Alkanes Flashcards

Active recall practice for fractional distillation, cracking, combustion reactions, and free radical substitution.

Properties and Bonding: read all 5 questions and answers

What is the general formula of alkanes?
\(\text{C}_n\text{H}_{2n+2}\)
Why are alkanes classified as saturated hydrocarbons?
They contain only single covalent carbon-carbon bonds, and consist of only carbon and hydrogen atoms.
Describe the trend in boiling points of alkanes as chain length increases.
Boiling point increases because longer chains have more electrons, creating stronger van der Waals forces between molecules which require more energy to overcome.
How does branching affect the boiling point of alkanes?
Branching decreases the boiling point because it prevents close packing of molecules, reducing surface contact and weakening van der Waals forces.
Describe fractional distillation of crude oil.
Crude oil is vaporised and passed up a fractionating column with a temperature gradient (hotter at the bottom). Fractions condense at different levels based on their boiling points.

Cracking & Combustion: read all 9 questions and answers

What is cracking?
The process of breaking down longer-chain hydrocarbons into shorter, more useful alkanes and alkenes.
What are the conditions for thermal cracking?
High temperature (700-1200 K) and high pressure (up to 7000 kPa). It produces a high percentage of alkenes.
What are the conditions for catalytic cracking?
High temperature (approx 720 K), slight pressure, and a zeolite catalyst. It produces branched alkanes, cycloalkanes, and aromatic compounds.
State the products of complete combustion of alkanes.
Carbon dioxide (CO₂) and water (H₂O).
State the products of incomplete combustion of alkanes.
Carbon monoxide (CO) and/or carbon/soot (C), and water (H₂O).
Why is carbon monoxide (CO) toxic?
It binds with very high affinity to haemoglobin in red blood cells, displacing oxygen and reducing their ability to transport oxygen.
How are nitrogen oxides (NOx) formed in car engines?
High temperatures inside the engine provide enough energy for nitrogen and oxygen from the air to react.
What is the role of a catalytic converter?
It uses platinum/palladium/rhodium catalysts to convert harmful engine gases (CO, \(\text{NO}_x\), hydrocarbons) into less harmful ones (CO₂, N₂, H₂O).
How is sulfur dioxide (SO₂) removed from factory flue gases?
By reacting it with calcium oxide or calcium carbonate (neutralisation) to form calcium sulfate.

Free Radical Substitution: read all 8 questions and answers

What is free radical substitution?
A reaction where a hydrogen atom in an alkane is replaced by a halogen atom, proceeding via a radical mechanism.
What condition is required to initiate the reaction between alkanes and halogens?
Ultraviolet (UV) light (to provide the energy to break the halogen-halogen bond).
State the three steps in free radical substitution.
Initiation, propagation, and termination.
Write the initiation step for the reaction of methane and chlorine.
\(\text{Cl}_2 \rightarrow 2\text{Cl}^\bullet\) (homolytic fission under UV light).
Write the first propagation step for the reaction of methane and chlorine.
\(\text{CH}_4 + \text{Cl}^\bullet \rightarrow {}^\bullet\text{CH}_3 + \text{HCl}\)
Write the second propagation step for the reaction of methane and chlorine.
\({}^\bullet\text{CH}_3 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet\)
Write a termination step for the reaction of methane and chlorine.
\(2\,{}^\bullet\text{CH}_3 \rightarrow \text{C}_2\text{H}_6\) (or \(\text{Cl}^\bullet + {}^\bullet\text{CH}_3 \rightarrow \text{CH}_3\text{Cl}\), or \(2\text{Cl}^\bullet \rightarrow \text{Cl}_2\)).
Why is free radical substitution not a good method to prepare a pure halogenoalkane?
Further substitution reactions occur, resulting in a mixture of different halogenated products.