AQA GCSE Chemistry 8462: Organic chemistry – Study Guide

This study guide teaches 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 assessed on Paper 2 (1 hour 45 minutes, 100 marks, 50% of the GCSE), which covers Topics 6 to 10 and can draw on ideas from sections 4.1 to 4.3. Both Foundation and Higher tier sit this topic. Three parts are Higher tier only – the weak-acid explanation in 4.7.2.4, condensation polymerisation (4.7.3.2) and amino acids (4.7.3.3) – and they are labelled below.

Use it with the Organic chemistry revision notes and the Organic chemistry practice questions. The course hub is AQA GCSE Chemistry and the printable checklist lists every statement.

What this topic covers

Spec	What you must be able to do	Tier
4.7.1.1	Describe crude oil; recognise alkanes (CnH2n+2); name methane to butane	Both
4.7.1.2	Explain fractional distillation by evaporation and condensation; name fuels and petrochemical products	Both
4.7.1.3	Recall trends in boiling point, viscosity and flammability; balance complete combustion equations	Both
4.7.1.4	Describe catalytic and steam cracking; bromine water test; balance cracking equations	Both
4.7.2.1–4.7.2.2	Alkenes (CnH2n), unsaturation, ethene to pentene; addition of hydrogen, water and halogens	Both
4.7.2.3	Alcohols: methanol to butanol, four reactions, uses, fermentation	Both
4.7.2.4	Carboxylic acids: methanoic to butanoic acid, three reactions, ethyl ethanoate	Both
4.7.2.4	Explain why carboxylic acids are weak acids	Higher tier only
4.7.3.1	Addition polymerisation; draw polymers from monomers	Both
4.7.3.2–4.7.3.3	Condensation polymerisation; amino acids and polypeptides	Higher tier only
4.7.3.4	DNA, proteins, starch and cellulose and their monomers	Both

You only need the names given in the spec: four alkanes, four alkenes, four alcohols, four carboxylic acids and one ester (ethyl ethanoate).

Crude oil and alkanes (4.7.1.1)

Crude oil is a finite resource found in rocks. It is the remains of an ancient biomass, mainly plankton, that was buried in mud. It is a mixture of a very large number of compounds. Most of them are hydrocarbons: molecules made of hydrogen and carbon atoms only.

Most hydrocarbons in crude oil are alkanes, general formula CnH2n+2. The first four are methane CH4, ethane C2H6, propane C3H8 and butane C4H10. Every carbon has four single bonds, so alkanes are saturated.

You must recognise an alkane from a molecular formula (C2H6), a condensed formula (CH3CH3) or a displayed formula:

      H   H
      |   |
  H - C - C - H        ethane, C2H6
      |   |
      H   H

Worked example 1. Which of C6H14, C5H10 and C9H20 are alkanes? Test each against CnH2n+2. C6H14: 2 × 6 + 2 = 14, so alkane. C5H10: 2 × 5 + 2 = 12, not 10, so not an alkane (it fits CnH2n). C9H20: 2 × 9 + 2 = 20, so alkane.

Fractional distillation and petrochemicals (4.7.1.2)

Fractional distillation separates crude oil into fractions. Each fraction contains molecules with a similar number of carbon atoms.

How it works, in terms of evaporation and condensation:

1. Crude oil is heated so that most of it evaporates.
2. The vapour enters a column that is hot at the bottom and cooler towards the top.
3. Vapour rises. When a substance reaches a level cooler than its boiling point, it condenses and is collected there.
4. Large molecules have high boiling points and condense low down. Small molecules have low boiling points and rise higher before condensing. The smallest leave the top as gases.

Fractions are processed into fuels – petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases – and into feedstock for the petrochemical industry, which makes solvents, lubricants, polymers and detergents. The huge range of carbon compounds exists because carbon atoms can form families of similar compounds. You do not need the names of other fractions.

Properties and combustion of hydrocarbons (4.7.1.3)

As molecules get larger:

• boiling point increases
• viscosity increases (thicker, flows less easily)
• flammability decreases (harder to ignite).

These trends decide use: small, runny, easily ignited hydrocarbons make good fuels.

Combustion releases energy. The carbon and hydrogen are oxidised. Complete combustion gives carbon dioxide and water.

Worked example 2. Balance the complete combustion of pentane, C5H12.

C5H12 + ?O2 → 5CO2 + 6H2O        (balance C, then H)
O atoms on right = 5 × 2 + 6 × 1 = 16  → 8 O2
C5H12 + 8O2 → 5CO2 + 6H2O

If the O2 count comes out as a half, double everything: 2C6H14 + 19O2 → 12CO2 + 14H2O.

Cracking and alkenes (4.7.1.4)

Cracking breaks large hydrocarbons into smaller, more useful molecules. Two methods in general terms:

• Catalytic cracking: the long-chain hydrocarbon is heated to vaporise it, and the vapour is passed over a hot catalyst.
• Steam cracking: the hydrocarbon vapour is mixed with steam and heated to a very high temperature.

The products are smaller alkanes and alkenes. Alkenes are more reactive than alkanes. Bromine water is the test for alkenes: it turns from orange to colourless. With an alkane it stays orange.

Why crack? Demand for fuels with small molecules is higher than the supply from distillation, so some cracked products are used as fuels. The alkenes are used to make polymers and as starting materials for many other chemicals. Modern life – transport, heating, plastics, medicines – depends on these uses of hydrocarbons.

Worked example 3. Complete: C13H28 → C9H20 + 2 ____ Carbon: 13 − 9 = 4, shared between two molecules, so 2 C each. Hydrogen: 28 − 20 = 8, so 4 H each. Missing product: C2H4 (ethene). Check: C9H20 is an alkane, C2H4 an alkene – the usual pattern.

Alkenes and their reactions (4.7.2.1–4.7.2.2)

Alkenes contain a C=C double bond. General formula CnH2n. They are unsaturated: two fewer hydrogen atoms than the alkane with the same number of carbons. The first four are ethene C2H4, propene C3H6, butene C4H8 and pentene C5H10.

      H   H   H
      |   |   |
      C = C - C - H     propene, C3H6 (CH2=CHCH3)
      |       |
      H       H

The functional group is C=C, and it is the functional group that decides how an organic compound reacts.

• Combustion: alkenes burn like other hydrocarbons, but tend to burn with smoky flames because of incomplete combustion.
• Addition: atoms add across the double bond, which becomes a single C–C bond.

Added	Conditions	Product from ethene
Hydrogen, H2	nickel catalyst, heated	ethane, C2H6
Water (as steam), H2O	high temperature, high pressure, catalyst (phosphoric acid)	ethanol, C2H5OH
Chlorine, bromine or iodine	room temperature, no catalyst	C2H4Cl2, C2H4Br2, C2H4I2

Worked example 4. Draw the product of propene with bromine. One Br goes on each carbon that was in the double bond. All bonds are now single.

      H   H   H
      |   |   |
  H - C - C - C - H     C3H6Br2
      |   |   |
     Br  Br   H

You must be able to draw displayed formulae of all four alkenes and their products with hydrogen, water, chlorine, bromine and iodine. Put the added atoms on the two carbons of the old double bond.

Alcohols (4.7.2.3)

Alcohols contain the functional group –OH. The first four are methanol CH3OH, ethanol CH3CH2OH, propanol and butanol. Watch the names: “-ol” means alcohol.

What happens when the first four alcohols:

• react with sodium: fizzing, hydrogen gas is given off
• burn in air: they burn cleanly, giving carbon dioxide and water, so they are used as fuels
• are added to water: they dissolve and form a neutral solution
• react with an oxidising agent: they are oxidised to a carboxylic acid (ethanol → ethanoic acid).

Main uses: fuels, solvents, and ethanol as the alcohol in alcoholic drinks.

Fermentation: aqueous ethanol is made by fermenting a sugar solution with yeast, in warm conditions (about 30 °C) with air kept out.

The only alcohol equations you must write are combustion equations: C2H5OH + 3O2 → 2CO2 + 3H2O.

Carboxylic acids (4.7.2.4)

Carboxylic acids contain –COOH. The first four are methanoic acid HCOOH, ethanoic acid CH3COOH, propanoic acid and butanoic acid.

• With carbonates: fizzing, carbon dioxide is given off; a salt and water form (ethanoic acid gives sodium ethanoate with sodium carbonate).
• In water: they dissolve to give an acidic solution with a pH above that of a strong acid at the same concentration.
• With alcohols: an acid catalyst is used and an ester forms. Ethanol + ethanoic acid → ethyl ethanoate + water.

You do not have to write balanced equations for carboxylic acid reactions.

Higher tier only – why they are weak acids. A carboxylic acid only partially ionises in water, so the concentration of H⁺ ions is lower than for a strong acid (which fully ionises) at the same concentration. That is why its pH is higher.

Polymers (4.7.3)

Addition polymerisation (4.7.3.1)

Many small monomers join to form very large polymers. Alkenes make addition polymers such as poly(ethene) and poly(propene). The C=C opens and the monomers link.

      H   H                (  H   H  )
      |   |                (  |   |  )
  n   C = C      →       —(— C - C —)—
      |   |                (  |   |  )
      H   H                (  H   H  ) n

The repeating unit has the same atoms as the monomer, because no other molecule is formed. To draw a polymer from any alkene: copy the monomer, make the C=C single, add bonds out through the brackets, and write n.

Worked example 5. A sample of poly(propene) has Mr 84 000. How many monomers joined? Mr of C3H6 = 3 × 12 + 6 × 1 = 42. Number of units = 84 000 ÷ 42 = 2000.

Condensation polymerisation (4.7.3.2) – Higher tier only

Each monomer has two functional groups. When they react, they join and usually lose a small molecule such as water. The simplest case uses two different monomers, each with two of the same group: ethanediol (two –OH) and hexanedioic acid (two –COOH) make a polyester. For each repeating unit, two water molecules are lost – one for each ester link.

Amino acids (4.7.3.3) – Higher tier only

Amino acids have two different functional groups: an amine group (–NH2) and a carboxylic acid group (–COOH). Glycine is H2NCH2COOH. Amino acids react by condensation polymerisation to form polypeptides, losing water at each link. Different amino acids combined in one chain make proteins.

DNA and natural polymers (4.7.3.4)

DNA encodes the genetic instructions for the development and functioning of living organisms and viruses. Most DNA molecules are two polymer chains in a double helix, made from four different monomers called nucleotides.

Natural polymer	Monomer
DNA	nucleotides (four different)
Protein	amino acids
Starch	glucose (a sugar)
Cellulose	glucose (a sugar)

Common errors

• Saying larger hydrocarbons are more flammable. The trend is the other way.
• Explaining distillation with “heavier molecules sink”. Use boiling point, evaporation and condensation.
• Writing bromine water goes “clear”. Clear means see-through; the answer is colourless.
• Adding water to an alkene without saying steam and a catalyst.
• Drawing a polymer without the extension bonds through the brackets, or leaving the C=C in the chain.
• Confusing oxidation of ethanol (gives ethanoic acid) with combustion (gives CO2 and water).
• Saying a weak acid is a dilute acid. Weak means partially ionised; dilute means low concentration.

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.