14.1 A2 Level

Alkanes

10 flashcards to master this topic

Key Concept Flip

Describe the reagents and conditions required to produce an alkane from an alkene via hydrogenation.

Answer Flip

Alkenes react with hydrogen gas (H₂(g)) in the presence of a platinum (Pt) or nickel (Ni) catalyst under heat. This is an addition reaction where hydrogen atoms add across the double bond, saturating the molecule to form an alkane.

Key Concept Flip

Outline the process of cracking a longer chain alkane, including the necessary conditions.

Answer Flip

Cracking involves breaking down large alkane molecules into smaller, more useful alkanes and alkenes. This is achieved by heating the alkane with aluminium oxide (Al₂O₃) as a catalyst.

Definition Flip

Describe the complete combustion of alkanes, including the products formed.

Answer Flip

Complete combustion of alkanes occurs when there is a sufficient supply of oxygen. The alkane reacts with oxygen to produce carbon dioxide (CO₂) and water (H₂O).

Definition Flip

Describe the incomplete combustion of alkanes, including the products formed and its environmental impact.

Answer Flip

Incomplete combustion occurs when there is a limited supply of oxygen. It produces carbon monoxide (CO), water (H₂O), and sometimes soot (carbon particles). Carbon monoxide is a toxic gas.

Definition Flip

Describe the initiation step in the free-radical substitution of ethane by chlorine.

Answer Flip

The initiation step involves the homolytic fission of a chlorine molecule (Cl₂) by ultraviolet (UV) light. This creates two chlorine free radicals (Cl•).

Definition Flip

Describe the propagation steps in the free-radical substitution of ethane by chlorine.

Answer Flip

Propagation involves two steps: 1) A chlorine free radical (Cl•) reacts with ethane (C₂H₆) to form an ethyl radical (C₂H₅•) and HCl. 2) The ethyl radical (C₂H₅•) reacts with another chlorine molecule (Cl₂) to form chloroethane (C₂H₅Cl) and another chlorine free radical (Cl•), which can then continue the chain reaction.

Definition Flip

Describe the termination steps in the free-radical substitution of ethane by chlorine.

Answer Flip

Termination steps involve the combination of two free radicals to form a stable molecule, removing free radicals from the reaction mixture and stopping the chain reaction.

Example: Cl• + Cl• → Cl₂ , C₂H₅• + Cl• → C₂H₅Cl, C₂H₅• + C₂H₅• → C₄H₁₀
Key Concept Flip

Explain how cracking is used to obtain more useful alkanes and alkenes of lower Mr from heavier crude oil fractions.

Answer Flip

Cracking breaks down long-chain hydrocarbons from crude oil into smaller, more valuable molecules. This increases the yield of gasoline and other useful chemicals, like alkenes for polymer production, which have a higher demand than the original long-chain alkanes.

Key Concept Flip

Explain the general unreactivity of alkanes towards polar reagents.

Answer Flip

Alkanes are generally unreactive due to the strong C–H bonds and their lack of polarity. Polar reagents are attracted to areas of high electron density or charge, which are absent in alkanes due to the similar electronegativity of carbon and hydrogen.

Key Concept Flip

Outline the environmental consequences of carbon monoxide (CO), oxides of nitrogen (NOx), and unburnt hydrocarbons from internal combustion engines.

Answer Flip

CO is a toxic gas that reduces oxygen transport in the blood. NOx contributes to acid rain and respiratory problems. Unburnt hydrocarbons contribute to smog and can be carcinogenic. Catalytic converters help to reduce these pollutants.

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13.2 Characteristic organic reactions 14.2 Alkenes