Chapter 6 / 6.1 Equilibrium In Physical Processes

Understanding Equilibrium in Physical Processes

What is Equilibrium? 🤔

Equilibrium is a dynamic state where two opposing processes happen at the same rate, so there’s no overall change. It’s like a dance where both partners move equally fast!

Types of Physical Equilibria ⚖️

1. Solid ⇌ Liquid (e.g., Ice-Water)

• At 273 K and 1 atm pressure, ice and water coexist in a thermos (no heat exchange).
• 💧 Dynamic process: Water molecules constantly melt ice and freeze back at equal rates:
  \[ \text{rate}_{\text{(ice→water)}} = \text{rate}_{\text{(water→ice)}} \]
• Temperature remains constant, and masses don’t change.
• This temperature is called the normal melting/freezing point.

2. Liquid ⇌ Vapour (e.g., Water-Steam)

• In a closed box with a manometer:
  • Water evaporates → pressure increases.
  • Vapour condenses → pressure stabilizes.
• ⚡ Equilibrium: When evaporation rate = condensation rate:
  \[ \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_2\text{O}(\text{vap}) \]
• Constant pressure at equilibrium is called vapour pressure.
• Normal boiling point 🌡️: Temperature where liquid/vapour equilibrium exists at 1.013 bar (e.g., water boils at 100°C).
• ⚠️ Open systems (e.g., water in an open dish) never reach equilibrium because vapour escapes!

3. Solid ⇌ Vapour (Sublimation)

• Example: Iodine crystals in a closed vial turn into violet vapour.
• 🌈 Equilibrium when colour intensity stabilizes:
  \[ \text{I}_2(\text{solid}) \rightleftharpoons \text{I}_2(\text{vapour}) \]
• Also seen in camphor or \(\text{NH}_4\text{Cl}\).

4. Solid ⇌ Solution (Dissolution)

• Saturated solution: No more solute dissolves (e.g., sugar in water).
• ⚖️ Dynamic equilibrium: Dissolution rate = crystallization rate:
  \[ \text{Sugar(solid)} \rightleftharpoons \text{Sugar(solution)} \]
• Proven using radioactive sugar! ☢️ Radioactivity appears in both solid and solution over time.

5. Gas ⇌ Solution (e.g., CO₂ in Soda)

• Governed by Henry’s Law: Mass of dissolved gas ∝ Pressure above solvent.
• 🥤 Soda bottle example:
  • Sealed: High CO₂ pressure → more gas dissolves.
  • Opened: CO₂ fizzes out until new equilibrium (lower pressure) is reached.
• Equilibrium: \[ \text{CO}_2(\text{gas}) \rightleftharpoons \text{CO}_2(\text{in solution}) \]
• Solubility ↓ as temperature ↑.

Key Features of Physical Equilibria 🔑

General Rules for Physical Equilibrium 📜

1. Only possible in closed systems (no escape route!).
2. Opposing processes occur at equal rates (dynamic but stable).
3. All measurable properties (mass, pressure, concentration) stay constant.
4. Each equilibrium has one constant parameter (see table above).
5. This parameter tells us “how far” the process went before balancing.

Chemical Equilibrium: A Quick Peek! ⚗️

• Same idea: Forward/reaction rates become equal:
  \[ \text{A} + \text{B} \rightleftharpoons \text{C} + \text{D} \]
• Concentrations stabilize (see graph below).
• Proven by Haber’s ammonia synthesis:
  • Using \(\text{D}_2\) (deuterium) instead of \(\text{H}_2\) gives same equilibrium, just with \(\text{ND}_3\)! 🤯

NEET Must-Knows! 🎯

1. Dynamic Equilibrium: Rates of forward/reverse processes are equal (no net change).
2. Vapour Pressure: Constant at liquid⇌vapour equilibrium; depends on temperature.
3. Henry’s Law: Gas solubility ∝ Pressure above liquid (e.g., CO₂ in soda).
4. Closed Systems: Essential for all physical equilibria (open systems can’t reach equilibrium!).
5. Saturated Solutions: Dynamic equilibrium between dissolved and undissolved solute.

Keep practicing — equilibrium is all about balance, just like studying! ✨