Chemistry of Sulfur (S) - Practice Questions

Multiple Choice Questions (MCQs)

Q1: Allotropes of Sulfur

Which allotrope of sulfur is stable at room temperature (25°C) and exists as S₈ rings? (A) Monoclinic sulfur (B) Rhombic sulfur (C) Plastic sulfur (D) Colloidal sulfur

Correct Answer: (B)

Explanation: Alpha-sulfur, also known as rhombic sulfur, is the most stable allotropic form of sulfur at temperatures below 95.6°C (room temperature). It consists of S₈ puckered ring structures, where each sulfur atom is bonded to two other sulfur atoms.

Q2: Redox Properties of SO₂

Sulfur dioxide (SO₂) can act as both an oxidizing agent and a reducing agent. Which of the following reactions best exemplifies its reducing nature? (A) SO₂ + 2H₂S → 3S + 2H₂O (B) SO₂ + Cl₂ → SO₂Cl₂ (C) SO₂ + 2NaOH → Na₂SO₃ + H₂O (D) SO₂ + 2Mg → 2MgO + S

Correct Answer: (B)

Explanation: For SO₂ to act as a reducing agent, sulfur’s oxidation state must increase.

• In reaction (A), sulfur’s oxidation state changes from +4 (in SO₂) to 0 (in S). This is a reduction, so SO₂ acts as an oxidizing agent.
• In reaction (B), sulfur’s oxidation state changes from +4 (in SO₂) to +6 (in SO₂Cl₂). This is an oxidation, meaning SO₂ acts as a reducing agent by donating electrons to Cl₂.
• In reaction (C), SO₂ reacts as an acidic oxide with a base, which is an acid-base reaction, not primarily a redox reaction.
• In reaction (D), sulfur’s oxidation state changes from +4 (in SO₂) to 0 (in S). This is a reduction, so SO₂ acts as an oxidizing agent.

Q3: Contact Process Detail

In the Contact Process for the manufacture of sulfuric acid, sulfur trioxide (SO₃) is absorbed in 98% H₂SO₄ instead of being directly absorbed in water. The primary reason for this is: (A) SO₃ is insoluble in water. (B) The direct reaction of SO₃ with water is highly exothermic, forming a dense fog of H₂SO₄ that is difficult to condense. (C) 98% H₂SO₄ acts as a better catalyst for the hydration of SO₃. (D) This method is economically less viable but produces ultra-pure H₂SO₄.

Correct Answer: (B)

Explanation: Direct absorption of SO₃ in water leads to a highly exothermic reaction, producing a fine mist of sulfuric acid particles (H₂SO₄ fog). This mist is difficult to condense and separate, leading to a significant loss of product and environmental concerns. By absorbing SO₃ in concentrated H₂SO₄, oleum (H₂S₂O₇) is formed: SO₃(g) + H₂SO₄(conc.) → H₂S₂O₇(l) (Oleum) This oleum can then be diluted with a calculated amount of water to obtain sulfuric acid of the desired concentration: H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l) This two-step process avoids the formation of acid mist and ensures efficient production.

Assertion-Reason Questions

Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Mark the correct option out of the choices given below: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.

Q1

Assertion (A): H₂S is a stronger reducing agent than H₂O. Reason (R): The bond dissociation enthalpy of H-S bond is lower than that of H-O bond.

Correct Answer: (A)

Explanation: As we move down Group 16 from oxygen to sulfur, the atomic size increases. Consequently, the H-S bond length is greater than the H-O bond length. This increased bond length leads to a decrease in bond dissociation enthalpy for the H-S bond compared to the H-O bond (H-S bond enthalpy is ~347 kJ/mol, H-O bond enthalpy is ~463 kJ/mol). A weaker H-S bond means it can be broken more easily, allowing sulfur to readily lose electrons (get oxidized) and thus act as a stronger reducing agent. Therefore, H₂S is a stronger reducing agent than H₂O, and the lower bond dissociation enthalpy of the H-S bond correctly explains this.

Q2

Assertion (A): Concentrated H₂SO₄ is a strong dehydrating agent. Reason (R): Concentrated H₂SO₄ has a strong affinity for water molecules.

Correct Answer: (A)

Explanation: Concentrated sulfuric acid has an extremely strong affinity for water, primarily due to the high enthalpy of hydration (i.e., strong exothermic reaction with water). This property allows it to effectively remove water from various substances, including organic compounds (e.g., charring of sugar, dehydration of ethanol) and even elements of water from compounds. This strong affinity for water molecules is precisely the reason why it acts as a powerful dehydrating agent.

Short Answer Questions

Q1: Catenation in Sulfur and Oxygen

Explain why sulfur exhibits catenation to a much greater extent than oxygen.

Model Answer: Catenation is the ability of an element to form bonds with atoms of itself, leading to the formation of chains or rings. Both oxygen and sulfur belong to Group 16, but their catenation abilities differ significantly:

1. Oxygen: Oxygen exhibits very limited catenation, primarily forming O₂ molecules and, to a lesser extent, ozone (O₃) and hydrogen peroxide (H₂O₂). The O-O single bond enthalpy (approx. 146 kJ/mol) is relatively low. This is attributed to the small size of oxygen atoms, which results in strong lone pair-lone pair repulsion between adjacent oxygen atoms in a chain, making longer chains unstable.

2. Sulfur: Sulfur displays extensive catenation, forming stable S₈ puckered rings (found in rhombic and monoclinic sulfur) and even long polymeric chains (as in plastic sulfur). The S-S single bond enthalpy (approx. 213 kJ/mol) is significantly higher than the O-O bond enthalpy. The larger atomic size of sulfur reduces the lone pair-lone pair repulsion between adjacent sulfur atoms. This allows for stronger and more stable S-S single bonds, facilitating the formation of diverse and stable polyatomic structures.

Q2: Acidic Nature of Sulfur Dioxide

Describe the acidic nature of sulfur dioxide (SO₂). Write relevant chemical equations.

Model Answer: Sulfur dioxide (SO₂) is a characteristic acidic oxide. Its acidic nature is evident from the following reactions:

1. Reaction with Water: SO₂ dissolves in water to form sulfurous acid (H₂SO₃), which is a weak diprotic acid. This is why an aqueous solution of SO₂ turns blue litmus red. SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq)

2. Reaction with Bases: Being an acidic oxide, SO₂ readily reacts with bases to form sulfites. For example, it reacts with sodium hydroxide to form sodium sulfite: SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l) If excess SO₂ is passed through the solution, the sulfite can react further to form a bisulfite: Na₂SO₃(aq) + H₂O(l) + SO₂(g) → 2NaHSO₃(aq)

3. Reaction with Basic Oxides: SO₂ also reacts with basic oxides to form sulfites. This property is utilized in industrial processes like flue gas desulfurization, where SO₂ is removed from exhaust gases by reacting it with calcium carbonate (limestone) or calcium hydroxide (slaked lime): SO₂(g) + CaO(s) → CaSO₃(s) SO₂(g) + CaCO₃(s) → CaSO₃(s) + CO₂(g)

These reactions clearly demonstrate the acidic character of sulfur dioxide.

High-Order Thinking Skills (HOTS) Question

Q: Drying H₂S Gas

Concentrated H₂SO₄ is generally not used for drying H₂S gas. Explain why, including relevant chemical equations. If not H₂SO₄, what other drying agent could be used?

Detailed Chemical Explanation: Concentrated sulfuric acid (H₂SO₄) is a powerful dehydrating agent, but it is unsuitable for drying hydrogen sulfide (H₂S) gas. The primary reason is that concentrated H₂SO₄ is also a strong oxidizing agent, while H₂S is a strong reducing agent. When these two substances come into contact, a redox reaction occurs instead of simple dehydration, leading to the consumption of H₂S and the formation of undesirable byproducts.

Chemical Equation: The reaction involves the oxidation of H₂S (sulfur’s oxidation state changes from -2 to 0) and the reduction of H₂SO₄ (sulfur’s oxidation state changes from +6 to +4). A common representation of this reaction is: H₂S(g) + H₂SO₄(conc.) → S(s) + SO₂(g) + 2H₂O(l) In this reaction, H₂S is oxidized to elemental sulfur (S), and H₂SO₄ is reduced to sulfur dioxide (SO₂). This not only fails to dry the H₂S but also produces solid sulfur and another gas (SO₂), complicating the process.

Alternative Drying Agents: Since H₂S is a reducing agent and also a weak acid, suitable drying agents must be:

1. Non-oxidizing: To prevent redox reactions.
2. Non-basic: To avoid acid-base reactions.

Therefore, neutral and non-oxidizing drying agents are preferred. Good choices include:

• Anhydrous Phosphorus Pentoxide (P₂O₅): This is a very effective dehydrating agent and does not react with H₂S under normal conditions.
• Anhydrous Magnesium Sulfate (MgSO₄): A neutral salt that acts as a good drying agent.
• Silica gel: A porous form of silicon dioxide that effectively absorbs moisture.

Fused anhydrous calcium chloride (CaCl₂) is generally avoided as it can form an unstable adduct (CaCl₂·xH₂S) with H₂S, although its reactivity is less severe than with H₂SO₄. The most commonly recommended and safest alternative for drying H₂S is anhydrous P₂O₅.