This MCQ module is based on: Equilibrium Physical Chemical
TOPIC 24 OF 28
Equilibrium Physical Chemical
🎓 Class 11
Chemistry
CBSE
Theory
Ch 6 – Equilibrium
⏱ ~14 min
🌐 Language: [gtranslate]
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Equilibrium in Physical and Chemical Processes
6.1 Introduction — What is Equilibrium?
Imagine a sealed bottle of water. Initially, only liquid water is present. Slowly, water molecules evaporate into the empty space above the liquid, raising the gas-phase concentration. Eventually, however, the water level stops dropping. Has evaporation stopped? No — molecules keep going back and forth. The system has reached a state of equilibrium.
Equilibrium: A state in which two opposing processes (forward and reverse) occur at the SAME RATE, so that the observed macroscopic properties (concentration, pressure, density) become time-independent — yet the system remains microscopically active (dynamic).
6.2 Equilibrium in Physical Processes
Equilibrium is not unique to chemical reactions. It also applies to physical changes such as phase changes and dissolution.
6.2.1 Solid–Liquid Equilibrium
Place an ice-water mixture in a thermally insulated container at 0 °C and 1 atm. Some ice melts; some water freezes. At 0 °C the rate of melting equals the rate of freezing → solid ⇌ liquid equilibrium. The masses of ice and water remain constant.
\[\text{H}_2\text{O(s)} \rightleftharpoons \text{H}_2\text{O(l)} \quad \text{at } 273.15 \text{ K, 1 atm}\]6.2.2 Liquid–Vapour Equilibrium
In a closed vessel at constant temperature, water (liquid) evaporates and water vapour condenses simultaneously. When the rates become equal, the gas-phase pressure becomes constant. This pressure is the saturated vapour pressure.
6.2.3 Solid–Vapour Equilibrium (Sublimation)
Iodine and camphor sublime — they go directly from solid to vapour. In a closed vessel:
\[\text{I}_2\text{(s)} \rightleftharpoons \text{I}_2\text{(g)}\]Vapour pressure becomes constant once equilibrium is reached.
6.2.4 Equilibrium of a Solute–Solvent System
Add solid sugar to water. Initially, sugar dissolves rapidly. As the solution becomes more concentrated, the rate of dissolution slows while the rate of crystallization (solute returning to solid) rises. At equilibrium, the solution is saturated:
\[\text{Sugar (s)} \rightleftharpoons \text{Sugar (aq)}\]6.2.5 Gases in Liquids — Henry's Law
The amount of gas dissolved in a liquid at constant T is proportional to the partial pressure of the gas above the liquid:
\[m \propto P_\text{gas} \quad \text{or} \quad P = K_H \cdot x_\text{gas}\]This is why a soda bottle fizzes when opened — pressure drops, dissolved CO₂ leaves solution.
6.3 General Characteristics of Physical Equilibria
| Process | Equilibrium | Constant feature |
|---|---|---|
| Liquid ⇌ Vapour | P(vapour) = constant at given T | Vapour pressure |
| Solid ⇌ Liquid | m.p. = freezing point | Temperature |
| Solid ⇌ Vapour | P(vapour) at given T constant | Vapour pressure |
| Solute (s) ⇌ Solute (aq) | Concentration of saturated soln. constant | Solubility |
| Gas (g) ⇌ Gas (aq) | K_H = P/x_gas | Henry's constant |
Common features of all equilibria:
- Achieved only in a closed system at given T.
- Dynamic — both forward and reverse processes continue.
- Can be approached from either side.
- Macroscopic properties (P, c, density) become constant.
- A catalyst doesn't change the equilibrium position — only the speed at which it is reached.
6.4 Equilibrium in Chemical Processes — Dynamic Equilibrium
Just like physical equilibrium, chemical reactions reach a state where forward and reverse rates are equal:
\[A + B \overset{k_f}{\underset{k_r}{\rightleftharpoons}} C + D\]Initially, [A] and [B] are high; [C] and [D] are zero. As reaction proceeds, forward rate falls while reverse rate rises. When rates become equal — equilibrium is reached, with all four species present in unchanging concentrations.
6.4.1 Haber Process — A Famous Reversible Reaction
\[\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}\]If we mix N₂ and H₂ in a closed container at fixed T and P, NH₃ is formed. Simultaneously, NH₃ decomposes back to N₂ and H₂. After some time, the concentrations of all three become constant — equilibrium is reached.
6.4.2 Evidence for Dynamic Nature
Use isotopic labelling! Take an equilibrium mixture of H₂ + I₂ + 2HI. Add a small amount of D₂ (deuterium). Even though concentrations don't change macroscopically, after some time D atoms appear in the HI. This proves that bonds are constantly being broken and re-formed at equilibrium.
🎯 Interactive: Approach to Equilibrium Simulator
Choose initial concentrations and an equilibrium constant. Watch how the system approaches equilibrium over time.
For A ⇌ B with [A]₀ = 1.0 M, K_c = 4
[A]_eq = 0.20 M | [B]_eq = 0.80 M
Equilibrium favours products (K > 1).
🧪 Activity 6.1 — Reversibility of FeCl₃ + KSCN
Setup: Mix dilute FeCl₃ (yellow) and KSCN (colourless) in a beaker. Note the deep blood-red colour of [Fe(SCN)]²⁺. Now divide the solution into 4 test tubes and add: (i) extra FeCl₃ (ii) extra KSCN (iii) NaOH (iv) leave alone.
Predict: What happens to the red colour intensity in each test tube? What does this tell you about equilibrium?
(i) +FeCl₃: Red colour deepens (more product forms).
(ii) +KSCN: Red colour deepens (more product forms).
(iii) +NaOH: Red fades (Fe³⁺ + 3OH⁻ → Fe(OH)₃↓ removes Fe³⁺ from equilibrium — left side runs in reverse).
(iv) Stays the same.
Conclusion: The reaction Fe³⁺ + SCN⁻ ⇌ [Fe(SCN)]²⁺ is reversible! Adding reactants drives equilibrium to products; removing them drives it back. This is Le Chatelier's principle in action.
Worked Example 6.1: Identify Equilibrium State
A 1-L sealed flask contains 0.4 mol N₂O₄ and 0.6 mol NO₂ at equilibrium. Is the system at equilibrium even though [NO₂] ≠ [N₂O₄]?
Equilibrium does NOT mean equal concentrations of reactants and products. It means the rates of forward and reverse reactions are equal, so concentrations remain constant.
The given values just describe one particular equilibrium state — the system is at equilibrium when [N₂O₄] and [NO₂] no longer change with time. The ratio [NO₂]²/[N₂O₄] = (0.6)²/0.4 = 0.9 M (this is K_c at that T).
The given values just describe one particular equilibrium state — the system is at equilibrium when [N₂O₄] and [NO₂] no longer change with time. The ratio [NO₂]²/[N₂O₄] = (0.6)²/0.4 = 0.9 M (this is K_c at that T).
Worked Example 6.2: Saturated Solution as Equilibrium
Excess solid CuSO₄·5H₂O is shaken with water. At 25 °C the saturated solution contains 22.0 g per 100 g water. Is the rate of dissolution zero?
NO! Dissolution and crystallization both proceed at finite rates — they are EQUAL. If we use radioactively labelled solid Cu*SO₄, the labelled atoms slowly appear in solution while ordinary Cu²⁺ from solution crystallizes onto the solid. The macroscopic concentrations remain constant (saturation), but molecules are constantly exchanging.
🎯 Competency-Based Questions
Q1. Which is true about equilibrium? L1 Remember
Answer: (b) Forward rate = reverse rate. Concentrations need NOT be equal; reaction does NOT stop; catalyst affects only the speed, not the position of equilibrium.
Q2. Why does a closed soda bottle keep its fizz, while an opened one goes flat? L2 Understand
Answer: In the closed bottle, gaseous CO₂ above the liquid is in equilibrium with dissolved CO₂: CO₂(g) ⇌ CO₂(aq). Henry's law sets a high P (typically ~3 atm), so a lot is dissolved. When opened, gas escapes; P_CO₂ drops to ~0.0004 atm (atmospheric); the equilibrium shifts dramatically left as dissolved CO₂ leaves solution. Eventually [CO₂(aq)] is very low — soda goes flat.
Q3. Saturated KCl(aq) at 20 °C has [KCl] = 4.4 M. Why does adding more solid KCl not raise the concentration? L3 Apply
Answer: The solution is at equilibrium with the solid: KCl(s) ⇌ K⁺(aq) + Cl⁻(aq). The dissolution rate equals the crystallization rate. Adding more solid increases the rate of crystallization (more surface) and the rate of dissolution proportionally, but the equilibrium concentration depends only on T (and the solubility product), so it stays at 4.4 M.
Q4. True / False: A reaction at equilibrium has stopped. L2 Understand
FALSE. Equilibrium is dynamic — molecules continue to react in both directions at equal rates. Macroscopic properties remain constant, but molecular activity continues. Isotope-labelling experiments show this directly.
Q5. HOT (Analyse): Sketch a graph of forward and reverse rates vs time for A + B → C + D. At what point does the system reach equilibrium? Mark the equilibrium time t_eq and explain why both rates remain non-zero at t_eq. L4 Analyse
Answer: Forward rate starts high (high [A], [B]) and decreases as reactants are consumed. Reverse rate starts at zero (no products) and increases as products accumulate. They meet at t_eq when both equal r_eq ≠ 0. Both remain non-zero because reactants and products are still present and continuously interconverting — the rates only become EQUAL, not zero. Thermodynamically, ΔG = 0 at this point.
🧠 Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Chemical equilibrium is dynamic in nature.
R: At equilibrium, both forward and reverse reactions stop.
Answer: (C). A is TRUE; R is FALSE — at equilibrium, both reactions still occur; only their RATES are equal.
A: The vapour pressure of a liquid in a closed container at constant T is always the same.
R: Saturated vapour pressure depends only on the nature of the liquid and temperature.
Answer: (A). Both true; R explains A. Saturated VP is independent of the amount of liquid present (provided some liquid remains).
A: Equilibrium can be approached from either direction.
R: The same equilibrium state is reached whether we start from reactants or products.
Answer: (A). Both true; R explains A. This is one of the defining features of equilibrium.
Frequently Asked Questions — Equilibrium in Physical and Chemical Processes
What is equilibrium and how is it different from completion?
Equilibrium is the state of a reversible reaction or physical change in which the rates of forward and reverse processes become equal so that no net change occurs over time. Macroscopic properties such as pressure, concentration and colour remain constant. Unlike a reaction going to completion (where one reactant is fully converted), equilibrium has both reactants and products present in fixed proportions. NCERT Class 11 Chemistry Chapter 6 emphasises that equilibrium is dynamic, not static — molecules continually convert in both directions but at equal rates. This is observable through isotope-tracer experiments.
What is the difference between physical and chemical equilibrium?
Physical equilibrium involves a physical process such as a phase change without any change in chemical composition. Examples include water ⇌ water vapour in a closed container, melting of ice in equilibrium with water at 0°C, and a saturated solution of NaCl in equilibrium with undissolved solid. Chemical equilibrium involves a chemical reaction where products and reactants coexist. Examples include N₂(g) + 3H₂(g) ⇌ 2NH₃(g) and H₂(g) + I₂(g) ⇌ 2HI(g). NCERT Class 11 Chemistry Chapter 6 covers both types because they obey the same equilibrium law and Le Chatelier's principle.
Why is equilibrium called dynamic?
Equilibrium is called dynamic because, although the system appears macroscopically still, the forward and reverse reactions continue at the molecular level — only their rates have become equal. This was confirmed by isotope-labelling experiments where, after equilibrium was reached, radioactively tagged molecules were found in both reactant and product positions, proving the reactions are ongoing. NCERT Class 11 Chemistry Chapter 6 highlights this dynamic nature as a defining feature, distinguishing equilibrium from a 'stopped' reaction. The dynamic picture also explains why equilibrium responds to disturbances (Le Chatelier's principle).
What are the characteristics of chemical equilibrium?
The five characteristics of chemical equilibrium in NCERT Class 11 Chemistry Chapter 6 are: (1) dynamic — forward and reverse reactions continue at equal rates; (2) reached only in closed systems; (3) achieved from either direction (forward or reverse); (4) accompanied by constant macroscopic properties (concentration, colour, pressure); (5) characterised by an equilibrium constant K that depends only on temperature. The equilibrium state is independent of the route taken and the time elapsed once reached. A catalyst speeds attainment of equilibrium but does not shift its position.
Give examples of physical equilibrium with explanation.
Physical equilibrium examples from NCERT Class 11 Chemistry Chapter 6: (1) solid-liquid: ice ⇌ water at 0°C and 1 atm — both phases coexist; (2) liquid-vapour: water ⇌ water vapour in a closed container — vapour pressure reaches saturation; (3) solid-solution: undissolved sugar ⇌ sugar in saturated solution — concentration reaches solubility limit; (4) gas-solution: dissolved CO₂ ⇌ CO₂ gas above the liquid in a sealed bottle — Henry's law applies. Each example involves no chemical change, only redistribution of matter between phases or states. Equilibrium constants can be defined for each.
How is equilibrium achieved in a chemical reaction?
When reactants are mixed, the forward reaction begins at a maximum rate that gradually decreases as reactant concentrations fall. Simultaneously, the reverse reaction starts at zero rate (no products yet) and gradually increases as products accumulate. Eventually the two rates become equal — this is the equilibrium point. After this, the concentrations of reactants and products remain constant. NCERT Class 11 Chemistry Chapter 6 illustrates this with concentration-time graphs showing how reactants decrease and products increase until both level off. Equilibrium can be approached from either direction with the same K value.
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