Physical Equilibrium

Physical equilibrium refers to a state in which a system's physical properties, such as pressure, temperature, and volume, remain constant over time. In this state, the rates of opposing processes, such as evaporation and condensation or melting and freezing, are equal, resulting in no net change in the system's overall properties.

1.0Introduction

Physical equilibrium refers to a state in which a system has a balance between opposing physical processes. This balance typically occurs when the rates of forward and reverse processes are equal, resulting in no net change in the system's properties over time. Physical equilibrium can manifest in various systems, such as phase transitions (e.g., solid-liquid equilibrium, liquid-gas equilibrium), chemical reactions involving physical changes (e.g., dissolution, evaporation), and other physical phenomena (e.g., osmosis).

2.0Types of Physical Equilibrium

Different types of physical equilibrium demonstrate how substances can exist in multiple states and how the transitions between these states can reach a balanced condition under specific conditions.

In each type of physical equilibrium, the rates of the forward and reverse processes are equal, resulting in no net change in the system's macroscopic properties.

Phase Equilibrium

Solid-Liquid Equilibrium:

• It Occurs when a solid and its liquid are in equilibrium at a specific temperature and pressure. Example: Ice and Water at 0°C
• Description: Ice and water coexist in equilibrium at 0°C and 1 atmosphere pressure. The rate at which water molecules freeze to form ice equals the rate at which ice melts to form water.
• Observation: There is no net change in the amount of ice and water, but the melting and freezing processes continue to occur simultaneously.

Liquid-Gas Equilibrium:

It occurs when a liquid and its vapour are in equilibrium at a specific temperature and pressure.

Example: Water and Steam at 100°C

• Description: Water and steam coexist in equilibrium at 100°C and 1 atmosphere pressure. The rate at which water molecules evaporate to form steam equals the rate at which steam condenses to form water.
• Observation: The amount of liquid water and steam remains constant, but evaporation and condensation occur simultaneously.

Solid-Gas Equilibrium:

• It occurs when a solid and its vapour are in equilibrium at a specific temperature and pressure.
• Example: Iodine and its Vapor in a Sealed Container
• Description: When iodine is placed in a sealed container at room temperature, it sublimes to form iodine vapour. Eventually, an equilibrium is reached where the sublimation rate of solid iodine to iodine vapour equals the deposition rate of iodine vapour back to solid iodine.
• Observation: The amounts of solid iodine and iodine vapour remain constant, but sublimation and deposition continue to occur simultaneously.

Solution Equilibrium

Solute-Solid Equilibrium:

• Solubility equilibrium occurs when a solute is in dynamic equilibrium between its dissolved state in a solvent and its undissolved solid state. Example: Salt in Water

Description:

• Dissolution Process: When salt (sodium chloride, NaCl) is added to water, it begins to dissolve. The solid salt dissociates into sodium (Na⁺) and chloride (Cl⁻) ions, which are dispersed uniformly throughout the water.

NaCl (solid)→Na+(aq)+Cl−(aq)

• Saturation Point: As more salt is added, the concentration of dissolved ions increases until the solution reaches its saturation point at a given temperature. At this point, the salt's dissolution rate equals the rate at which the dissolved ions recombine to form solid salt and precipitate out of the solution..

Na⁺ (aq) +Cl⁻(aq)  →  NaCl (solid)

• Dynamic Equilibrium: At saturation, the system is in dynamic equilibrium. This means that the processes of dissolving and precipitating occur simultaneously at the same rate, resulting in no net change in the amount of dissolved salt and the undissolved solid salt.

 NaCl (solid)  ⇌  Na⁺(aq)+Cl⁻(aq)

• Observation: The amount of dissolved salt in the water and the amount of undissolved solid salt remain constant over time, but individual salt particles continuously dissolve and precipitate.
• This equilibrium can be represented by the solubility product constant (K_sp​), which is a measure of the solubility of a compound under specific conditions. 

For sodium chloride, it would be: 

K_sp =  [Na⁺] [Cl⁻]

• Solubility equilibrium in the case of salt in water demonstrates the balance between the dissolved ions in the solution and the undissolved solid, maintaining a constant concentration of dissolved ions at equilibrium.

Gas-Liquid Equilibrium

Henry's Law Equilibrium

Henry's Law describes the solubility of gases in liquids at a constant temperature. According to Henry's Law, the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of the gas above the liquid.

Example: Carbon Dioxide in Carbonated Beverages

• Description: Carbon dioxide (CO₂) is dissolved in the liquid under high pressure in carbonated beverages. When the pressure is reduced (e.g., by opening the bottle), the solubility of CO₂ decreases, and the gas escapes from the liquid, forming bubbles.
• Henry's Law Equation: C = k_H⋅P
• • C is the concentration of the dissolved gas.
  • k_H is Henry's Law constant for the gas in the solvent.
  • P is the partial pressure of the gas.

Liquid-Liquid Equilibrium

Miscibility equilibrium occurs when two immiscible or partially miscible liquids are in equilibrium. In such systems, each liquid dissolves to a limited extent in the other.

Example: Oil and Water

• Description: Oil and water are immiscible liquids that do not mix uniformly. When they are combined, they form two distinct layers. The small amount of each liquid dissolves in the other, representing the miscibility equilibrium.
• Observation: When oil and water are shaken and then allowed to settle, two layers form with a small interfacial region where some oil is dissolved in water and some water is dissolved in oil.

3.0Characteristics of Equilibrium in Physical Processes

• During equilibrium, opposing processes occur at the same rate, resulting in a dynamic yet stable condition.
• Equilibrium in physical processes can only be established in a closed system at a given temperature.
• At a specific temperature, physical equilibrium is characterized by a constant value of one of its parameters.
• The value of this parameter at any given stage indicates the extent to which a physical process has progressed before reaching equilibrium.
• All measurable properties of the system remain constant at equilibrium.

4.0Sample Question on Physical Equilibrium

Ques.1 Differentiate between physical and chemical equilibrium.

Ans. We can differentiate physical and chemical equilibrium in the following manner