Organic Chemistry and Fuels
Organic chemistry and fuels
This domain tests hydrocarbons, homologous series, functional groups, combustion, cracking and polymerisation. GCE O-Level questions often ask you to name compounds, draw structures, distinguish alkanes from alkenes and discuss fuel effects.
Core rules
- Alkanes are saturated hydrocarbons with general formula CnH2n+2. They undergo substitution with halogens in ultraviolet light and burn to form carbon dioxide and water.
- Alkenes are unsaturated hydrocarbons with general formula CnH2n. They contain a carbon-carbon double bond and undergo addition reactions, including decolourising bromine water.
- Alcohols contain the -OH group; ethanol can be made by fermentation of glucose or by hydration of ethene. Carboxylic acids contain -COOH and react with carbonates to produce carbon dioxide.
- Complete combustion gives carbon dioxide and water; incomplete combustion gives carbon monoxide and carbon, which are pollutants. Carbon dioxide is a greenhouse gas; sulfur dioxide causes acid rain.
- Cracking breaks larger alkanes into smaller alkenes and alkanes. Addition polymerisation joins alkene monomers.
Worked example
Ethene gas is bubbled through bromine water. State the observation and explain the reaction.
Reasoning: bromine water is orange-brown and decolourises when shaken with ethene. Ethene has a carbon-carbon double bond, so addition occurs: C2H4 + Br2 -> C2H4Br2. The double bond opens and each carbon bonds to one bromine atom. This is a test for unsaturation; an alkane such as ethane does not decolourise bromine water in the dark.
Traps that cost marks
Do not use the bromine water test for alkanes. Do not write CnH2n for alkanes. Incomplete combustion is not the same as complete combustion. Ethanol fermentation uses yeast and produces carbon dioxide as well as ethanol. Polymers have repeating units; do not call ethene and poly(ethene) the same substance.
Key points
- Alkanes are saturated; alkenes are unsaturated.
- Alkenes decolourise bromine water by addition.
- Complete combustion gives CO2 and H2O; incomplete gives CO and soot.
- Cracking produces smaller alkenes and alkanes.
- Addition polymerisation uses alkene monomers.
Practical Chemistry Skills, Qualitative Analysis and Laboratory Safety
Practical chemistry skills, qualitative analysis and laboratory safety
This domain tests safe handling, accurate measurement, observation and identification of gases, cations and anions. GCE O-Level practical questions expect precise tests, correct observations and conclusions, not vague statements.
Core rules
- Gas tests: hydrogen gives a squeaky pop with a lighted splint; oxygen relights a glowing splint; carbon dioxide turns limewater milky; ammonia turns damp red litmus blue; chlorine bleaches damp litmus.
- Cation tests use sodium hydroxide and ammonia solutions. Many metal hydroxides are insoluble precipitates with characteristic colours. Some precipitates dissolve in excess reagent.
- Flame tests: sodium gives yellow, potassium lilac, calcium brick-red, copper blue-green. Clean the wire with concentrated hydrochloric acid between tests.
- Anion tests: carbonate gives effervescence with dilute acid and turns limewater milky; sulfate gives a white precipitate with acidified barium chloride; chloride gives a white precipitate with acidified silver nitrate; nitrate gives brown gas with Devarda's alloy and alkali when heated.
- Safety: wear goggles, tie back hair, use a fume cupboard for chlorine or concentrated ammonia, heat test tubes at the mouth, and never point a tube at anyone.
Worked example
An unknown solution gives a white precipitate with sodium hydroxide solution that dissolves in excess sodium hydroxide. With ammonia solution it gives a white precipitate that does not dissolve in excess. Identify the cation.
Reasoning: the white precipitate is a metal hydroxide. Solubility in excess sodium hydroxide shows amphoteric behaviour, which is characteristic of Al3+ or Zn2+. However, zinc hydroxide also dissolves in excess ammonia, forming a soluble complex. Aluminium hydroxide is insoluble in excess ammonia. Therefore the cation is Al3+. A flame test is not useful because aluminium gives no distinctive flame colour.
Traps that cost marks
Acidify silver nitrate with dilute nitric acid, not hydrochloric acid, before testing for chloride. Acidify barium chloride with dilute hydrochloric acid before testing for sulfate. Do not report a precipitate colour without naming the reagent and excess condition. Do not smell gases directly. Record observations, then the conclusion.
Key points
- Use specific gas tests and record the exact observation.
- Cation tests depend on precipitate colour and solubility in excess reagent.
- Flame tests require a clean wire and concentrated acid.
- Acidify reagents correctly for anion tests.
- Safety procedures are part of the assessed practical skills.