Short Q&A for Chemical and Ionic Equilibrium (Class 11 NCERT)

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Chemical and Ionic Equilibrium

112 Short Q&A • NCERT Class 11 Chemistry

Class 11 JEE / NEET

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112 Short Q&A for Equilibrium

Part 1: Chemical Equilibrium (General Concepts)
1. Q1: Define a Reversible Reaction.
   A: A reaction that can proceed in both forward and reverse directions.
2. Q2: What is a State of Chemical Equilibrium?
   A: The state where the rate of the forward reaction equals the rate of the reverse reaction.
3. Q3: Is chemical equilibrium static or dynamic?
   A: Dynamic (reactions continue, but concentrations remain constant).
4. Q4: How is the Equilibrium Constant (K) expressed in terms of concentration?
   A: K_c (Concentration in mol L⁻¹).
5. Q5: How is the Equilibrium Constant (K) expressed in terms of partial pressure?
   A: K_p (Pressure in bar or atm).
6. Q6: Write the relationship between K_p and K_c.
   A: K_p = K_c(RT)^Δn_g.
7. Q7: What does Δn_g represent in the K_p and K_c relationship?
   A: (Moles of gaseous products) - (Moles of gaseous reactants).
8. Q8: When is K_p = K_c?
   A: When Δn_g = 0.
9. Q9: Does the magnitude of K indicate the speed of a reaction?
   A: No, K indicates the extent of the reaction, not the speed.
10. Q10: What does a large value of K (≈10³) indicate?
    A: The reaction proceeds almost to completion (products are favoured).
11. Q11: What does a very small value of K (≈10⁻³) indicate?
    A: The reaction hardly proceeds (reactants are favoured).
12. Q12: What is the Reaction Quotient (Q)?
    A: The expression for K_c evaluated at any point during the reaction, not necessarily at equilibrium.
13. Q13: If Q_c < K_c, which direction will the reaction proceed?
    A: Forward direction (to increase products and reach equilibrium).
14. Q14: If Q_c > K_c, which direction will the reaction proceed?
    A: Reverse direction (to decrease products and reach equilibrium).
15. Q15: State Le Chatelier's Principle.
    A: When a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system shifts to counteract the change.
16. Q16: How does increasing the concentration of a reactant affect equilibrium?
    A: The equilibrium shifts in the forward direction.
17. Q17: How does increasing the pressure affect equilibrium (for Δn_g ≠ 0)?
    A: It shifts towards the side with the fewer number of gaseous moles.
18. Q18: For an exothermic reaction, how does increasing the temperature affect K?
    A: K decreases (equilibrium shifts in the reverse direction).
19. Q19: For an endothermic reaction, how does increasing the temperature affect K?
    A: K increases (equilibrium shifts in the forward direction).
20. Q20: Does adding an inert gas at constant volume affect equilibrium?
    A: No, it only changes total pressure, not partial pressures.
21. Q21: Does adding a catalyst affect the value of K?
    A: No, a catalyst only helps attain equilibrium faster.
22. Q22: What is a Homogeneous Equilibrium?
    A: All reactants and products are in the same phase (e.g., all gases or all liquids).
23. Q23: What is a Heterogeneous Equilibrium?
    A: Reactants and products are in different phases (e.g., solid and gas).
24. Q24: How are pure solids and liquids treated in the K expression?
    A: Their concentration is taken as unity (1).
Part 2: Ionic Equilibrium (Acids, Bases, and Salts)
25. Q25: What is an Electrolyte?
    A: A substance that dissociates into ions in an aqueous solution.
26. Q26: Name the three theories for acids and bases.
    A: Arrhenius, Brønsted-Lowry, and Lewis.
27. Q27: Define an Arrhenius Acid.
    A: A substance that produces H⁺ ions in water.
28. Q28: Define an Arrhenius Base.
    A: A substance that produces OH⁻ ions in water.
29. Q29: Define a Brønsted-Lowry Acid.
    A: A substance that is a proton (H⁺) donor.
30. Q30: Define a Brønsted-Lowry Base.
    A: A substance that is a proton (H⁺) acceptor.
31. Q31: What is a Conjugate Acid-Base Pair?
    A: A pair that differs by only one proton (H⁺).
32. Q32: What is the conjugate acid of NH₃?
    A: NH₄⁺ (Ammonium ion).
33. Q33: What is the conjugate base of H₂O?
    A: OH⁻ (Hydroxide ion).
34. Q34: What is an Amphoteric Substance?
    A: A substance that can act as both an acid and a base (e.g., H₂O).
35. Q35: Define a Lewis Acid.
    A: An electron pair acceptor.
36. Q36: Give one example of a Lewis Acid.
    A: BF₃ or AlCl₃ (electron deficient molecules).
37. Q37: Define a Lewis Base.
    A: An electron pair donor.
38. Q38: Give one example of a Lewis Base.
    A: NH₃ or H₂O (molecules with lone pairs).
39. Q39: What is the significance of the ionization constant (K_a or K_b)?
    A: It measures the strength of an acid (K_a) or base (K_b).
40. Q40: What is the relationship between pK_a and acid strength?
    A: Smaller the pK_a, stronger the acid.
41. Q41: What is the Ionization Constant of Water (K_w)?
    A: K_w = [H₃O⁺][OH⁻].
42. Q42: What is the value of K_w at 298 K?
    A: 1.0 × 10⁻¹⁴ (mol² L⁻²).
43. Q43: What is the relationship between pK_a, pK_b, and pK_w?
    A: pK_a + pK_b = pK_w (= 14 at 298 K).
44. Q44: Define pH scale.
    A: pH = −log₁₀[H⁺] (or [H₃O⁺]).
45. Q45: What is the pH of a neutral solution at 298 K?
    A: 7
46. Q46: What is the pOH of a solution with [OH⁻] = 10⁻⁴ M?
    A: 4 (pOH = −log(10⁻⁴)).
47. Q47: What is the pH of the solution in the previous question?
    A: 10 (pH + pOH = 14).
48. Q48: What is the minimum condition for an acid to be considered a strong acid?
    A: It must be almost completely ionized in an aqueous solution.
49. Q49: What is the Ostwald's Dilution Law used for?
    A: To calculate the degree of dissociation (α) of a weak electrolyte.
50. Q50: What is Hydrolysis of a Salt?
    A: The reaction of the salt's ion (cation or anion) with water to produce acid and base.
51. Q51: What kind of salt produces an acidic solution upon hydrolysis?
    A: Salt of a strong acid and weak base (e.g., NH₄Cl).
52. Q52: What kind of salt produces a basic solution upon hydrolysis?
    A: Salt of a weak acid and strong base (e.g., CH₃COONa).
53. Q53: What kind of salt produces a neutral solution upon hydrolysis?
    A: Salt of a strong acid and strong base (e.g., NaCl).
Part 3: Buffer Solutions and Solubility Product
54. Q54: Define a Buffer Solution.
    A: A solution that resists change in its pH upon the addition of small amounts of acid or base.
55. Q55: What is a Simple Buffer?
    A: A solution of a salt of a weak acid and weak base (rarely used).
56. Q56: What is an Acidic Buffer composed of?
    A: A weak acid and its salt with a strong base (e.g., CH₃COOH + CH₃COONa).
57. Q57: What is a Basic Buffer composed of?
    A: A weak base and its salt with a strong acid (e.g., NH₃ + NH₄Cl).
58. Q58: Write the Henderson-Hasselbalch equation for an acidic buffer.
    A: pH = pK_a + log([Salt]/[Acid]).
59. Q59: Write the Henderson-Hasselbalch equation for a basic buffer.
    A: pOH = pK_b + log([Salt]/[Base]).
60. Q60: What is the pH of an acidic buffer when [Salt] = [Acid]?
    A: pH = pK_a.
61. Q61: Define Solubility Product Constant (K_sp).
    A: The equilibrium constant for the equilibrium between a sparingly soluble ionic solid and its ions in a saturated solution.
62. Q62: If the ionic product (Q_sp) is less than K_sp, what is the state of the solution?
    A: Unsaturated (more salt can dissolve).
63. Q63: If the ionic product (Q_sp) is greater than K_sp, what happens?
    A: Precipitation occurs (supersaturated).
64. Q64: Write the K_sp expression for AgCl.
    A: K_sp = [Ag⁺][Cl⁻].
65. Q65: Write the K_sp expression for BaSO₄.
    A: K_sp = [Ba²⁺][SO₄²⁻].
66. Q66: Write the K_sp expression for CaF₂.
    A: K_sp = [Ca²⁺][F⁻]².
67. Q67: What is the effect of the Common Ion Effect on the solubility of a sparingly soluble salt?
    A: The solubility decreases.
68. Q68: Why is HCl gas passed into a solution before precipitating Group II metal sulphides?
    A: To suppress the ionization of H₂S (common ion H⁺) and control [S²⁻] to precipitate only Group II sulphides.
69. Q69: What is the relationship between solubility (s) and K_sp for AgCl?
    A: K_sp = s².
70. Q70: What is the relationship between solubility (s) and K_sp for CaF₂?
    A: K_sp = 4s³ (s × (2s)²).
Part 4: Numerical and Conceptual Questions (Mixed)
71. Q71: If K for a reaction is 4 × 10²⁰, what is the sign of ΔG°?
    A: Negative (ΔG° = −RTlnK).
72. Q72: What is the effect of a catalyst on the activation energy of the forward reaction (E_a)?
    A: E_a decreases.
73. Q73: What is the pH of 10⁻⁸ M HCl?
    A: 6.96 (Must consider H⁺ from water due to low concentration).
74. Q74: In the reaction H₂ + I₂ ⇌ 2HI, Δn_g = ?
    A: 0 (2 − (1+1)).
75. Q75: In the reaction PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), how does increasing pressure affect PCl₅ amount?
    A: PCl₅ amount increases (shifts to fewer moles).
76. Q76: In the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH < 0), how does increasing T affect NH₃ yield?
    A: NH₃ yield decreases (shifts to reverse for exothermic reaction).
77. Q77: In a buffer solution, which component consumes the added acid (H⁺)?
    A: The conjugate base (salt component).
78. Q78: Why is the pH of NaCl solution 7?
    A: Neither Na⁺ nor Cl⁻ undergo hydrolysis.
79. Q79: Which one is the weaker acid: one with pK_a = 5 or one with pK_a = 3?
    A: The acid with pK_a = 5 (higher pK_a means weaker acid).
80. Q80: Is O²⁻ a strong or weak Brønsted base?
    A: A strong Brønsted base (readily accepts H⁺ to form OH⁻).
81. Q81: What is the relationship between the strength of an acid and its conjugate base?
    A: A strong acid has a weak conjugate base, and vice versa.
82. Q82: How does dilution affect the degree of ionization (α) of a weak acid?
    A: α increases (Ostwald's Law).
83. Q83: What does K_sp really represent in terms of concentration?
    A: The maximum product of ion concentrations in a saturated solution.
84. Q84: What is the main reason for the Common Ion Effect?
    A: The shift in equilibrium due to the addition of an ion already present in the system (Le Chatelier's Principle).
85. Q85: Why must pure water ionization always be considered for very dilute strong acid/base solutions?
    A: The concentration of H⁺ from water becomes comparable to the added acid/base concentration.
Part 5: More Definitions and Concepts
86. Q86: If an overall reaction is the sum of two steps, how is the overall K related to K₁ and K₂?
    A: K_overall = K₁ × K₂.
87. Q87: If an equilibrium reaction is reversed, how does the new K' relate to the original K?
    A: K' = 1 / K.
88. Q88: If an equilibrium reaction is doubled, how does the new K'' relate to the original K?
    A: K'' = K².
89. Q89: What is the value of pK_w at 298 K?
    A: 14 (pK_w = −logK_w).
90. Q90: What is the common ion in the CH₃COOH / CH₃COONa buffer?
    A: CH₃COO⁻ (Acetate ion).
91. Q91: What is the effect of pressure on solubility of gases in liquids (Henry's Law)?
    A: Solubility increases with increasing pressure.
92. Q92: Define Buffer Capacity.
    A: The amount of acid or base a buffer can absorb before its pH changes significantly.
93. Q93: What is the pH of a solution when [H⁺] = [OH⁻]?
    A: Neutral pH (usually 7).
94. Q94: What is the relationship between the degree of hydrolysis (h) and K_h for a weak acid/strong base salt?
    A: K_h = Ch² (C is concentration).
95. Q95: Write the expression for the hydrolysis constant (K_h) of NH₄Cl.
    A: K_h = K_w / K_b.
96. Q96: Write the expression for the hydrolysis constant (K_h) of CH₃COONa.
    A: K_h = K_w / K_a.
97. Q97: What is the primary species in water responsible for making a solution basic?
    A: OH⁻ (Hydroxide ion).
98. Q98: What is the primary species in water responsible for making a solution acidic?
    A: H₃O⁺ (Hydronium ion, or simplified H⁺).
99. Q99: What is the minimum pH attainable by an aqueous solution?
    A: Theoretically 0 or less (for highly concentrated strong acids).
100. Q100: Why must K_sp always be determined in a saturated solution?
     A: Because K_sp is an equilibrium constant.
101. Q101: Does adding an inert gas at constant pressure affect equilibrium?
     A: Yes, it increases volume, decreasing concentration/pressure of reactants, shifting equilibrium to the side with more gaseous moles.
102. Q102: What is the maximum pH attainable by an aqueous solution?
     A: Theoretically 14 or more (for highly concentrated strong bases).
103. Q103: What is the relationship between ΔG° and K at equilibrium?
     A: ΔG° = −RTlnK (Standard Gibbs Free Energy equals zero at equilibrium).
104. Q104: Which theory is the most general for acids and bases?
     A: Lewis Theory (includes species without H⁺ or OH⁻).
105. Q105: Does the value of K_w depend on temperature?
     A: Yes, K_w increases with temperature (ionization of water is endothermic).
106. Q106: How does the solubility of an ionic solid typically change with increasing temperature?
     A: Solubility increases (most dissolution processes are endothermic).
107. Q107: What is the common ion used to precipitate Group III hydroxides (Al³⁺, Fe³⁺)?
     A: OH⁻ (from NH₄OH and NH₄Cl buffer).
108. Q108: Is BF₃ a better Lewis acid or a Brønsted acid?
     A: A Lewis Acid (electron deficient, lacks H⁺).
109. Q109: What type of salt solution requires both K_a and K_b of its parent acid/base to determine pH?
     A: Salt of a weak acid and weak base.
110. Q110: If pK_a = pK_b for the parent acid/base of a salt, what is the pH of the salt solution?
     A: 7 (Neutral).
111. Q111: What is the minimum K_a an acid must have to be practically considered a weak acid?
     A: K_a < 1 (typically 10⁻³ to 10⁻¹⁰).
112. Q112: What is the value of ΔG when a reaction is at equilibrium?
     A: ΔG = 0.

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