Enhance your understanding of hydrocarbons with solved exercises. Cover key topics like types of hydrocarbons, reactions, and properties to reinforce your knowledge and excel in exams.
Short Questions:
- Differentiate between saturated and unsaturated hydrocarbons.
- Saturated hydrocarbons: Contain only single bonds between carbon atoms (e.g., alkanes).
- Unsaturated hydrocarbons: Contain one or more double or triple bonds between carbon atoms (e.g., alkenes and alkynes).
- A compound consisting of four carbon atoms has a triple bond in it. How many hydrogen atoms are present in it?
- The compound is butyne (C₄H₆). There are 6 hydrogen atoms.
- Why are the alkanes called ‘paraffins’?
- Alkanes are called paraffins because they are relatively unreactive due to the presence of only single bonds between carbon atoms, making them resistant to chemical reactions.
- What do you know about hydrogenation of alkenes?
- Hydrogenation of alkenes is a chemical reaction in which hydrogen (H₂) is added across the double bond of alkenes, converting them into alkanes.
- How are alkyl halides reduced?
- Alkyl halides can be reduced by reagents such as zinc and hydrochloric acid (Zn/HCl) to alkanes.
- Why are the alkanes used as fuel?
- Alkanes are used as fuels because they release a large amount of energy when burned, producing carbon dioxide and water.
- How can you prepare ethene from alcohol and ethyl bromide?
- From alcohol: Ethene can be prepared by dehydrating ethanol (C₂H₅OH) using concentrated sulfuric acid (H₂SO₄).
- From ethyl bromide: Ethene can be prepared by treating ethyl bromide (C₂H₅Br) with alcoholic KOH, leading to elimination of HBr.
- Identify propane from propene with a chemical test.
- Propene decolorizes bromine water, while propane does not react with bromine water.
- Why are the alkenes called ‘olefins’?
- Alkenes are called olefins because they form oily liquids when reacted with halogens such as chlorine.
- Why alkane can’t be oxidized with KMnO₄ solution?
- Alkanes are saturated hydrocarbons and do not have any reactive sites like double or triple bonds, so they do not react with strong oxidizing agents like potassium permanganate (KMnO₄).
- What are the addition reactions? Explain with an example.
- Addition reactions: In these reactions, atoms or groups of atoms are added to the double or triple bond of unsaturated compounds.
Example: Ethene (C₂H₄) reacts with bromine (Br₂) to form 1,2-dibromoethane (C₂H₄Br₂).
- Addition reactions: In these reactions, atoms or groups of atoms are added to the double or triple bond of unsaturated compounds.
- Justify that alkanes give substitution reactions.
- Alkanes undergo substitution reactions where a hydrogen atom is replaced by a halogen in the presence of light (e.g., methane reacts with chlorine to form chloromethane and hydrogen chloride).
- Both alkenes and alkynes are unsaturated hydrocarbons. State the one most significant difference between them.
- Alkenes contain at least one double bond, while alkynes contain at least one triple bond.
- Write the molecular, dot and cross, and structural formula of ethyne.
- Molecular formula: C₂H₂
- Dot and cross diagram: Two carbon atoms with a triple bond between them, each bonded to a hydrogen atom.
- Structural formula: H−C≡C−H
- Why are hydrocarbons soluble in organic solvents?
- Hydrocarbons are nonpolar molecules and are soluble in organic solvents, which are also nonpolar, following the principle “like dissolves like.”
- Give the physical properties of alkanes.
- Alkanes are colorless, odorless (for the lower members), nonpolar, insoluble in water, and have low boiling and melting points.
- How can you identify ethene from ethane?
- Ethene decolorizes bromine water, whereas ethane does not.
- Why does the color of bromine water discharge on the addition of ethene in it?
- The double bond in ethene reacts with bromine, leading to the formation of a colorless dibromo compound, thus discharging the color of bromine water.
- State one important use of each:
- Ethene: Used in the production of polyethylene (a plastic).
- Acetylene: Used in welding (oxyacetylene welding).
- Chloroform: Used as a solvent in laboratories.
- Carbon tetrachloride: Used as a cleaning agent and fire extinguisher.
Extensive Questions:
- What type of reactions are given by alkanes? Explain with reference to halogenation of alkanes.
- Alkanes undergo substitution reactions, particularly halogenation, where a hydrogen atom is replaced by a halogen atom. For example, methane reacts with chlorine under UV light to form chloromethane and hydrogen chloride.
- Alkanes are a source of heat. Explain it.
- Alkanes are used as fuels because, when combusted, they release large amounts of heat due to the exothermic nature of the combustion reaction, producing carbon dioxide and water.
- Prepare the following as directed:
- (i) Ethene from ethane: Ethene can be prepared by dehydrogenation of ethane.
- (ii) Acetylene from tetrahalide: Acetylene can be prepared by treating a tetrahalide like 1,2-dibromoethane with alcoholic KOH.
- (iii) Carbon tetrachloride from methane: Methane reacts with chlorine in the presence of UV light to form carbon tetrachloride.
- (iv) Ethylene glycol from ethene: Ethene reacts with water in the presence of oxygen to form ethylene glycol.
- (v) 1,2-Dibromoethane from ethene: Ethene reacts with bromine to form 1,2-dibromoethane.
- (vi) Glyoxal from acetylene: Acetylene can be oxidized with potassium permanganate to form glyoxal (C₂H₂O₂).
4. Explain the oxidation of acetylene.
- Acetylene (C₂H₂) can undergo oxidation in different ways:
- Controlled oxidation: Under controlled conditions, acetylene can be oxidized to form products like acetaldehyde or acetic acid using catalysts.
- Combustion: When acetylene is burnt in air or oxygen, it undergoes complete oxidation to form carbon dioxide (CO₂) and water (H₂O).
2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O
5. Write balanced chemical equations for the following reactions. Also, name the products that are formed.
- (i) A mixture of ethyne and hydrogen is passed over heated nickel:
- C₂H₂ + 2H₂ →{Ni} C₂H₆
Product: Ethane (C₂H₆). - (ii) Ethyne is treated with chlorine:
C₂H₂ + Cl₂ → C₂H₂Cl₂
Product: 1,2-Dichloroethene (if further chlorination occurs, it can form tetrachloroethane). - (iii) Ethyne is burnt in air:
2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O
Products: Carbon dioxide (CO₂) and water (H₂O). - (iv) Ethyne is passed through bromine water:
C₂H₂ + Br₂ → C₂H₂Br₂
Product: 1,2-Dibromoethene.
6. Explain briefly:
- (i) Why butane undergoes substitution reactions?
- Butane, as a saturated hydrocarbon (alkane), has single bonds, making it more likely to undergo substitution reactions rather than addition reactions.
- (ii) There are millions of organic compounds.
- The vast number of organic compounds is due to the ability of carbon atoms to form long chains, branched structures, and rings, along with multiple types of bonds (single, double, triple) and the presence of various functional groups.
- (iii) Acetylene undergoes addition reactions in two stages.
- Acetylene (C₂H₂) has a triple bond, allowing it to add atoms in two stages: first converting the triple bond to a double bond (forming a di-substituted alkene), and then to a single bond (forming a tetra-substituted alkane).
- (iv) Alkynes are more reactive than alkanes.
- Alkynes have a triple bond, which is more reactive than the single bonds in alkanes. The electron density in the triple bond makes alkynes more susceptible to addition reactions.
