Combustion, pollutants and cracking
A fuel burns to release heat energy. Complete combustion gives carbon dioxide + water; incomplete (limited oxygen) gives carbon monoxide and soot. Carbon monoxide is toxic — it reduces the blood's capacity to carry oxygen. Sulfur impurities burn to sulfur dioxide, and oxides of nitrogen form in engines; both cause acid rain. Cracking breaks long alkanes (heat + catalyst) into short-chain alkanes (fuels) and alkenes (polymers).
Crude oil: mixture separated by distillation
Crude oil = a mixture of hydrocarbons (not chemically combined); a hydrocarbon is hydrogen and carbon only. Fractional distillation separates it: the oil is heated and vaporised; vapours rise a column that is hot at the bottom, cooler at the top; each fraction condenses at its own boiling point — large, high-boiling molecules low down, small ones high up. "It splits into fractions" is a bare statement and scores nothing.
Fractions and the trends down the column
From the top (small molecules) to the bottom (large) the main fractions are refinery gases, gasoline (petrol), kerosene, diesel oil, fuel oil and bitumen. Going down the column (larger molecules) the fractions become darker, have higher boiling points, are more viscous, less volatile and harder to ignite; going up, the opposite. The cause is stronger intermolecular forces in the larger molecules.
Drawn from real examiner reports.
Describe distillation, don't just name it
Asked to describe fractional distillation, restating that the oil "splits into fractions" earns almost nothing — the mark scheme wants the mechanism: the oil is heated and vaporised; vapours rise a column hotter at the bottom, cooler at the top; each fraction condenses at its boiling point; the fractions are collected at different heights.
Jun24 1CR Q8a: "crude oil is heated" scored, but "it splits into fractions" was only a statement; at least four points describing how the separation happens are needed for full marks.
Acid rain is sulfur dioxide, not CO2
In the acid-rain question, answers about carbon dioxide and global warming score zero — carbon dioxide does not cause acid rain. The mark-earning chain: fuels contain sulfur it burns to sulfur dioxide () this dissolves in rain to cause acid rain. Omitting the sulfur-dioxide step loses the middle mark.
Jun24 1CR Q8c: many wrote about carbon dioxide / global warming rather than the sulfur impurity that burns to sulfur dioxide and causes acid rain.
SO2, not SO4 or "sulfite"
Sulfur dioxide is — not , and not "sulfite", "sulfide" or "sulfur (tetra)oxide". Remember oxygen in the air is diatomic (), so the equation is . A wrong formula or name loses the mark even when the acid-rain idea is right.
Nov24 1C Q11ai: some gave sulfur dioxide the formula SO4, and some were unaware that oxygen is diatomic (O2).
Fuel = burns AND releases heat
A fuel is "a substance that, when burned, releases heat energy". Both elements are required: "it produces heat" without saying it burns/combusts caps you at one mark, and "a substance that burns" without the energy release also drops a mark. "A source of energy" alone is too vague.
Jun24 1CR Q6a: many could not define a fuel; both "burns/combusts" and "releases heat energy" are needed, and "a source of energy" is too vague.
Alkenes make polymers, not fuels
Cracking makes short-chain alkanes (in high demand as fuels) and alkenes (used to make polymers). A common error is stating that the alkenes are used as fuels — this is not correct and gains nothing. Be clear: the short-chain alkanes are the more useful fuels; the alkenes go into polymers.
Jun24 1CR Q8b: "alkenes are used as fuels" was noted as incorrect, while "greater demand for short-chain alkanes" earned the mark.
"Limited" oxygen, not "no" oxygen
For incomplete combustion the cause is a limited/insufficient supply of oxygen — "no oxygen" is not credited, because with none there would be no reaction at all. The tell-tale product is carbon monoxide (), not carbon dioxide; writing describes complete combustion instead.
Jun24 1CR Q2ci: "not enough oxygen/air" was credited but "no oxygen" was not.
Balance combustion: C, then H, then O last
Balance a combustion equation in order: fix carbon (set the coefficient), then hydrogen (set ), then oxygen last by counting the O atoms on the right. Leaving oxygen till last avoids the fractions that trip candidates up.
One distinct point per mark
"Describe" and "explain" questions on crude oil are usually worth 3-4 marks — plan one distinct point per mark and make each a separate step or reason, not the same idea reworded. Single-statement answers cap the marks.
Name or formula — read the command
Read whether the command wants a name or a formula and give that form only. Writing when the name "sulfur dioxide" was asked (or the reverse) scores nothing even when the chemistry is right.
Sulfur dioxide (formula SO2) is the gas that causes acid rain. Calculate its relative formula mass (Mr). (Ar: S = 32, O = 16.)
Propane () is used as a bottled fuel. It burns completely in plenty of oxygen to form carbon dioxide and water.
Write the balanced equation for the complete combustion of propane.
