Crude oil and fractional distillation
Crude oil is a mixture of hydrocarbons (hydrogen and carbon only), not chemically combined, so it separates physically. Fractional distillation uses their boiling points: the oil is heated and vaporised, the vapour enters the column (hot bottom, cool top), and each fraction condenses at its boiling point. Fractions run refinery gases (top) to bitumen (bottom). Down the column they get darker, higher-boiling, more viscous and less flammable.
Combustion, carbon monoxide and acid rain
A fuel releases heat when burned. Complete combustion (plenty of oxygen) gives carbon dioxide and water, e.g. CH₄ + 2O₂ → CO₂ + 2H₂O. Incomplete combustion (limited oxygen) gives carbon monoxide (CO) and/or soot. CO is poisonous: it reduces the blood's capacity to carry oxygen. Two acid-rain routes: sulfur impurities burn to sulfur dioxide (S + O₂ → SO₂); and in a hot engine N₂ and O₂ form oxides of nitrogen. Both cause acid rain.
Cracking long chains into useful products
Cracking converts long-chain alkanes (low demand) into shorter-chain alkanes and alkenes (high demand). Catalytic cracking uses a silica or alumina catalyst at 600-700 °C. The equation must balance, e.g. C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene). It is needed because distillation over-supplies long-chain fractions: short-chain alkanes are fuels (petrol), and alkenes make polymers. Name these uses — not just "supply and demand".
Drawn from real examiner reports.
"Heated in the column" loses marks
For fractional distillation, give three stages: (1) the oil is heated and vaporised; (2) the vapour enters the column (hot bottom, cool top); (3) each fraction condenses at its boiling point. The classic error is writing the oil is heated inside the column — it must be vaporised before it enters. Naming a fraction given in the stem also scores nothing.
June 2024 Paper 2C Q3(b)(ii): candidates lost marks for stating the crude oil was heated IN the column; the mark scheme required vaporising the oil and feeding the vapour into the column, then the fraction condensing and being collected. Naming a fraction already in the stem was not creditworthy.
Cracking: name the products and uses
When asked why cracking is useful, "more supply than demand" alone caps the marks. You must name which products form and their uses: short-chain alkanes are in demand as fuels (petrol/gasoline), and alkenes (e.g. ethene) are used to make polymers. Restating stem words such as "short-chain alkanes are more useful" earns nothing.
June 2024 Paper 2C Q3(c)(ii): many candidates mentioned supply and demand but did not name any products, gaining only one mark; the full answer needed short-chain alkanes used for petrol AND alkenes used for polymers.
Carbon monoxide: be precise
Carbon monoxide is poisonous because it reduces the capacity of the blood to carry oxygen (it binds in place of oxygen). Vague answers — "it is toxic", "it stops you breathing" — score zero without reference to the oxygen-carrying capacity of the blood. (Binding with haemoglobin is accepted, but haemoglobin is not required by the spec.)
June 2023 Paper 2C Q7(a): a vague carbon-monoxide answer scored zero; the mark required reference to reducing the blood capacity to carry oxygen, or CO binding with haemoglobin.
A mixture is not chemically combined
Crude oil is a mixture: two or more substances not chemically combined, which is exactly why it can be separated by the physical process of fractional distillation. Answers like "different things" or "a compound of hydrocarbons" are wrong — a compound has its elements chemically bonded in fixed proportions, whereas the hydrocarbons in crude oil are simply mixed.
(common Edexcel chemistry error)
CO2 vs CO — combustion products
CO₂ is the complete-combustion product (plenty of oxygen); CO and/or carbon (soot) are the incomplete-combustion products (limited oxygen). Writing CO for complete combustion, or CO₂ for incomplete, loses the mark. Note "limited" oxygen, not "no" oxygen — with no oxygen there is no combustion at all.
(common Edexcel chemistry error)
Cracking vs fractional distillation
Fractional distillation only separates the hydrocarbons already present — a physical change; cracking breaks large molecules into smaller ones — a chemical change. Do not describe distillation as making new, smaller molecules, and do not describe cracking as merely separating the molecules that were already there.
(common Edexcel chemistry error)
Read the column for the trends
Picture the column: small molecules leave the top (low boiling point, pale, runny, flammable); large molecules leave the bottom (high boiling point, dark, viscous). Always state the trend AND its direction — "viscosity increases down the column", not just "it changes".
Balance combustion and cracking equations
For any combustion or cracking equation, balance C first, then H, then O last, and check the atom totals match on both sides. Complete combustion → CO₂ + H₂O; incomplete → CO and/or C. If the oxygen count is odd (a fraction of O₂), double every coefficient to clear it.
Name products in "explain" answers
For "why is cracking useful" and "describe fractional distillation", write the process step by step and name the products/fractions and their uses. Marks come from the linked stages and the named products, not from paraphrasing the question stem.
What is crude oil?
Octane, , is one of the hydrocarbons found in the gasoline (petrol) fraction of crude oil.
Calculate its relative formula mass (). Use : , .
