2. Give short answer.
(i) Give three examples of saturated hydrocarbons.
Answer: Three examples of saturated hydrocarbons (alkanes):
1. Methane (CH₄): The simplest alkane.
2. Ethane (C₂H₆): A two-carbon alkane.
3. Propane (C₃H₈): A three-carbon alkane. These hydrocarbons contain only single covalent bonds between carbon atoms and are called saturated because they contain the maximum possible number of hydrogen atoms.
(ii) Draw structure for ethane.
Answer: (Diagram Description) Ethane (C₂H₆) consists of two carbon atoms bonded by a single covalent bond. Each carbon atom is bonded to three hydrogen atoms. Structure: H₃C-CH₃ or a full displayed formula showing all bonds: H-C-C-H with three H atoms attached to each C.
(iii) Draw structural formulas of an alkane containing five carbon atoms.
Answer: The alkane with five carbon atoms is pentane (C₅H₁₂). It has three isomers:
1. n-Pentane: CH₃-CH₂-CH₂-CH₂-CH₃ (straight chain).
2. Isopentane (2-methylbutane): CH₃-CH(CH₃)-CH₂-CH₃ (branched).
3. Neopentane (2,2-dimethylpropane): C(CH₃)₄ (highly branched).
(iv) What do you mean by hydrogenation reaction? Give one example.
Answer: Hydrogenation is a chemical reaction in which hydrogen (H₂) is added to an unsaturated compound (alkene or alkyne) in the presence of a metal catalyst (Ni, Pt, Pd) to convert it into a saturated compound (alkane).
Example: Hydrogenation of ethene to ethane:
CH₂=CH₂ + H₂ [source symbol unavailable] (Ni catalyst) [source symbol unavailable] CH₃-CH₃
(v) What is meant by cracking?
Answer: Cracking is the process of breaking down large hydrocarbon molecules into smaller, more useful molecules (e.g., petrol, diesel) by applying heat and pressure, often in the presence of a catalyst. It is used in petroleum refining to increase the yield of lighter fractions from heavier crude oil fractions. Example: C₁₀H₂₂ [source symbol unavailable] C₈H₁₈ + C₂H₄.
3. Discuss methods for the preparation ethane.
Answer: Ethane (C₂H₆) can be prepared by several methods:
1. Hydrogenation of Ethene: Ethene reacts with hydrogen in the presence of a nickel catalyst.
CH₂=CH₂ + H₂ [source symbol unavailable] (Ni) [source symbol unavailable] CH₃-CH₃
2. Hydrogenation of Ethyne: Ethyne reacts with hydrogen in the presence of a nickel catalyst.
HC[source symbol unavailable]CH + 2H₂ [source symbol unavailable] (Ni) [source symbol unavailable] CH₃-CH₃
3. Reduction of Ethyl Chloride: Ethyl chloride is reduced with Zn/HCl.
CH₃-CH₂Cl + 2[H] [source symbol unavailable] (Zn/HCl) [source symbol unavailable] CH₃-CH₃ + HCl
4. Wurtz Reaction: Ethyl bromide reacts with sodium metal in dry ether.
2CH₃-CH₂Br + 2Na [source symbol unavailable] (dry ether) [source symbol unavailable] CH₃-CH₂-CH₂-CH₃ + 2NaBr (This produces butane, not ethane. For ethane, we use Kolbe's electrolysis.)
5. Kolbe's Electrolysis: Sodium acetate solution is electrolyzed.
2CH₃COONa + 2H₂O [source symbol unavailable] (electrolysis) [source symbol unavailable] C₂H₆ + 2CO₂ + H₂ + 2NaOH
4. Describe properties of alkanes.
Answer: Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. Their properties include:
Physical State: C₁-C₄ are gases, C₅-C₁₇ are liquids, and C₁₈+ are solids at room temperature. Solubility: Insoluble in water but soluble in organic solvents. Density: Less dense than water. Boiling Point: Increases with increasing molecular mass. Chemical Reactivity: Relatively unreactive due to strong C-C and C-H sigma bonds. They undergo substitution reactions (e.g., halogenation) and combustion. Combustion: Burn in oxygen to produce CO₂, H₂O, and heat. Halogenation: React with halogens in the presence of UV light to form haloalkanes.
5. Write a chemical equation to show the preparation of an alkane from an alkene and an alkyne.
Answer:
From Alkene (Hydrogenation): CH₂=CH₂ + H₂ [source symbol unavailable] (Ni) [source symbol unavailable] CH₃-CH₃ From Alkyne (Hydrogenation): HC[source symbol unavailable]CH + 2H₂ [source symbol unavailable] (Ni) [source symbol unavailable] CH₃-CH₃
