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Ionic-bond formation factors and lattice enthalpy: NEET MCQs with answers

90 questions on this topic of Chemical Bonding and Molecular Structure (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following electronic configurations of a neutral atom X corresponds to the easiest formation of a cation during the formation of an ionic solid?

  1. [Ne] 3s¹
  2. [He] 2s¹
  3. [Ar] 4s¹
  4. [Kr] 5s¹
Show answer and explanation

Answer: (D) [Kr] 5s¹

The formation of a cation requires low ionization enthalpy. Down the group among alkali metals, atomic size increases and ionization enthalpy decreases systematically ([Kr] 5s¹ corresponds to Rb, which has the lowest ionization enthalpy among the given options).

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2. An ionic compound A₂B is formed from element A (alkali metal) and element B (chalcogen). If the lattice enthalpy of A₂B is 2200 kJ mol⁻¹ and its hydration enthalpy is -2150 kJ mol⁻¹, which of the following statements is correct regarding its dissolution in water?

  1. The standard enthalpy of solution is +50 kJ mol⁻¹ and dissolution is endothermic
  2. The compound is insoluble because hydration enthalpy is more negative than lattice enthalpy
  3. The process of dissolution is exothermic and the compound is likely freely soluble
  4. The process has an enthalpy of solution of -50 kJ mol⁻¹
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Answer: (A) The standard enthalpy of solution is +50 kJ mol⁻¹ and dissolution is endothermic

The enthalpy of solution is given by Δ_solH° = Δ_latticeH° + Δ_hydH° = +2200 kJ mol⁻¹ + (-2150 kJ mol⁻¹) = +50 kJ mol⁻¹. Because Δ_solH° is positive, the dissolution process is endothermic.

hard

3. Given that the lattice enthalpies of LiF, NaF, and KF are +1036, +923, and +821 kJ mol⁻¹ respectively, and their respective interionic distances (r₀) are 201, 231, and 268 pm. Which of the following best explains the observed variation?

  1. Lattice enthalpy is predominantly governed by the covalent character of the bond
  2. Lattice enthalpy is inversely proportional to interionic distance for ions of identical charge
  3. Lattice enthalpy decreases linearly with increasing atomic mass of the alkali metal
  4. Lattice enthalpy increases as the polarizability of the cation increases
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Answer: (B) Lattice enthalpy is inversely proportional to interionic distance for ions of identical charge

According to the Born-Landé or Kapustinskii relation, for salts with identical ion charges (Li⁺, Na⁺, K⁺ and F⁻ all have |z₊ · z₋| = 1), the lattice enthalpy is inversely proportional to the equilibrium interionic separation r₀ (r₊ + r₋). As r₀ increases from LiF to KF, lattice enthalpy decreases accordingly.

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4. Given that the lattice enthalpy of LiF is +1036 kJ mol⁻¹ and its hydration enthalpy (Δ_hydH°) is -1005 kJ mol⁻¹, what is the standard enthalpy of solution (Δ_solH°) of LiF(s) in water, and what does it indicate about its initial dissolution process?

  1. -31 kJ mol⁻¹; exothermic
  2. +2041 kJ mol⁻¹; endothermic
  3. +31 kJ mol⁻¹; endothermic
  4. -2041 kJ mol⁻¹; exothermic
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Answer: (C) +31 kJ mol⁻¹; endothermic

The enthalpy of solution is given by Δ_solH° = Δ_latticeH° + Δ_hydH°. Here, Δ_solH° = (+1036 kJ mol⁻¹) + (-1005 kJ mol⁻¹) = +31 kJ mol⁻¹. Since Δ_solH° is positive, the dissolution process is endothermic.

easy

5. Read the following two statements: Statement I: A large negative electron gain enthalpy of a non-metal favours the formation of a stable ionic compound. Statement II: The qualitative ease of ionic bond formation depends primarily on the low ionization enthalpy of the metal and the high lattice enthalpy of the resulting crystal.

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

Both statements are correct according to NCERT. Ionic bonds are readily formed between elements with low ionization enthalpies and elements with high negative electron gain enthalpies, while the overall driving force and stability are heavily sustained by a high lattice enthalpy.

hard

6. Read the following two statements regarding lattice enthalpy and solubility: Statement I: An ionic solid is readily soluble in water only if the magnitude of its hydration enthalpy is greater than or equal to its lattice enthalpy. Statement II: Lattice enthalpy accounts for the net attractive forces between ions of opposite charge as well as repulsive interactions between ions of like charge and electron clouds in the three-dimensional crystal. Choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

For dissolution of an ionic solid to occur spontaneously (considering enthalpy balance), the energy released on hydration (|Δ_hydH|) must overcome the lattice enthalpy (|Δ_latticeH|). Statement II is also correct; lattice enthalpy represents the net result of all attractive and repulsive electrostatic forces throughout the entire infinite crystal lattice.

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7. Consider the following two statements regarding ionic bond formation: Statement I: A large negative electron gain enthalpy of the non-metal atom always completely compensates for the energy absorbed during the ionization of the metal atom. Statement II: The stability of an ionic compound is primarily due to its high positive lattice enthalpy released during crystal formation. In the light of the above statements, choose the most appropriate answer:

  1. Both Statement I and Statement II are correct.
  2. Statement I is correct but Statement II is incorrect.
  3. Statement I is incorrect but Statement II is correct.
  4. Both Statement I and Statement II are incorrect.
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Answer: (C) Statement I is incorrect but Statement II is correct.

Statement I is incorrect because the electron gain enthalpy of a non-metal is generally smaller in magnitude than the ionization enthalpy of the metal (ionization requires more energy than electron gain releases). Statement II is correct because the release of large lattice energy during crystal lattice formation provides the main thermodynamic driving force for ionic compound formation.

easy

8. Which among the following alkaline earth metal oxides exhibits the lowest magnitude of lattice enthalpy?

  1. BaO
  2. MgO
  3. CaO
  4. BeO
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Answer: (A) BaO

All alkaline earth oxides contain divalent cations (M²⁺) and the oxide anion (O²⁻). Since the ionic charges are identical, lattice enthalpy depends inversely on the interionic distance (r₊ + r₋). Because Ba²⁺ has the largest ionic radius, BaO has the largest interionic separation and therefore the lowest lattice enthalpy.

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9. Given the following thermochemical data: Sublimation enthalpy of K(s) = +89 kJ mol⁻¹ Ionization enthalpy of K(g) = +419 kJ mol⁻¹ Dissociation enthalpy of Cl₂(g) = +242 kJ mol⁻¹ Electron gain enthalpy of Cl(g) = -349 kJ mol⁻¹ Enthalpy of formation of KCl(s) = -436 kJ mol⁻¹ What is the lattice enthalpy (defined as the energy required to dissociate 1 mole of KCl(s) into gaseous ions) of KCl(s)?

  1. +652 kJ mol⁻¹
  2. +716 kJ mol⁻¹
  3. +824 kJ mol⁻¹
  4. +584 kJ mol⁻¹
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Answer: (B) +716 kJ mol⁻¹

Applying Born-Haber cycle: Δ_fH° = Δ_subH°(K) + IE(K) + (1/2)Δ_bondH°(Cl₂) + Δ_egH°(Cl) - U_lattice. Here -436 = 89 + 419 + (1/2)(242) + (-349) - U_lattice. -436 = 89 + 419 + 121 - 349 - U_lattice = 280 - U_lattice. Thus, U_lattice = 280 + 436 = +716 kJ mol⁻¹.

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10. Read the following two statements: Statement I: The first electron gain enthalpy of oxygen is exothermic, but its second electron gain enthalpy is strongly endothermic. Statement II: Formation of oxide-containing ionic solids such as CaO(s) is thermodynamically favourable primarily due to their exceptionally high lattice enthalpy. Choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

The addition of a second electron to O⁻ experiences strong electrostatic repulsion, making Δ_egH₂ endothermic (+780 kJ mol⁻¹). Despite this energy requirement, divalent oxides like CaO are highly stable because the divalent ions (Ca²⁺ and O²⁻) generate very high lattice enthalpies that easily compensate for the positive enthalpy terms.

easy

11. Ionic compounds conduct electricity:

  1. In the solid state only
  2. Never
  3. Only as dry powders
  4. In the molten state or in aqueous solution
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Answer: (D) In the molten state or in aqueous solution

In solid ionic compounds ions are fixed in the lattice. In molten or dissolved form the ions are mobile and carry current.

NCERT Class 11

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12. Given the following thermochemical values for sodium fluoride, NaF(s): Enthalpy of sublimation of Na(s) = +108 kJ mol⁻¹ Ionization enthalpy of Na(g) = +496 kJ mol⁻¹ Bond dissociation enthalpy of F₂(g) = +158 kJ mol⁻¹ Electron gain enthalpy of F(g) = -328 kJ mol⁻¹ Standard enthalpy of formation of NaF(s) = -574 kJ mol⁻¹ The lattice enthalpy (dissociation) of NaF(s) is:

  1. +929 kJ mol⁻¹
  2. +850 kJ mol⁻¹
  3. +769 kJ mol⁻¹
  4. +651 kJ mol⁻¹
Show answer and explanation

Answer: (A) +929 kJ mol⁻¹

Applying the Born-Haber cycle: Δ_fH° = Δ_subH° + Δ_iH° + 1/2 Δ_bondH° + Δ_egH° - Δ_latticeH°. Substituting values: -574 = 108 + 496 + 79 - 328 - Δ_latticeH° = 355 - Δ_latticeH°. Thus, Δ_latticeH° = 355 + 574 = +929 kJ mol⁻¹.

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