3.3.2 Alkanes

Recognise an alkane from two things: it is a hydrocarbon and every C–C bond is a single σ bond. For an open-chain alkane, that gives the general formula CₙH₂ₙ₊₂.

“Saturated” is specifically about the absence of C=C or C≡C carbon–carbon multiple bonds. An alkane can still react — combustion and free-radical substitution are the important reactions here.

Picture fractional distillation of crude oil: no covalent bonds are being broken. The hydrocarbons separate physically because they have different boiling ranges, so they condense at different heights in the column.

Shorter molecules generally have weaker London forces and lower boiling points, so they condense higher up. No covalent bonds are broken during the distillation itself.

For cracking, think carbon–carbon bond breaking in a larger hydrocarbon. You end up with smaller hydrocarbons, including alkenes, which turns less useful long-chain feedstock into more useful products.

The product mixture depends on the cracking process, so do not assume one unique pair of products. In an equation, conserve the total numbers of C and H atoms.

For thermal cracking, I’d remember the combination: very high temperature, high pressure and an alkene-rich product mixture. You do not need a cracking mechanism for this line.

No cracking mechanism is required here. What matters is distinguishing the conditions and broad product pattern from catalytic cracking.

Catalytic cracking uses a zeolite catalyst, high temperature and slight pressure. Its main products are useful motor fuels and aromatic hydrocarbons.

Keep the zeolite with catalytic cracking and distinguish its conditions from thermal cracking. No cracking mechanism is required here.

Start with product demand when you explain why cracking is economically useful. Refineries can turn surplus long-chain fractions into shorter fuel-range hydrocarbons and alkene feedstocks that are more useful and valuable.

The economic value therefore comes from changing the product distribution, not from creating extra carbon. Cracking rearranges the same hydrocarbon feedstock into a more valuable mixture.

For why alkanes work as fuels, connect the use to their strongly exothermic combustion. Complete combustion converts carbon to CO₂ and hydrogen to H₂O and releases energy as the product bonds form.

Their usefulness as fuels is separate from the environmental consequences of fossil carbon and combustion pollutants, which need to be considered when the question asks for an evaluation.

For complete combustion, I’d expect CO₂ and H₂O. Balance C first, then H, then O; if oxygen leaves you with a fraction, multiply the whole equation at the end.

Complete combustion with excess O₂ gives CO₂ and H₂O. Limited oxygen can give CO and/or carbon particulates as well as water, so product identity depends on oxygen supply.

When you look at engine pollutants, attach each one to its source. CO and carbon particulates come from incomplete combustion, unburnt hydrocarbons escape oxidation, and NOₓ forms when N₂ and O₂ react at the high temperatures in the engine.

Attach each pollutant to both its origin and its effect when asked: CO is toxic, particulates harm respiratory health, and NOₓ/unburnt hydrocarbons contribute to atmospheric pollution such as smog.

Give the catalyst a chemical job in a catalytic-converter explanation: it provides a surface and a lower-energy route for redox reactions that turn harmful exhaust gases into less harmful products. Balance whatever species the question actually gives you.

Converters promote redox reactions that turn CO and unburnt hydrocarbons into CO₂/H₂O and reduce NOₓ towards N₂. The catalyst changes rate, not the overall stoichiometric need to balance atoms.

If the fuel contains sulfur compounds, combustion can oxidise that sulfur to SO₂. Keep this source separate from NOₓ: the sulfur starts in the fuel, while most engine NOₓ comes from N₂ and O₂ reacting at high temperature.

SO₂ is an air pollutant because it can form acidic species in atmospheric water and contribute to acid deposition. Keep that environmental consequence connected to sulfur already present in the fuel.

Treat SO₂ as an acidic gas and the calcium compound as the base that traps it in flue-gas desulfurisation. CaO or CaCO₃ reacts with SO₂ so much less sulfur dioxide leaves in the flue gas.

Useful equations are CaO + SO₂ → CaSO₃ and CaCO₃ + SO₂ → CaSO₃ + CO₂. The essential idea is acid–base neutralisation that traps the sulfur in a solid calcium compound.

For methane chlorination, keep the overall chemistry simple first: CH₄ + Cl₂ → CH₃Cl + HCl under UV light. Then remember that further substitution can happen because the products still contain C–H bonds.

UV light is there to generate radicals by homolytic fission of Cl₂; it is not a catalyst in the ordinary sense.

Free-radical substitution has initiation, propagation and termination stages. Homolytic fission makes radicals; propagation consumes one radical and produces another; termination removes radicals by combining them.

I find the easiest check is to count radicals through the chain: initiation creates them, each propagation step consumes one and makes one, and termination removes two. For methane chlorination, write the two propagation equations explicitly before adding a termination example.