GCSE Chemistry: Organic & Analysis — Deep Revision Workbook

Inside GCSE Chemistry: Organic & Analysis — Deep Revision Workbook

name hydrocarbons and use CnH2n+2, follow crude oil through fractional distillation and the fractions, tell complete from incomplete combustion, explain cracking and test alkenes with bromine water, build addition polymers, sort pure substances, mixtures and formulations, calculate Rf values and carry out the four gas tests, then write a full exam-style six-mark answer. About 45 minutes.

Learning objectives

• crude oil, hydrocarbons and alkanes — crude oil as a finite mixture of hydrocarbons, the alkane general formula CnH2n+2, saturation, and the first alkanes;
• fractional distillation and combustion — separation by boiling point, the property trends down the column, the uses of the fractions, and complete versus incomplete combustion;
• cracking, alkenes and polymers — thermal (steam) and catalytic cracking, supply and demand, the alkene formula CnH2n, the bromine-water test and addition polymerisation;
• pure substances, mixtures and formulations — single elements or compounds versus mixtures made in exact amounts;
• chemical analysis — chromatography and the Rf calculation, and the four gas tests (hydrogen, oxygen, carbon dioxide and chlorine).

Activities in this book

1. Section 1 — Crude Oil, Hydrocarbons & Alkanes

   crude oil is a finite resource and a mixture of hydrocarbons — molecules of hydrogen and carbon only. Alkanes are saturated hydrocarbons with the general formula CnH2n+2. The first four are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10).

2. Section 2 — Distillation, Fractions & Combustion

   fractional distillation separates crude oil into fractions by boiling point. Going down the column, boiling point and viscosity increase while flammability decreases. Fractions include petrol, kerosene (aircraft fuel), diesel and bitumen (roads). Complete combustion gives carbon dioxide and water; incomplete combustion can give carbon monoxide and soot (carbon).

3. Section 3 — Cracking, Alkenes & Polymers

   cracking breaks long-chain alkanes into shorter alkanes and alkenes. It is done because demand for short-chain fuels is greater than supply, and because alkenes are needed to make polymers. Thermal (steam) cracking uses high temperature and steam; catalytic cracking passes vapour over a hot catalyst. Alkenes (CnH2n) have a C=C double bond and decolourise bromine water. Monomers join by addition polymerisation to make a polymer.

4. Section 4 — Pure Substances, Chromatography & Gas Tests

   a pure substance is a single element or compound; a formulation is a mixture made in exact amounts as a useful product (e.g. paint, fuel, medicine). In chromatography, the solvent is the mobile phase and Rf = distance moved by the substance / distance moved by the solvent (always 0 to 1). Gas tests: hydrogen gives a squeaky pop, oxygen relights a glowing splint, carbon dioxide turns limewater milky, chlorine bleaches damp litmus paper.

5. Section 5 — Cracking, Alkenes & Polymers (6 marks)

   this is an exam-style writing task, so it is not auto-marked. Aim for six clear points, using the correct keywords (long-chain alkane, cracking, alkene, C=C double bond, bromine water, monomer, addition polymer, supply and demand) with one clear idea per line.

Suggestions for teaching

• Drill the general formulae. Fix CnH2n+2 for alkanes and CnH2n for alkenes so learners can write any formula from the number of carbons.
• Follow the column. Model fractional distillation as boiling-point separation, and rehearse the property trends (boiling point and viscosity up; flammability down) with the uses of each fraction.
• Contrast the combustions. Keep complete combustion (CO₂ + water) firmly apart from incomplete combustion (carbon monoxide and soot).
• Explain cracking with supply and demand. Link cracking of long-chain alkanes into short alkanes and alkenes to the greater demand for fuels and polymers, and separate steam from catalytic cracking.
• Practise the Rf sum. Rehearse Rf = distance moved by substance ÷ distance moved by solvent (always 0 to 1), and secure the four gas tests as quick recall facts.