alkanes

Why are alkanes considered unreactive?

Alkanes are unreactive because they have strong, non-polar C–C and C–H bonds, making them resistant to electrophilic or nucleophilic attack under normal conditions.

What is the general formula for alkanes and what type of hydrocarbon are they?

Alkanes follow the general formula CₙH₂ₙ₊₂ and are saturated hydrocarbons, meaning they contain only single covalent bonds and are fully 'saturated' with hydrogen atoms.

How does boiling point of alkanes vary with chain length and branching?

Boiling point increases with chain length due to stronger van der Waals forces. Branching lowers boiling point because it reduces surface area for intermolecular interactions.

What is the main method used to separate the components of crude oil?

Fractional distillation is used to separate crude oil into fractions based on boiling point. Components condense at different heights in the distillation column depending on their size and volatility.

Describe the conditions and purpose of catalytic cracking.

Catalytic cracking uses a zeolite catalyst at about 450°C and slight pressure. It breaks long-chain alkanes into shorter, more valuable alkanes and alkenes, used as fuels and chemical feedstocks.

Compare thermal and catalytic cracking.

Thermal cracking operates at high temperatures (~900°C) and pressures (~70 atm), producing mostly alkenes. Catalytic cracking uses lower temperature and pressure with a zeolite catalyst, producing branched alkanes and aromatic hydrocarbons.

Why are short-chain hydrocarbons more in demand than long-chain ones?

Short-chain hydrocarbons are more flammable and volatile, making them more useful as fuels. They combust more efficiently and are better suited for transport fuels like petrol.

Define complete and incomplete combustion of alkanes.

Complete combustion occurs with excess oxygen and produces CO₂ and H₂O. Incomplete combustion occurs with limited oxygen, producing CO or C (soot) and less energy.

Why is carbon monoxide dangerous and how can its formation be minimised?

Carbon monoxide is a toxic, colorless, odorless gas that binds to haemoglobin, reducing oxygen transport. Its formation is minimized by ensuring complete combustion and using catalytic converters in engines.

How does sulfur dioxide form from combustion, and how is it removed?

Sulfur dioxide (SO₂) forms when sulfur-containing fuels are burned. It's removed by flue gas desulfurisation, where SO₂ reacts with calcium oxide or carbonate to form calcium sulfite (CaSO₃).