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Saturated and Unsaturated Hydrocarbons – Long Answer Questions

Medium Level (Application & Explanation)

Q1. Explain the differences between saturated and unsaturated hydrocarbons with respect to bonding, general formula, reactivity, typical reactions, and uses. Give suitable examples.

Answer:

  • Saturated hydrocarbons (alkanes) have only single C–C bonds and follow the general formula CₙH₂ₙ₊₂. Because every carbon has the maximum hydrogen attached, they are called “saturated.”
  • Unsaturated hydrocarbons contain at least one double (C=C) or triple (C≡C) bond. Alkenes follow CₙH₂ₙ and alkynes follow CₙH₂ₙ₋₂. They carry fewer hydrogens compared to alkanes with the same carbon number.
  • In terms of reactivity, alkanes are less reactive and mainly undergo substitution reactions. Alkenes and alkynes are more reactive and undergo addition reactions where atoms add across multiple bonds.
  • Uses: Alkanes (e.g., methane, propane, butane) are common fuels (CNG, LPG). Alkenes/alkynes (e.g., ethene, ethyne) are used for plastics, chemicals, welding.
  • Examples: Ethane C₂H₆ (alkane), Ethene C₂H₄ (alkene), Ethyne C₂H₂ (alkyne).

Q2. Why are alkanes less reactive than alkenes and alkynes? Explain using the nature of bonds and give examples of typical reactions each class undergoes.

Answer:

  • Alkanes contain only single C–C and C–H sigma bonds which are strong and stable, making them less reactive. Due to this stability, they generally resist addition and instead show substitution reactions, such as gradual halogenation (e.g., chlorination of methane, CH₄).
  • Alkenes and alkynes possess π-bonds in C=C or C≡C which are electron-rich and easier to break, so they are more reactive. They readily undergo addition reactions where reagents add across the multiple bond.
  • Examples: Ethene (C₂H₄) undergoes addition of bromine (decolorizes bromine water), hydrogen (forms ethane), or polymerization (forms polyethylene). Ethyne (C₂H₂) adds hydrogen stepwise to give ethene then ethane.
  • Thus, the presence or absence of multiple bonds largely governs reactivity and reaction type.

Q3. Derive and use the general formulas CₙH₂ₙ₊₂, CₙH₂ₙ, and CₙH₂ₙ₋₂ to compare hydrogen content for compounds with five carbon atoms. What does this reveal about saturation?

Answer:

  • For n = 5:
    • Alkane (saturated): C₅H₁₂ using CₙH₂ₙ₊₂
    • Alkene (one double bond): C₅H₁₀ using CₙH₂ₙ
    • Alkyne (one triple bond): C₅H₈ using CₙH₂ₙ₋₂
  • The hydrogen count decreases as we go from alkane → alkene → alkyne, reflecting increasing unsaturation (more multiple bonds means fewer hydrogens).
  • This shows why alkanes are called “saturated”—they hold the maximum number of hydrogens for a given carbon chain. In contrast, unsaturated hydrocarbons have space to add more hydrogens via addition reactions (e.g., hydrogenation).
  • In practice, this difference impacts properties and uses: C₅H₁₂ (pentane) is a fuel component, while C₅H₁₀ and C₅H₈ can serve as reactive feedstocks for making chemicals and polymers due to their multiple bonds.

Q4. Describe the bromine water test used to distinguish saturated and unsaturated hydrocarbons. Include materials, steps, observations, and the chemical reasoning behind the color change.

Answer:

  • Materials: Test tubes, bromine water (orange), hexane (saturated), hexene (unsaturated), droppers.
  • Steps:
    1. Take two test tubes with ~2 mL bromine water each.
    2. Add a few drops of hexane to tube 1 and hexene to tube 2.
    3. Shake gently and observe.
  • Observations:
    • With hexane, the orange color persists—no visible reaction.
    • With hexene, the orange color disappears—solution becomes colorless.
  • Reason: Unsaturated hydrocarbons undergo an addition reaction with bromine across the C=C bond, effectively consuming bromine and causing decolorization. Saturated hydrocarbons lack such reactive multiple bonds, so no addition occurs and color remains.
  • This simple test quickly identifies unsaturation and is widely used in school labs and basic organic analysis.

Q5. Relate structure to use: Explain why methane, propane, and butane are widely used as fuels, whereas ethene and ethyne are preferred as chemical feedstocks and for specialized uses.

Answer:

  • Methane (CH₄), propane (C₃H₈), and butane (C₄H₁₀) are alkanes with single C–C bonds, making them stable, less reactive fuels. They burn cleanly with sufficient oxygen to yield CO₂ and H₂O, which is why they’re used in CNG and LPG for domestic and transport fuel. Their stability also aids safe storage and handling.
  • Ethene (C₂H₄) and ethyne (C₂H₂) are unsaturated. Ethene is highly valuable as a building block for plastics (e.g., polyethylene) via addition polymerization, and for making many chemicals. Ethyne has a triple bond and high flame temperature, making it ideal for welding and metal cutting.
  • Thus, bonding and reactivity dictate application: saturated for combustion, unsaturated for chemical synthesis and specialized high-temperature uses.

High Complexity (Analytical & Scenario-Based)

Q6. A colorless gas follows the formula CₙH₂ₙ and rapidly decolorizes bromine water. Predict its class, propose two likely reactions it can undergo, and justify one industrial use.

Answer:

  • The formula CₙH₂ₙ and bromine water decolorization indicate an alkene (unsaturated hydrocarbon). A common example is ethene (C₂H₄).
  • Likely reactions:
    • Addition of bromine across the C=C bond, causing decolorization—a diagnostic test for unsaturation.
    • Addition of hydrogen (hydrogenation) to form the corresponding alkane (e.g., C₂H₄ → C₂H₆).
    • It can also undergo addition polymerization to form long-chain polymers.
  • Industrial use: Ethene is a key feedstock for making polyethylene (plastic bags, containers), produced by polymerization of the double bond. Its reactive double bond allows multiple molecules to join, giving materials with useful strength, flexibility, and chemical resistance, underpinning large-scale plastic manufacturing.

Q7. Your city wants a household fuel that is easy to store, safe to transport, and burns cleanly. Evaluate LPG (propane/butane) versus acetylene and recommend with reasons.

Answer:

  • LPG (propane C₃H₈ / butane C₄H₁₀) are saturated alkanes. They are less reactive, liquefiable under moderate pressure, and easy to store/transport in cylinders. On combustion with adequate oxygen, they burn cleanly, producing mainly CO₂ and H₂O, with minimal soot—ideal for domestic kitchens.
  • Acetylene (ethyne, C₂H₂) is an unsaturated alkyne with a triple bond. It is more reactive, supports very high flame temperatures, and is preferred for welding/cutting, not routine cooking. It requires careful handling and is typically stored dissolved (e.g., in acetone) to reduce hazards.
  • Recommendation: Choose LPG due to its stability, safety, easy handling, and clean combustion. Acetylene is specialized for industrial flames, not general household use.

Q8. A food technologist wants to convert liquid vegetable oil into a semi-solid spread. Using your understanding of saturated and unsaturated hydrocarbons, explain the process and the structural change involved.

Answer:

  • Vegetable oils are rich in unsaturated chains (contain C=C double bonds). These multiple bonds make the molecules less tightly packed, keeping oils liquid at room temperature.
  • Hydrogenation is used to convert unsaturated oils into more saturated forms by adding hydrogen across C=C bonds, turning them into C–C single bonds. As unsaturation decreases, the chains can pack more closely, raising the melting point and yielding semi-solid spreads like margarine.
  • This process exemplifies addition reactions typical of unsaturated hydrocarbons, where atoms add across double bonds.
  • Conceptually, the change is from CₙH₂ₙ (or lower hydrogen content) toward CₙH₂ₙ₊₂-like saturated structures, increasing stability and altering physical properties. Thus, controlling degree of saturation tunes texture and spreadability.

Q9. You receive two gases: Gas A is C₂H₆ and Gas B is C₂H₄. Design a simple test and predict results. Also, recommend which gas suits plastic manufacturing and which suits home fuel, with reasons.

Answer:

  • Test: Use bromine water. Bubble each gas through separate bromine water samples.
    • Gas A (C₂H₆, ethane): No reaction; orange color persists—indicates a saturated alkane.
    • Gas B (C₂H₄, ethene): Decolorizes bromine water—indicates an unsaturated alkene via addition across the C=C bond.
  • Recommendations:
    • Plastic manufacturing: Choose ethene (C₂H₄) because its double bond enables addition polymerization to form polyethylene, a major plastic.
    • Home fuel: Use ethane (C₂H₆) as part of CNG/LPG blends of alkanes due to clean combustion, stability, and ease of handling. Alkanes are less reactive, making them safer for storage and routine domestic use.

Q10. A lab sample has formula C₄H₆. It rapidly reacts with bromine water and adds hydrogen to become C₄H₁₀. Analyze what this reveals about its structure, reactivity, and classification.

Answer:

  • The conversion C₄H₆ → C₄H₁₀ by adding hydrogen shows the sample has fewer hydrogens than an alkane, indicating unsaturation. The rapid bromine decolorization further confirms a multiple bond.
  • Since the alkane with four carbons is C₄H₁₀, and our c...

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