GCSE Chemistry: Organic Chemistry & Analysis — Revision Workbook

Inside GCSE Chemistry: Organic Chemistry & Analysis — Revision Workbook

work out alkane names and formulae from the general formula CnH2n+2, follow crude oil through fractional distillation, tell complete from incomplete combustion, explain cracking and test alkenes with bromine water, build addition polymers and name the alcohol and carboxylic-acid functional groups, sort pure substances from mixtures and formulations, calculate an Rf value from a chromatogram, and carry out the four gas tests. About 30 minutes.

Learning objectives

• crude oil, hydrocarbons and alkanes — crude oil as a finite mixture of hydrocarbons, the alkane general formula CnH2n+2, and the names and formulae of methane, ethane, propane and butane;
• fractional distillation and combustion — separation by boiling point and the trend in properties down the fractions, plus complete combustion (carbon dioxide + water) versus incomplete combustion (carbon monoxide);
• cracking and alkenes — why cracking is done, catalytic and steam cracking, the alkene general formula CnH2n, and the bromine-water test for a C=C double bond;
• addition polymers, alcohols and carboxylic acids — monomers joining to make polymers, and the functional groups –OH (alcohols) and –COOH (carboxylic acids) as homologous series;
• chemical analysis — pure substances, mixtures and formulations, chromatography and the Rf calculation, and the four gas tests (hydrogen, oxygen, carbon dioxide and chlorine).

Activities in this book

1. Task 1 — Crude Oil, Hydrocarbons & Alkanes

   crude oil is a finite resource and a mixture of hydrocarbons — molecules made 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. Task 2 — Distillation, Combustion, Cracking & Alkenes

   fractional distillation separates crude oil into fractions by boiling point; going down the column, boiling point, viscosity and colour increase while flammability decreases. Complete combustion gives carbon dioxide and water; incomplete combustion can give carbon monoxide. Cracking breaks long alkanes into shorter alkanes and alkenes; alkenes decolourise bromine water.

3. Task 3 — Polymers, Alcohols & Carboxylic Acids

   many small alkene molecules (monomers) join to make a very large molecule (a polymer) by addition polymerisation. Alcohols are a homologous series with the functional group -OH (e.g. ethanol). Carboxylic acids are a homologous series with the functional group -COOH (e.g. ethanoic acid).

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

   a pure substance is a single element or compound; a mixture is two or more substances not chemically joined; a formulation is a mixture made in exact amounts as a useful product (e.g. paint, fuel). In chromatography, Rf = distance moved by the substance / distance moved by the solvent. Gas tests: hydrogen gives a squeaky pop with a lighted splint, oxygen relights a glowing splint, carbon dioxide turns limewater milky, and chlorine bleaches damp litmus paper.

5. Task 5 — Six-Mark Exam Task

   this is exam practice for a 6-mark extended-response question. Plan three or four clear points, use precise chemistry vocabulary, and link cause to effect. There is no single right answer — mark it against the answer-sheet notes.

Suggestions for teaching

• Drill the general formula. 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 (viscosity, colour and boiling point up; flammability down) going down the fractions.
• Contrast the combustions. Keep complete combustion (CO₂ + water) firmly apart from incomplete combustion (carbon monoxide, a toxic gas).
• 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.
• Practise the Rf sum. Rehearse Rf = distance moved by substance ÷ distance moved by solvent, and secure the four gas tests as quick recall facts.