Alkanes (Edexcel International A Level (IAL) Chemistry): Flashcards

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

Thermal cracking requires high temperatures (up to 1000 °C) and high pressure (up to 70 atmospheres). It produces a mixture of alkanes and a large proportion of alkenes.

Catalytic cracking uses a lower temperature (600–700 °C) and slight pressure in the presence of a catalyst such as zeolite or aluminium oxide. It mainly produces aromatic hydrocarbons rather than alkenes.

True.

Cracking breaks down excess heavier fractions from fractional distillation into smaller alkanes and alkenes with lower relative formula masses, which are in higher demand.

Straight-chain alkanes in petrol can cause knocking in a car engine, which makes combustion less efficient.

Straight-chain alkanes are more likely to explode rather than combust smoothly inside an engine, causing knocking. They are reformed into branched alkanes or cycloalkanes, which burn more efficiently.

Reforming typically uses a platinum catalyst to convert straight-chain alkanes into branched alkanes or cycloalkanes.

False.

Thermal cracking operates at higher temperatures (up to 1000 °C). Catalytic cracking uses a lower temperature of 600–700 °C, relying on a catalyst instead.

In incomplete combustion, the oxygen supply is limited so carbon cannot be fully oxidised to carbon dioxide. Instead, carbon is only partially oxidised to form carbon monoxide (CO).

Carbon monoxide binds strongly to haemoglobin, preventing it from carrying oxygen to the organs or removing carbon dioxide as waste. This can cause dizziness, loss of consciousness and eventually death.

Solid carbon particles released during incomplete combustion clump together to form soot which gradually falls to the ground.

Sulfur dioxide (SO₂) and sulfur trioxide (SO₃) dissolve in atmospheric water to form sulfurous acid (H₂SO₃) and sulfuric acid (H₂SO₄). These acids fall as acid rain, damaging aquatic life, crops and metallic structures.

False.

Nitrogen is normally too unreactive to combine with oxygen. It is only the high temperatures and pressures inside a car engine that provide enough energy to oxidise nitrogen to NO and NO₂.

Photochemical smog is a type of air pollution formed when nitrogen oxides react with volatile organic compounds (VOCs) in the atmosphere. Its main pollutant is peroxyacetyl nitrate (PAN), which is harmful to the lungs, eyes and plant life.

True.

Crude oil fractions can contain sulfur atoms. When these are combusted, the sulfur is oxidised to form sulfur dioxide (SO₂) and sulfur trioxide (SO₃).

Acid rain can cause:

1. Damage or death to aquatic life as lakes and rivers become too acidic

2. Corrosion of metallic structures and damage to buildings made from carbonate rocks

Precious metals including platinum, rhodium and palladium are coated onto a honeycomb structure to provide a large surface area, maximising contact with exhaust gases.

The reaction is:

2CO (g) + 2NO (g) → 2CO₂ (g) + N₂ (g)

This converts the toxic pollutants into carbon dioxide and unreactive nitrogen gas.

The three main types of biofuel are biodiesel, bioethanol and biogas.

True.

Biofuels are derived from living or recently living matter (biological sources), which can be replaced over a short period of time, making them renewable.

Carbon neutral describes a fuel or process that has a net-zero effect on the amount of carbon dioxide in the atmosphere.

When hydrogen combusts, the only product is water (2H₂ (g) + O₂ (g) → 2H₂O (l)), so no carbon dioxide is released. This makes it carbon neutral in terms of combustion.

1. Its low density means large volumes are needed to deliver the same energy as fossil fuels, creating storage problems

2. Producing hydrogen is expensive, limiting its widespread use

Although the CO₂ released when a biofuel burns was absorbed during the plant's growth, the full production cycle involves additional CO₂ emissions from growing, harvesting, processing and transporting the biofuel. These are not accounted for in a simple carbon neutrality claim.

False.

Fossil fuels are not carbon neutral. They are releasing CO₂ that was absorbed millions of years ago when atmospheric CO₂ levels were much higher than today, so burning them increases current atmospheric CO₂ levels.

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

Heterolytic fission is the breaking of a covalent bond in which the more electronegative atom takes both electrons, forming a negative ion and a positive ion.

A free radical is a species that contains an unpaired electron.

A single-headed (fishhook) arrow is used to show the movement of one electron in homolytic fission. A double-headed arrow is used for the movement of an electron pair in heterolytic fission.

False.

In heterolytic fission, the more electronegative atom takes both electrons from the bond. This produces a negative ion and a positive ion, not free radicals.

In heterolytic fission, the positive ion formed is electron-deficient and can accept a pair of electrons, making it an electrophile.

The negative ion is a nucleophile. It is electron-rich because it gained both electrons from the broken bond, enabling it to donate a pair of electrons to an electron-deficient species.

False.

Homolytic fission of a Cl-Cl bond produces two chlorine free radicals, each with one unpaired electron. Ions are produced by heterolytic fission, not homolytic.

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

1. A Cl radical attacks methane: CH₄ + Cl• → •CH₃ + HCl (a C-H bond breaks homolytically, forming a methyl radical)

2. The methyl radical attacks Cl₂: •CH₃ + Cl₂ → CH₃Cl + Cl• (chloromethane forms and a Cl radical is regenerated)

The termination step is the stage at which the chain reaction stops, when two free radicals collide and combine to form a single unreactive molecule.

Free radical substitution is not ideal for preparing a specific halogenoalkane because a mixture of substitution products is formed.

False.

The propagation step is a chain reaction in which each step regenerates a free radical. A Cl radical is used up but a new alkyl radical is produced, which then regenerates a Cl radical in the next step.

Two methyl radicals can combine in the termination step to form ethane (CH₃CH₃). This is an impurity as it is not the desired halogenoalkane product.

Further substitution occurs, progressively replacing more hydrogen atoms. The products can include CH₂Cl₂, CHCl₃ and CCl₄, in addition to the initial product CH₃Cl.

True.

The position of the radical dot in the propagation step must show the electron-deficient carbon correctly. For substitution on the middle carbon of propane, the dot is shown on C2: CH₃•CHCH₃.

Each propagation step consumes one free radical but produces another. This means the reactions are self-sustaining: the radical generated in one step triggers the next, continuing the chain until a termination event occurs.