Chemistry of Zinc (Zn) - Practice Questions

Explanation:

• A) Zinc exhibits a single stable oxidation state of +2. This is correct. In its compounds, Zinc almost exclusively shows a +2 oxidation state, where its electron configuration becomes [Ar] 3d¹⁰.
• B) Zinc belongs to Group 12 of the d-block elements. This is correct. Zinc (atomic number 30) is the first element in Group 12, followed by Cadmium and Mercury.
• C) Zinc forms coloured compounds due to d-d transitions. This is incorrect. In the +2 oxidation state, Zinc has a completely filled d-subshell (3d¹⁰). As there are no empty d-orbitals available, d-d electronic transitions are not possible. Therefore, most Zinc compounds are diamagnetic and colourless.
• D) Zinc metal reacts with both acids and strong bases. This is correct. Zinc is an amphoteric metal. It reacts with acids to produce hydrogen gas (e.g., Zn + H₂SO₄ → ZnSO₄ + H₂) and with strong bases to form zincates (e.g., Zn + 2NaOH + 2H₂O → Na₂[Zn(OH)₄] + H₂).

Explanation: The reaction of metals with nitric acid is complex and depends on the concentration of the acid, the activity of the metal, and temperature.

• Very dilute nitric acid reacts with active metals like Zinc to produce nitrous oxide (N₂O). 4Zn + 10HNO₃ (very dilute) → 4Zn(NO₃)₂ + 5H₂O + N₂O
• Dilute nitric acid often produces nitric oxide (NO) or even hydrogen gas with very active metals. 3Zn + 8HNO₃ (dilute) → 3Zn(NO₃)₂ + 4H₂O + 2NO
• Concentrated nitric acid typically produces nitrogen dioxide (NO₂). Zn + 4HNO₃ (conc.) → Zn(NO₃)₂ + 2H₂O + 2NO₂ However, with dilute nitric acid and Zinc, the formation of N₂O is a common observation for moderate dilution. For very dilute nitric acid, H₂ can be produced, but N₂O is more characteristic of moderately dilute conditions for moderately active metals like Zinc. Given the options, N₂O is a primary product for dilute nitric acid with Zinc.

Explanation:

• A) Zinc blende (Sphalerite): This is Zinc sulfide (ZnS), the most important ore of Zinc worldwide.
• B) Calamine: This refers to Zinc carbonate (ZnCO₃), also known as smithsonite. It’s a significant ore of Zinc. (Note: Historically, the term calamine also referred to a mixture of zinc oxide and ferric oxide, but in metallurgy, it typically refers to ZnCO₃).
• C) Zincite: This is Zinc oxide (ZnO), another important ore of Zinc.
• D) Bauxite: Bauxite is an ore of Aluminium, primarily consisting of hydrated aluminium oxides like gibbsite (Al(OH)₃), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH)). It does not contain Zinc.

Explanation:

• Assertion (A) is true. The IUPAC definition for transition elements states that they are elements which have incompletely filled d-orbitals in their ground state or in any of their common oxidation states.
• Reason (R) is also true and correctly explains A. Zinc’s electronic configuration is [Ar] 3d¹⁰ 4s². In its most common and stable oxidation state of +2, it loses the two 4s electrons, resulting in the configuration [Ar] 3d¹⁰. Since neither the ground state nor the +2 oxidation state has an incompletely filled d-orbital, Zinc does not fit the definition of a transition element, even though it is a d-block element.

Explanation:

• Assertion (A) is true. Zinc in its common +2 oxidation state has a 3d¹⁰ configuration. All the electrons in the d-orbitals are paired, leaving no unpaired electrons. Therefore, Zinc compounds are diamagnetic.
• Reason (R) is also true. Zinc(II) is a d¹⁰ ion and can form tetrahedral or octahedral complexes with various ligands, including strong field ligands (e.g., [Zn(NH₃)₄]²⁺, [Zn(CN)₄]²⁻).
• However, the reason (forming complexes with strong field ligands) does not explain the diamagnetic nature. The diamagnetism arises from the completely filled d-subshell (3d¹⁰) in the Zn²⁺ ion, which means there are no unpaired electrons, irrespective of the ligand strength. The strong field ligands affect crystal field splitting, but for d¹⁰ ions, this splitting does not lead to unpaired electrons or paramagnetism.

According to the IUPAC definition, transition elements are those which have incompletely filled d-orbitals in their ground state or in any of their common oxidation states.

• Electronic Configuration: In its ground state, Zinc has a completely filled 3d¹⁰ subshell. In its only common and stable oxidation state, +2, it loses the two 4s electrons to form Zn²⁺, which has the configuration [Ar] 3d¹⁰. Since neither the ground state nor the +2 oxidation state possesses an incompletely filled d-orbital, Zinc does not meet the IUPAC definition of a transition element.
• Properties: This electronic configuration (3d¹⁰) leads to several properties that differentiate Zinc from typical transition elements:
  1. Variable Oxidation States: Unlike most transition elements which exhibit multiple oxidation states due to the participation of d-electrons, Zinc exhibits only a +2 oxidation state.
  2. Colour of Compounds: Zinc compounds are generally colourless (white) in the solid state or in solution because there are no unpaired d-electrons to undergo d-d electronic transitions, which are responsible for colour in most transition metal compounds.
  3. Catalytic Activity: Typical transition metals often act as catalysts due to their variable oxidation states and ability to form unstable intermediates. Zinc, primarily restricted to a +2 state, exhibits limited catalytic activity compared to true transition metals.

b) Zinc oxide with dilute hydrochloric acid: Zinc oxide is an amphoteric oxide. It reacts with acids to form a salt and water. ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)

1. Electronic Configuration:

   • Zinc has an atomic number of 30. Its ground state electronic configuration is [Ar] 3d¹⁰ 4s².
   • In its common and stable oxidation state, Zinc forms the Zn²⁺ ion by losing its two 4s electrons. The electronic configuration of Zn²⁺ is [Ar] 3d¹⁰.

2. Colour of Compounds (White):

   • The colour in most transition metal compounds arises from d-d electronic transitions. When a transition metal ion with an incompletely filled d-subshell is placed in a ligand field (e.g., in a complex or a crystal lattice), the d-orbitals split into different energy levels.
   • Electrons can absorb specific wavelengths of visible light to jump from a lower energy d-orbital to a higher energy d-orbital (d-d transition). The colour observed is the complementary colour of the light absorbed.
   • In the Zn²⁺ ion (3d¹⁰), the d-subshell is completely filled. There are no empty d-orbitals available for electrons to transition into. Therefore, d-d transitions are not possible.
   • As a result, Zn²⁺ compounds do not absorb visible light via d-d transitions and thus appear white or colourless because they transmit or reflect all wavelengths of visible light. Any colour observed in some Zinc compounds might be due to charge transfer transitions involving the ligand or impurities, but not due to Zn²⁺ itself.

3. Magnetic Properties (Diamagnetic):

   • Paramagnetism occurs in substances that contain one or more unpaired electrons. These unpaired electrons have a magnetic moment and are attracted to an external magnetic field.
   • Diamagnetism occurs in substances where all electrons are paired. Paired electrons have opposite spins, cancelling out their magnetic moments. Diamagnetic substances are weakly repelled by an external magnetic field.
   • Since the Zn²⁺ ion has a 3d¹⁰ electronic configuration, all ten d-electrons are paired in the five d-orbitals. There are no unpaired electrons in Zn²⁺.
   • Consequently, Zinc(II) compounds are diamagnetic because they lack unpaired electrons.

In summary, the completely filled 3d¹⁰ electronic configuration of the Zn²⁺ ion is the fundamental reason why Zinc(II) compounds are typically white (due to the absence of d-d transitions) and diamagnetic (due to the absence of unpaired electrons).