AQA GCSE Chemistry 8462: Organic chemistry – Revision Notes

These notes condense Topic 7, Organic chemistry (sections 4.7.1 to 4.7.3), of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 with exams from June 2018 (version 1.1). The topic is examined on Paper 2 at Foundation and Higher tier. Higher tier only content is the weak-acid explanation in 4.7.2.4, condensation polymerisation (4.7.3.2) and amino acids (4.7.3.3); it is labelled below.

For full explanations and worked examples, use the Organic chemistry study guide. Then test yourself with the Organic chemistry practice questions. The AQA GCSE Chemistry hub and the printable checklist show where this topic sits in the course.

Key definitions

• Hydrocarbon: a compound of hydrogen and carbon only.
• Crude oil: a finite mixture of very many compounds, mostly hydrocarbons; the remains of ancient biomass (mainly plankton) buried in mud.
• Homologous series: a family of compounds with the same general formula and the same functional group.
• Saturated: only single C–C bonds (alkanes). Unsaturated: contains C=C (alkenes).
• Fraction: a group of hydrocarbons with a similar number of carbon atoms.
• Cracking: breaking large hydrocarbons into smaller, more useful molecules.
• Functional group: the part of a molecule that decides how it reacts (C=C, –OH, –COOH).
• Monomer / polymer: small molecules / the very large molecule made when many join.
• Repeating unit: the section that repeats along a polymer chain.

The four homologous series

Series	General formula / group	First four	Example formula
Alkanes	CnH2n+2	methane, ethane, propane, butane	C3H8
Alkenes	CnH2n, C=C	ethene, propene, butene, pentene	C4H8
Alcohols	–OH	methanol, ethanol, propanol, butanol	CH3CH2OH
Carboxylic acids	–COOH	methanoic, ethanoic, propanoic, butanoic acid	CH3COOH

Only ester you need: ethyl ethanoate.

Recognising a compound, in steps:

1. Only C and H? It is a hydrocarbon. Check the H count: 2n + 2 means alkane; 2n means alkene.
2. Contains O? Look for –OH on its own (alcohol, name ends “-ol”) or –COOH (carboxylic acid, name ends “-oic acid”).
3. In a displayed formula, look for C=C (alkene), O–H joined to a carbon with no C=O (alcohol), or C=O and O–H on the same carbon (carboxylic acid).

Displayed formulae to learn

      H   H                        H   O
      |   |                        |   //
  H - C - C - O - H            H - C - C
      |   |                        |    \
      H   H                        H     O - H
     ethanol                     ethanoic acid

Why modern life depends on hydrocarbons: fuels for transport, heating and power stations, and feedstock for plastics, solvents, lubricants and detergents all come from crude oil.

Crude oil to useful products

Fractional distillation, in steps:

1. Heat crude oil so it evaporates.
2. Vapour rises up a column: hot at the bottom, cooler at the top.
3. Each hydrocarbon condenses when it reaches a level below its boiling point.
4. Long chains (high boiling points) collect low; short chains collect high; the smallest leave as gases.

Fuels from crude oil: petrol, diesel oil, kerosene, heavy fuel oil, liquefied petroleum gases. Petrochemical products: solvents, lubricants, polymers, detergents.

Trends with increasing molecular size

Property	Trend
Boiling point	increases
Viscosity	increases
Flammability	decreases

Complete combustion: hydrocarbon + oxygen → carbon dioxide + water. C and H are oxidised; energy is released.

Balancing combustion, in steps: balance C with CO2 → balance H with H2O → count O atoms on the right → halve for O2 → if you get a half, double every coefficient.

Cracking

Method	Conditions (general terms)
Catalytic cracking	hydrocarbon vaporised and passed over a hot catalyst
Steam cracking	hydrocarbon vapour mixed with steam at a very high temperature

• Products: smaller alkanes + alkenes.
• Test for alkene: bromine water, orange → colourless.
• Reasons: high demand for small-molecule fuels; alkenes make polymers and other chemicals.
• Balancing: carbon and hydrogen atoms on the left must equal the totals on the right.

Worked reminder: C15H32 → C9H20 + 2C3H6. Left: 15 C, 32 H. Right: 9 + 6 = 15 C and 20 + 12 = 32 H. Balanced, and the products are one alkane and one alkene.

Drawing an addition product, in steps:

1. Draw the alkene with its C=C.
2. Break the double bond to a single bond.
3. Put one added atom (H, Cl, Br or I) on each of the two carbons that were double-bonded. For water, put H on one carbon and –OH on the other.
4. Count: every carbon must have four bonds.

Alkene reactions

All additions go across the C=C, which becomes C–C.

Reactant	Conditions	Ethene gives
H2	nickel catalyst, heat	ethane
H2O (steam)	high temperature and pressure, catalyst	ethanol
Cl2 / Br2 / I2	room temperature	C2H4Cl2 / C2H4Br2 / C2H4I2
O2 (burning)	ignite	CO2 + H2O, but smoky flame (incomplete combustion)

Worked reminder: butene + hydrogen → butane, C4H8 + H2 → C4H10. Butene + water → butanol.

Alcohols – four reactions

With	What you see / get
Sodium	fizzing; hydrogen given off
Air (burning)	burns; carbon dioxide + water
Water	dissolves; neutral solution
Oxidising agent	carboxylic acid forms (propanol → propanoic acid)

Uses: fuels, solvents; ethanol in alcoholic drinks. Fermentation: sugar solution + yeast, warm (about 30 °C), air kept out → aqueous ethanol. Equations required only for combustion, e.g. 2CH3OH + 3O2 → 2CO2 + 4H2O.

Carboxylic acids – three reactions

With	Result
Carbonate	fizzing; CO2 + salt + water
Water	dissolves; weakly acidic solution
Alcohol (acid catalyst)	ester + water; ethanoic acid + ethanol → ethyl ethanoate

Higher tier only: weak acid = only partially ionised in water, so lower H⁺ concentration and higher pH than a strong acid at the same concentration.

Polymers

Addition (both tiers): alkene monomers; the C=C opens; repeating unit has the same atoms as the monomer; nothing else forms.

Drawing method: copy monomer → change C=C to C–C → bonds out through brackets → write n after the bracket.

          H   CH3              (  H   CH3 )
          |   |                (  |   |   )
     n    C = C      →       —(— C - C —)—
          |   |                (  |   |   )
          H   H                (  H   H   ) n
      propene                  poly(propene)

Condensation (Higher tier only): monomers with two functional groups each; small molecule (usually water) lost per link. Ethanediol + hexanedioic acid → polyester + water.

Worked reminder (Higher tier only): ethanediol (Mr 62) and hexanedioic acid (Mr 146) join and two water molecules (2 × 18 = 36) are lost per repeating unit, so the repeating unit has Mr 62 + 146 − 36 = 172. In an addition polymer, by contrast, the repeating unit has exactly the same Mr as the monomer.

Amino acids (Higher tier only): two different groups (–NH2 and –COOH) in one molecule; glycine H2NCH2COOH; condensation → polypeptides; different amino acids → proteins.

Natural polymers: DNA (nucleotides, four types; two chains, double helix), proteins (amino acids), starch and cellulose (glucose, a sugar).

Must-know distinctions

• Alkane vs alkene: CnH2n+2, saturated, bromine water stays orange vs CnH2n, unsaturated, orange → colourless.
• Addition vs condensation polymer: one product only vs polymer + small molecule; monomer with C=C vs monomer with two functional groups.
• Oxidation vs combustion of an alcohol: carboxylic acid vs CO2 + water.
• Weak vs dilute: degree of ionisation vs amount per volume.
• Distillation vs cracking: physical separation vs chemical reaction making new molecules.

Quick self-test

1. Give the formula of the alkane with 7 carbon atoms.
2. Is C12H24 an alkane or an alkene?
3. State how viscosity changes as hydrocarbon molecules get larger.
4. Balance: C4H10 + O2 → CO2 + H2O.
5. Complete the cracking equation: C18H38 → C10H22 + 2 ____
6. State the colour change when propene is shaken with bromine water.
7. Name the product when ethene reacts with steam, and give the conditions.
8. What forms when butanol reacts with an oxidising agent?
9. Name the gas given off when ethanoic acid reacts with calcium carbonate.
10. A sample of poly(ethene) contains 2500 monomer units. Calculate its Mr. (Ar: C = 12, H = 1)
11. Name the monomers of starch and of DNA.
12. Higher tier only. Glycine (C2H5NO2) forms a polypeptide. Calculate the Mr of one repeating unit. (Ar: C = 12, H = 1, N = 14, O = 16)

Answers

1. C7H16 (2 × 7 + 2 = 16).
2. Alkene: 24 = 2 × 12, so it fits CnH2n.
3. Viscosity increases.
4. 2C4H10 + 13O2 → 8CO2 + 10H2O.
5. C4H8. Carbon 18 − 10 = 8, so 4 each; hydrogen 38 − 22 = 16, so 8 each.
6. Orange to colourless.
7. Ethanol; high temperature, high pressure and a catalyst.
8. Butanoic acid (a carboxylic acid).
9. Carbon dioxide.
10. Mr of C2H4 = 28; 28 × 2500 = 70 000.
11. Starch: glucose. DNA: nucleotides.
12. Mr glycine = 75; one water (18) is lost per link, so 75 − 18 = 57.

Where marks are usually lost

• Writing “clear” instead of colourless for bromine water.
• Reversing the flammability trend – small molecules are the more flammable ones.
• Explaining fractional distillation by weight instead of boiling point, evaporation and condensation.
• Leaving a half in a combustion equation instead of doubling all coefficients.
• Forgetting “catalyst” for hydrogenation or hydration of alkenes.
• Missing the “n” or the extension bonds when drawing an addition polymer.
• Writing a formula for an ester or carboxylic acid equation when only a word description is needed – and then getting it wrong. Keep to what is asked.
• Calling a carboxylic acid weak “because it is dilute” (Higher tier).
• Saying a condensation polymer’s repeating unit has the same atoms as the monomers. Water has been lost.

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016, GCSE exams June 2018 onwards, version 1.1, published by AQA – section 4.7 Organic chemistry.