Alkanes (Edexcel A Level Chemistry): Flashcards

False.

Crude oil is a mixture of hydrocarbons including alkanes, cycloalkanes, and arenes. It must be separated by fractional distillation before use.

Cracking is the process of breaking down large, heavy hydrocarbon fractions into smaller, more useful alkanes and alkenes with lower relative formula mass.

Thermal cracking requires temperatures up to 1000 °C and pressures up to 70 atmospheres, and produces a lot of alkenes.

Reforming is the conversion of straight-chain alkanes into branched alkanes or cycloalkanes to reduce knocking in engines. It typically uses a platinum catalyst.

Catalytic cracking uses a temperature of around 450°C, slight pressure, and a catalyst such as a zeolite or aluminium oxide. It mainly produces aromatic hydrocarbons.

False.

Thermal cracking produces alkanes and a large proportion of alkenes. It is catalytic cracking that mainly produces aromatic hydrocarbons.

Smaller hydrocarbon fractions such as gasoline are in higher demand than heavier fractions. Cracking converts excess heavy fractions into more useful, lower-Mr alkanes and alkenes.

Complete combustion occurs when an alkane burns in excess oxygen, producing only carbon dioxide and water.

Nitrogen oxides (NOx) react with volatile organic compounds (VOCs) in the atmosphere to form peroxyacetyl nitrate (PAN), the main pollutant in photochemical smog.

True.

Carbon monoxide binds strongly to haemoglobin, preventing it from transporting oxygen to organs and removing carbon dioxide as waste. This can cause death.

In a car engine, nitrogen and oxygen react at high temperatures and pressures to form oxides of nitrogen, released in exhaust fumes.

Nitrogen oxides dissolve and react with water and oxygen in the atmosphere to form nitric acid, which falls as acid rain, corroding buildings and harming aquatic life.

Incomplete combustion occurs when an alkane burns in a limited supply of oxygen, producing carbon monoxide and/or carbon (soot) alongside water.

True.

Inside a car engine the oxygen supply is restricted, so incomplete combustion takes place, producing toxic carbon monoxide and carbon soot alongside water.

Carbon particulates can cause respiratory damage when inhaled, coat buildings (accelerating corrosion), and cause global dimming by reflecting sunlight back into space.

Photochemical smog is atmospheric pollution formed when nitrogen oxides react with volatile organic compounds (VOCs) in sunlight to produce peroxyacetyl nitrate (PAN).

A catalytic converter is a device fitted to car exhausts that uses precious metals (such as platinum) on a honeycomb surface to convert toxic exhaust gases into less harmful products.

Biofuels are renewable and roughly carbon neutral. They can also reduce waste going to landfill (for biodiesel and biogas) and provide economic opportunities for less developed countries.

In a catalytic converter, carbon monoxide and nitrogen monoxide react to form carbon dioxide and nitrogen gas, which is unreactive and harmless to the atmosphere.

False.

Biofuels are often considered carbon neutral because plants absorb CO₂ as they grow. However, CO₂ is also produced during refining and transport, so they are not fully carbon neutral.

The honeycomb structure provides a very large surface area for the precious metal catalyst (e.g. platinum), maximising contact with exhaust gases and improving the efficiency of reactions.

A biofuel is a renewable fuel derived from living matter. The three main types are biodiesel, bioethanol, and biogas.

The cost of converting engines to run on biofuels is high. In developed countries, land needed to grow biofuel crops conflicts with food production.

False.

The platinum in a catalytic converter is a heterogeneous catalyst: it is in a different phase (solid) from the gaseous exhaust reactants. It is coated on a honeycomb to maximise surface area.

Bioethanol is produced by fermentation of plant sugars. It is a renewable biofuel that can be used as a substitute for or additive to petrol.

A free radical is a highly reactive species with an unpaired electron, formed by homolytic fission of a covalent bond.

Free radical substitution produces a mixture of products because further substitution can occur if excess halogen is present, and because substitution can occur at different carbon atoms in the chain.

Homolytic fission is the breaking of a covalent bond in which each atom receives one electron from the shared pair, producing two free radicals.

The three steps are initiation (homolytic fission by UV light), propagation (chain reaction forming halogenoalkane), and termination (two radicals combine to stop the reaction).

False.

The propagation step regenerates free radicals in each cycle, allowing the chain reaction to continue. Free radicals are only consumed permanently in the termination step.

In propagation, a halogen radical attacks an alkane, breaking a C-H bond by homolytic fission to form an alkyl radical, which attacks a halogen molecule to regenerate a halogen radical.

The initiation step is where UV light provides energy to break the Cl-Cl or Br-Br bond by homolytic fission, producing two halogen free radicals.

False.

Alkanes are very unreactive, so UV light (sunlight) is required to provide the energy needed to initiate free radical substitution by breaking the halogen-halogen bond.

The termination step is where two free radicals combine to form a single unreactive molecule, ending the chain reaction.

If two methyl radicals (CH₃•) combine, ethane (CH₃CH₃) is formed as an impurity instead of the target chloroalkane.