice. When melted, water molecules become more disordered.
• Evaporation: Liquid water converts to vapor, increasing molecular randomness.
• Protein Denaturation: Loss of protein structure leads to increased randomness of amino acid chains.

Summary of Key Points

 Entropy measures the degree of disorder in a system.
 Dissolution of NaCl increases entropy due to the breakdown of the crystal lattice.
 The increase in entropy compensates for the small endothermic enthalpy, making the process spontaneous.
 Biological systems rely on entropy-driven processes for ion transport, enzyme activity, and protein folding.