Carbon is the only element that gets a whole branch of chemistry to itself. Because carbon atoms can form four strong covalent bonds — including bonds to other carbon atoms — they build chains and rings of almost any size, and that ability makes a near-limitless family of compounds possible. The name organic chemistry comes from the original sources of these compounds: living, or once-living, material. C7 follows that carbon from its biggest modern source — crude oil — through the fuels in cars and aircraft, to the alcohols, acids and polymers that the petrochemical industry builds from it.
C7 (4.7) has the biggest Combined/Triple split of any chemistry topic, so check what you actually need:
- Everyone — sections 1–4: crude oil, hydrocarbons and alkanes, fractional distillation, properties of hydrocarbons and combustion, and cracking (including the bromine water test for alkenes).
- Triple (Chemistry only) T — sections 5–11: the structure and reactions of alkenes, alcohols, carboxylic acids and esters, and all of the polymer chemistry.
- Higher Tier H (within Triple) — explaining why carboxylic acids are weak acids (section 8), and the whole of condensation polymerisation and amino acids (section 10).
There is no required practical in C7 — but fuels questions love to borrow from other topics: pollutants from burning fuels (C9), energy released by burning fuels (C5) and intermolecular forces (C2) all turn up in organic chemistry questions, and they’re flagged where they do.
1Crude Oil, Hydrocarbons & Alkanes
Crude oil is a finite resource found in rocks. It is the remains of an ancient biomass — mostly plankton — that was buried in mud and compressed over millions of years. “Finite” matters: it is being used far faster than it forms, so on any human timescale it cannot be replaced.
- Crude oil — a finite resource found in rocks; the remains of an ancient biomass, consisting mainly of plankton, that was buried in mud.
- Hydrocarbon — a molecule made up of hydrogen and carbon atoms only. The word only is usually a mark on its own: ethanol (C2H5OH) contains carbon and hydrogen but is not a hydrocarbon, because it also contains oxygen.
- Mixture — crude oil is a mixture of a very large number of compounds, most of which are hydrocarbons. Because it’s a mixture, its components keep their own properties and can be separated physically (section 2).
Alkanes — the main hydrocarbons in crude oil
Most of the hydrocarbons in crude oil belong to one family: the alkanes. Alkanes are saturated hydrocarbons — every bond is a single covalent bond, so each molecule holds as many hydrogen atoms as it possibly can. They form a homologous series with the general formula:
CnH2n+2
Put in the number of carbon atoms (n) and the formula tells you the number of hydrogens. The first four alkanes — the only ones you need to know by name — are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). The prefixes meth- (1C), eth- (2C), prop- (3C) and but- (4C) repeat across every family in this topic — alkenes, alcohols, carboxylic acids — so learn them once with a memory hook (Monkeys Eat Peanut Butter) and they pay out four times.
The first four alkanes. Every bond is a single covalent bond — the molecules are saturated — and each one fits CnH2n+2.
Alkanes are a homologous series — a family of compounds that:
- share the same general formula (CnH2n+2 for alkanes),
- differ from the next member by CH2 (count up the table: CH4, C2H6, C3H8…),
- have similar chemical reactions, and
- show gradually changing physical properties (boiling point rises with size — section 3).
Exam questions exploit the pattern: given four successive members with one formula missing, fill the gap by adding CH2 — if C4H10 and C6H14 are members, the one between them is C5H12.
Recognising alkanes from formulae
You can be shown a molecule three ways — a name, a molecular formula (C2H6) or a displayed formula (every atom and every bond drawn out, as in the figure above) — and you must recognise alkanes in all of them. With a formula, test it against CnH2n+2:
- C6H14: n = 6, so 2n+2 = 14. The hydrogens match — alkane.
- C7H14: n = 7, so an alkane would need 16 hydrogens. This molecule has two fewer — it fits CnH2n instead, so it’s an alkene (section 4).
- In a displayed formula, look for the giveaway instead: all single bonds means alkane; a C=C double bond means alkene.
- Writing C4H12 for the alkane after propane. The general formula gives C4H10 — always calculate 2n+2 rather than guessing the pattern.
- Saying crude oil is “made of dinosaurs”. The mark scheme wants ancient biomass / plankton, buried in mud.
🧪 Exam-style questions
Which of these compounds is an alkane? Tick (✓) one box.
Ethane, propane and butane are successive members of the alkane homologous series. The next member is pentane. What is the formula of pentane? Tick (✓) one box.
What is the simplest whole-number ratio of carbon : hydrogen atoms in a molecule of butane (C4H10)? Complete the ratio 2 : …
Show answer
- C : H = 4 : 10. Divide both by 2 to get the simplest form: 2 : 5. 1 mark
Octane (C8H18) is found in petrol. Which statement explains why octane is a hydrocarbon? Tick (✓) one box.
2Fractional Distillation
Crude oil straight out of the ground is nearly useless — a thick black mixture of thousands of different hydrocarbons. It becomes useful when it is separated into fractions: groups of hydrocarbons with a similar number of carbon atoms, and therefore similar boiling points. The separation happens in a fractionating column, and the whole process runs on just two physical changes: evaporation and condensation.
How the column works — the answer examiners want
This process is one of the most reliable extended-answer questions on Paper 2 (anything from 2 to 6 marks), and the mark scheme is built from a fixed chain of ideas. Learn it as a sequence:
- The crude oil is heated until it evaporates (vaporises) — around 350 °C in the furnace.
- The vapours enter a column that is hot at the bottom and cool at the top (a temperature gradient).
- Vapours rise up the column and cool down.
- Each fraction condenses when it reaches the level where the temperature equals its boiling point, and is piped off.
- Hydrocarbons with high boiling points (large molecules) condense near the bottom; those with low boiling points (small molecules) rise higher before condensing.
- The smallest molecules — the refinery gases — never condense: they leave the top of the column still as gases. Bitumen, with the highest boiling point, drains from the very bottom.
Notice what makes the separation possible in the first place: the fractions have different boiling points because boiling point depends on molecular size — bigger molecules are held together by stronger intermolecular forces, a structure-and-bonding idea from C2 that section 3 recaps and links back to. And keep the vocabulary physical, not chemical — no bonds inside molecules are broken anywhere in this process. Fractional distillation is a physical separation of a mixture, which is exactly why it works: the hydrocarbons in crude oil are not chemically combined.
One more linking idea: each fraction is still a mixture — its molecules have similar boiling points, not identical ones — so a fraction boils over a range of temperatures. A pure substance boils at one sharp temperature (the purity test you’ll meet in C8), which is why data tables always give fractions a boiling point range.
Worked example — placing a fraction
A question gives you boiling point ranges — kerosene 180–260 °C, diesel oil 260–320 °C, heavy fuel oil 320–400 °C — and asks: explain why diesel oil collects above heavy fuel oil but below kerosene (2 marks).
- Diesel oil’s boiling point is lower than heavy fuel oil’s, so its vapour rises further up the column before it condenses — it collects above heavy fuel oil. 1 mark
- Diesel oil’s boiling point is higher than kerosene’s, so it condenses at a hotter level, lower down than kerosene. 1 mark
- The pattern for any “where does it collect” question: match the boiling point to the temperature of the level in the column.
Fuels and petrochemical feedstock
Most fractions are processed into fuels — much of modern life runs on them: petrol for cars, kerosene for aircraft, diesel oil for lorries and trains, heavy fuel oil for ships and power stations, and liquefied petroleum gases (LPG) for camping stoves and heating. You don’t need to memorise other fraction names — the exam will supply any table it wants you to use.
Crude oil is also the feedstock (raw material) for the petrochemical industry, which makes solvents, lubricants, polymers and detergents. That short list of four is worth learning — “which two are produced from petrochemical feedstock?” is a recurring multiple-choice question, with alloys, ceramics and fertilisers as the distractors.
The vast array of natural and synthetic carbon compounds exists because carbon atoms can form families of similar compounds — chains and rings linked by C–C bonds, sorted into homologous series like the alkanes, alkenes and alcohols in this topic.
🧪 Exam-style questions
Three fractions have these boiling point ranges: fraction A 200–300 °C, fraction B 100–150 °C, fraction C below 30 °C. The column is 30 °C at the top and 400 °C at the bottom. Where is each fraction collected? Tick (✓) one box.
Which two of these are produced by the petrochemical industry from crude oil feedstock? Tick (✓) two boxes, then press Check.
A sample of crude oil is 16.5% petrol, 11.5% diesel oil and 72% other fractions by mass. Petrol and diesel oil are both used as car fuel. Calculate the total mass of car fuel that can be produced from 2000 kg of this crude oil.
Show answer
- Total percentage used as car fuel = 16.5 + 11.5 = 28%. 1 mark
- Mass = 0.28 × 2000 1 mark
- = 560 kg. 1 mark
Most of the compounds in crude oil are hydrocarbons; those with the smallest molecules are the most volatile. Using the fractionating column above, describe and explain how petrol is separated from the mixture of hydrocarbons in crude oil. This is a levels-of-response question — you are also assessed on clear written communication, so organise your answer logically and use specialist terms.
Show a model answer
How it is marked — marks depend on the quality of written communication as well as the science:
- Level 3 (5–6): a reasonable explanation of how petrol / the fractions are separated from crude oil using evaporating AND condensing.
- Level 2 (3–4): some description of heating / evaporating crude oil, AND either that fractions have different boiling points OR that there is a temperature difference up the column.
- Level 1 (1–2): a statement that crude oil is heated OR that substances are cooled, with little detail.
Example chemistry points:
- the crude oil / mixture of hydrocarbons is heated (to about 350 °C) so that most of the hydrocarbons evaporate / form vapours
- the column has a temperature gradient — hot at the bottom, cooler at the top
- the vapours rise up the column and cool as they rise
- a fraction condenses when it cools to its boiling point; the condensed fraction (petrol) separates from the vapours and flows out through a pipe
- hydrocarbons with low boiling points (like petrol) are collected near the top; those with high boiling points near the bottom
- the process is fractional distillation
Note: if cracking / a catalyst is given as part of the separation process, the answer is capped at Level 2.
Source: AQA GCSE Chemistry.
3Properties of Hydrocarbons & Combustion
Why is petrol runny and easy to light, while heavy fuel oil is gloopy and reluctant to burn? Same family of molecules — different sizes. Exactly three properties change with molecular size, and all three move in a predictable direction.
As molecules get bigger: boiling point and viscosity increase, flammability decreases. Small molecules make the best fuels.
- Boiling point increases — bigger molecules are harder to boil, which is exactly what fractional distillation exploits.
- Viscosity increases — bigger molecules flow less easily (compare runny petrol with sticky bitumen).
- Flammability decreases — bigger molecules are harder to ignite.
These properties decide how each fraction is used as a fuel: the small, runny, easy-to-light molecules (petrol, kerosene) power engines, while a fraction like bitumen — very viscous and very hard to ignite — isn’t used as a fuel at all.
Complete combustion
Hydrocarbons are fuels because their combustion releases energy — strongly exothermic reactions. Burn a hydrocarbon in plenty of air and you get complete combustion: both elements in the fuel are fully oxidised. The carbon becomes carbon dioxide, the hydrogen becomes water — and the oxygen comes from the air.
hydrocarbon + oxygen → carbon dioxide + water
You already know how to balance an equation, and complete combustion just applies it. One ordering trick makes it painless: balance carbon first, then hydrogen, then oxygen last. Because the oxygen on the left is on its own (as O2), it can soak up whatever number the carbon dioxide and water end up needing. If that oxygen total comes out odd, balance with a half — C2H6 + 3½O2 → 2CO2 + 3H2O — then double everything for whole numbers.
Balance the equation for the complete combustion of propane.💡 Hint: balance carbon first, then hydrogen, then oxygen last. Type a number in each box (leave it as 1 if no number is needed), then press Check — any correct set of numbers is accepted.
Show answer
- Carbons: 3 → 3CO2. Hydrogens: 8 → 4H2O.
- Right-hand oxygens: (3×2) + (4×1) = 10, so 5O2: C3H8 + 5O2 → 3CO2 + 4H2O.
In a limited supply of air, combustion is incomplete: the products can include carbon monoxide (a toxic gas) and carbon itself — the soot that blackens a beaker held over a smoky flame. A smoky, sooty flame is the visual signature of incomplete combustion, and it’s why alkenes (which burn with smoky flames) make poor fuels (section 6). The pollutant chemistry — carbon monoxide, particulates, sulfur dioxide and oxides of nitrogen — is examined fully in C9 Chemistry of the Atmosphere, but fuels questions in C7 borrow it freely: particulates cause global dimming, sulfur dioxide causes acid rain, and the fix examiners accept for city air pollution is fuel-efficient engines and electric vehicles.
🧪 Exam-style questions
The hydrocarbon C18H38 is cracked to produce C6H14, C4H8 and C2H4. Compared with the three products, C18H38 has… Tick (✓) one box.
Which two substances are produced by the complete combustion of any hydrocarbon? Tick (✓) two boxes, then press Check.
Balance the equation for the complete combustion of methane.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check. Any correct set of numbers is accepted.
Show answer
- 1 carbon → 1CO2; 4 hydrogens → 2H2O. 1 mark
- Right-hand oxygens: 2 + 2 = 4, so 2O2: CH4 + 2O2 → CO2 + 2H2O. 1 mark
Petrol contains the hydrocarbon C9H20. Balance the equation for its complete combustion.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check.
Show answer
- 9 carbons → 9CO2; 20 hydrogens → 10H2O. 1 mark
- Right-hand oxygens: 18 + 10 = 28, so 14O2: C9H20 + 14O2 → 9CO2 + 10H2O. 1 mark
When a hydrocarbon was burned, the bottom of the beaker above the flame turned black. Why? Tick (✓) one box.
Propene (C3H6) burns in a limited supply of air and incomplete combustion occurs. Which equation correctly represents this reaction? Tick (✓) one box.
Successive alkanes have these boiling points: butane 0 °C, pentane ?, hexane 69 °C, heptane 98 °C. Which is the best estimate of the boiling point of pentane? Tick (✓) one box.
4Cracking & the Test for Alkenes
Fractional distillation has an economics problem: the mix of fractions a barrel of crude oil contains is not the mix the world wants to buy. There is high demand for fuels with small molecules — petrol above all — and limited demand for the large, viscous, hard-to-ignite fractions. The fix is cracking: breaking large hydrocarbon molecules down into smaller, more useful ones. Cracking is a chemical reaction — a thermal decomposition — unlike the physical separation in section 2.
Cracking, drawn as displayed formulae: one large, low-demand alkane breaks into a smaller alkane (fuel) plus an alkene (the C=C, in red, makes it reactive — used for polymers). Atoms are conserved — count the carbons: 10 = 8 + 2.
You only need the conditions “in general terms”:
- Catalytic cracking — the fraction is heated to vaporise it, and the vapours are passed over a hot catalyst, which speeds the reaction up without being used up (the rate idea from C6).
- Steam cracking — the vaporised fraction is mixed with steam and heated to a very high temperature.
Asked for two conditions for cracking? High temperature + catalyst (or high temperature + steam) are the safe pairings. And a nice “suggest” question: why must air be kept out of the reactor? Because the hot hydrocarbon vapour would combust.
The products — and balancing cracking equations
Cracking produces two kinds of hydrocarbon: smaller alkanes, useful as fuels because small molecules are in high demand, and alkenes — a new homologous series with general formula CnH2n and a C=C double bond. Alkenes are more reactive than alkanes, and are used to make polymers (section 9) and as starting materials for many other chemicals.
The diagram above names one alkene — ethene (C2H4) — as an example, but Combined Science students do not need the names or formulae of individual alkenes. What everyone needs from this section is that cracking produces alkenes, that alkenes contain a C=C double bond, and the bromine-water test. The names and formulae of specific alkenes — ethene, propene, butene, pentene — are Triple Chemistry only T and are covered in section 5.
Balancing a cracking equation is pure atom-counting — no new atoms appear and none vanish:
C15H32 → C12H26 + ?
- Carbons: 15 − 12 = 3. Hydrogens: 32 − 26 = 6.
- The missing product is C3H6 (propene) — and it fits CnH2n, an alkene, as expected: cracking one alkane into another alkane always leaves an alkene’s worth of atoms over.
- Watch for coefficients: in C10H22 → C4H10 + 3C2H4, the 3 in front of ethene multiplies it to C6H12-worth of atoms. Check: C 10 = 4+6 ✓, H 22 = 10+12 ✓.
A long-chain alkane is cracked as shown. Work out the formula of the alkene that forms.💡 Hint: atoms are conserved, so subtract the alkane on the right from the starting molecule. The leftover always fits CnH2n — an alkene. Type the two subscripts, then press Check.
Show answer
- Carbons: 14 − 10 = 4. Hydrogens: 30 − 22 = 8.
- The alkene is C4H8 (butene) — and it fits CnH2n, as every cracked-off alkene does.
The test for alkenes — bromine water
Alkanes and alkenes can have identical-looking formulae in a list, so chemists tell them apart with a one-step test tube reaction. Shake the hydrocarbon with bromine water (orange):
Bromine water: orange → colourless with an alkene; stays orange with an alkane.
The colour change is orange → colourless. “Clear” is rejected — orange bromine water is already clear (you can see through it); what changes is the colour. And it’s the bromine water that changes colour, not the alkene. Spot alkenes in formula lists by CnH2n: of C7H14, C7H16, C8H16 and C8H18, only C7H14 and C8H16 would turn bromine water colourless.
A common 6-mark question stitches sections 2 and 4 into one answer: “Explain how alkenes are produced from crude oil using fractional distillation followed by cracking.” A Level 3 response covers both processes in order, with the right vocabulary:
- Heat the crude oil until it evaporates; the vapours rise the column, which is hot at the bottom, cool at the top.
- Each fraction condenses at its boiling point and is collected — small molecules higher up, large molecules lower down.
- A large-molecule fraction is then heated to vaporise it and passed over a hot catalyst (or mixed with steam at very high temperature).
- The large alkanes break down (thermally decompose) into smaller alkanes and alkenes.
Six linked steps, in sequence, with the right vocabulary — that’s the whole game.
🧪 Exam-style questions
Which hydrocarbon could be cracked to produce octane (C8H18)? Tick (✓) one box.
Decane is cracked: C10H22 → C5H10 + C3H8 + ? What is the formula of the third product? Tick (✓) one box.
Balance the equation for cracking decane into butane and ethene.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check.
Show answer
- Butane takes 4 carbons, leaving 6 for ethene molecules: 6 ÷ 2 = 3C2H4. Hydrogens check: 10 + (3×4) = 22 ✓. C10H22 → C4H10 + 3C2H4 1 mark
Cracking also produces hydrocarbons with five and six carbon atoms. Which pair would both turn bromine water colourless? Tick (✓) one box.
Which two conditions are used to crack large alkane molecules? Tick (✓) two boxes, then press Check.
5Structure of Alkenes T
Combined Science students met alkenes only as “the reactive products of cracking”. Triple Chemistry students study them in detail: names, formulae and structures. Alkenes are hydrocarbons with a carbon–carbon double bond, written C=C. That double bond is the functional group — the part of the molecule where the chemistry happens — and it defines the homologous series:
CnH2n
Because of the double bond, an alkene carries two fewer hydrogen atoms than the alkane with the same number of carbons. The molecule is unsaturated — it does not contain the maximum possible hydrogen, and the spare bonding capacity at the C=C is exactly what makes alkenes reactive (section 6). The first four alkenes — the only names you need — are ethene (C2H4), propene (C3H6), butene (C4H8) and pentene (C5H10). There is no “methene” — a double bond needs two carbons, so the series starts at ethene.
The first four alkenes. One C=C double bond each (in red); every formula fits CnH2n. Adding one CH2 each time steps along the homologous series.
| Alkanes | Alkenes | |
|---|---|---|
| General formula | CnH2n+2 | CnH2n |
| Bonds | All single bonds | One C=C double bond |
| Saturation | Saturated | Unsaturated |
| First member | Methane, CH4 | Ethene, C2H4 |
| Reactivity | Less reactive | More reactive (at the C=C) |
| Bromine water | Stays orange | Orange → colourless |
A functional group is the atom or group of atoms that controls how a compound reacts — the “business end” of the molecule. It comes down to one sentence worth understanding: it is the generality of reactions of functional groups that determines the reactions of organic compounds. Every member of a homologous series carries the same functional group, so every member reacts the same way — learn what C=C does once, and you know the chemistry of every alkene from ethene to pentene. The same logic powers the alcohols (–OH, section 7) and carboxylic acids (–COOH, section 8).
🧪 Exam-style questions
Hexene is an alkene with six carbon atoms. Complete the formula: C6H? — how many hydrogen atoms does hexene have?
Show answer
- Alkenes fit CnH2n, so hexene is C6H12. 1 mark
Why are alkenes described as unsaturated hydrocarbons? Tick (✓) one box.
A hydrocarbon comes from the homologous series with general formula CnH2n. Which feature must its displayed formula show? Tick (✓) one box.
6Addition Reactions of Alkenes T
Alkenes do burn — they react with oxygen like any hydrocarbon — but they burn badly: in air they tend to undergo incomplete combustion and produce smoky flames, which is one reason they’re used as chemical building blocks rather than fuels. Their useful chemistry happens at the C=C functional group instead, and it is all one type of reaction: addition.
In an addition reaction, a small molecule adds across the carbon–carbon double bond: the double bond opens to a single C–C bond, and one new atom (or group) attaches to each of the two carbons. Everything ends up in one product — nothing else is made. Alkenes add hydrogen, water (as steam) and the halogens (chlorine, bromine, iodine).
Ethene’s three addition reactions. The C=C opens, and one atom or group joins each carbon: H2 → ethane; steam → ethanol; bromine → dibromoethane.
| Added molecule | Conditions | Product type | Example (from ethene) |
|---|---|---|---|
| Hydrogen, H2 | Nickel catalyst, heat (≈150 °C) | Alkane (“hydrogenation”) | Ethane, C2H6 |
| Water, H2O (as steam) | Catalyst, high temperature & pressure | Alcohol | Ethanol, C2H5OH |
| Halogens (Cl2, Br2, I2) | Room temperature — no catalyst needed | Di-halo compound | Dibromoethane, C2H4Br2 |
The steam reaction is industrially important — it’s one of the two ways ethanol is manufactured (the other is fermentation, section 7) — and the bromine reaction is the chemistry behind the bromine water test: the orange bromine is used up as it adds across the double bond, leaving a colourless product.
Drawing the products
You need to be able to draw fully displayed formulae for the first four alkenes reacting with hydrogen, water, chlorine, bromine and iodine. One method covers every case:
- Draw the alkene’s carbon skeleton and replace the double bond with a single bond.
- Split the added molecule in two: H–H gives H and H; H–OH gives H and OH; Br–Br gives Br and Br.
- Attach one piece to each of the two carbons that used to share the double bond. Every other atom stays exactly where it was.
- Sanity-check by counting: propene + Br2 → C3H6Br2 — all six hydrogens kept, two bromines gained, no atoms lost.
- Leaving the double bond in the product. After addition the molecule is saturated — if your drawing still shows C=C, an atom count will catch it.
- Adding both halogen atoms to the same carbon. One goes on each carbon of the old double bond.
- Writing H2O as a product of the steam reaction. Addition reactions have one product only — the water is absorbed into the alcohol molecule.
🧪 Exam-style questions
Propene reacts with hydrogen in the presence of a nickel catalyst. What is the product? Tick (✓) one box.
Ethanol can be manufactured from ethene. Which reagent and conditions are used? Tick (✓) one box.
Which is the correct formula of the product when butene (C4H8) reacts with bromine (Br2)? Tick (✓) one box.
Why do alkenes tend to burn with smoky flames? Tick (✓) one box.
A few drops of bromine water are added to an alkene and the tube is shaken. What colour change is seen? Tick (✓) one box.
7Alcohols & Fermentation T
Swap one hydrogen of an alkane for an –OH group and you leave the hydrocarbons behind: the molecule now contains oxygen, and it belongs to the alcohols. The –OH is the functional group, so every alcohol shares the same chemistry. The first four are methanol (CH3OH), ethanol (C2H5OH, often written CH3CH2OH to show the structure), propanol and butanol.
Ethanol. Write the formula as CH3CH2OH (or C2H5OH) so the –OH functional group stays visible — C2H6O counts the same atoms but hides what makes it an alcohol.
The four reactions of alcohols
There are four reactions to know for the first four alcohols, and describing “what happens” in each is enough — you only ever need a balanced equation for combustion:
| Reaction | What you observe / what forms |
|---|---|
| …react with sodium | Fizzing — hydrogen gas is given off (gentler than sodium + water) |
| …burn in air | Clean flame; complete combustion to carbon dioxide + water, releasing energy |
| …are added to water | They dissolve, giving a neutral solution (pH 7) — no fizzing, no reaction |
| …react with an oxidising agent | They are oxidised to the matching carboxylic acid (ethanol → ethanoic acid) |
That last row is why an opened bottle of wine slowly turns to vinegar: oxygen from the air (helped by microbes) oxidises the ethanol to ethanoic acid.
Balance the equation for the complete combustion of ethanol. Same routine as section 3 — but don’t forget ethanol brings one oxygen atom of its own:
- Carbons: 2 → 2CO2. Hydrogens: 6 → 3H2O.
- Oxygens needed on the right: 4 + 3 = 7. Ethanol supplies 1, so O2 supplies 6 — that’s 3O2.
C2H5OH + 3O2 → 2CO2 + 3H2O
The uses of the first four alcohols follow from their properties: fuels (ethanol in spirit burners and as a biofuel blended into petrol; methanol as a fuel and chemical feedstock), solvents (ethanol dissolves substances water can’t — it carries perfumes, mouthwashes and methylated spirits), and ethanol is the alcohol in alcoholic drinks.
Fermentation — making ethanol with yeast
Industry makes ethanol two ways. Hydration of ethene (section 6) is fast and continuous but uses a crude-oil feedstock; fermentation uses renewable plant sugars and runs at gentle conditions. Exam questions ask for the conditions, so learn all three:
sugar (glucose) yeast⟶ ethanol + carbon dioxide
- Yeast — its enzymes catalyse the reaction, which is exactly why the temperature matters (next point).
- Warm, around 30 °C — too cold and the enzymes work slowly; too hot and they denature. (Offered 0 °C / 25–35 °C / 450 °C in a multiple-choice, take the middle.)
- No oxygen (anaerobic conditions) — air is kept out.
Fermentation produces an aqueous (dilute) solution of ethanol — the carbon dioxide bubbles off (it turns limewater milky, a favourite link to C8 gas tests). To concentrate the ethanol, the mixture is fractionally distilled — the same separation idea as section 2, now exploiting the boiling point difference between ethanol (78 °C) and water (100 °C).
The two routes to ethanol, side by side
Ethanol is manufactured two ways: fermentation of plant sugars (above) and hydration of ethene — ethene + steam over a catalyst (section 6). The same molecule, from completely different starting materials — which is why “compare the two methods” is one of C7’s most predictable long-answer questions (there’s one to try below). The marks come from making paired points: say something about each method on the same idea — raw material, then conditions, then rate — rather than everything about one and then the other.
| Fermentation | Hydration of ethene | |
|---|---|---|
| Raw material | Sugar from plants — renewable | Ethene from crude oil — finite |
| Equation | glucose → ethanol + CO2 | ethene + steam → ethanol |
| Conditions | Yeast, warm (≈30 °C), no oxygen | Catalyst, high temperature & pressure |
| Process & rate | Batch — relatively slow | Continuous — fast |
| Product | Dilute ethanol — needs fractional distillation | Pure ethanol, high yield |
| Energy & resources | Low temperature → less energy; renewable, but uses farmland | Uses finite crude oil; more energy (heat + pressure) |
🧪 Exam-style questions
Which gas is produced when sodium is added to ethanol? Tick (✓) one box.
Describe how ethanol is produced from sugar solution, naming the process. Build the answer, then compare with the model below.
Show answer
- Add yeast to the sugar solution. 1 mark
- Keep it warm (≈30 °C) in the absence of oxygen (anaerobic). 1 mark
- The process is fermentation — producing an aqueous solution of ethanol and carbon dioxide. 1 mark
Balance the equation for the complete combustion of ethanol.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check. Remember ethanol’s own oxygen atom.
Show answer
- 2 carbons → 2CO2; 6 hydrogens → 3H2O. 1 mark
- Right side needs 7 O; ethanol provides 1, so 3O2: C2H5OH + 3O2 → 2CO2 + 3H2O. 1 mark
Ethanol is used in alcoholic drinks. Which two of these are other main uses of ethanol? Tick (✓) two boxes, then press Check.
Propanol is warmed with an oxidising agent. What is the organic product? Tick (✓) one box.
Ethanol can be produced by fermentation or by the hydration of ethene. Compare these two methods of producing ethanol. This is a levels-of-response question — make paired points (raw material, conditions, rate, product) and reach a justified conclusion. Plan, then compare with the model answer.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): relevant points (about both methods) identified, given in detail and logically linked to form a clear account — ending in a justified conclusion.
- Level 2 (3–4): relevant points identified, with attempts at logical linking, but the account is not fully clear.
- Level 1 (1–2): points identified and stated simply, relevance not clear, no logical linking.
Marks come from comparing the methods point-by-point, not from listing everything about one then the other. Credit-worthy comparisons:
- Raw material: fermentation uses sugar from plants, which is renewable; hydration uses ethene from crude oil, which is finite.
- Conditions / energy: fermentation is warm (≈30 °C) with yeast; hydration needs a catalyst at high temperature and pressure, so it uses more energy.
- Rate & process: fermentation is a slow, batch process; hydration is fast and continuous.
- Product: fermentation makes a dilute solution that must be fractionally distilled; hydration makes pure ethanol in high yield.
Conclusion (needed for Level 3): fermentation is the more sustainable route (renewable, low energy); hydration is better for fast, pure, large-scale manufacture. 6 marks
8Carboxylic Acids T
Oxidise an alcohol (section 7) and you arrive at the last homologous series of the topic: the carboxylic acids, with the functional group –COOH. The first four are methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid and butanoic acid. Ethanoic acid is the one you’ve met: it’s the acid in vinegar.
Ethanoic acid. Count the –COOH carbon in the name: ethanoic acid has two carbons in total, one of them inside the functional group.
The three reactions to describe
| Reaction | What you observe / what forms |
|---|---|
| …react with carbonates | Fizzing — carbon dioxide is given off (plus a salt and water). The salts are named -anoates: ethanoic acid makes ethanoates |
| …dissolve in water | An acidic solution forms (pH below 7) — they behave as typical acids, just weak ones |
| …react with alcohols | With an acid catalyst (e.g. concentrated sulfuric acid), they form esters + water |
You are not expected to write balanced symbol equations for any of these — describing the reaction and naming the products is what the marks are for.
The ester to know by name is ethyl ethanoate, made from ethanol + ethanoic acid. Esters smell pleasant and are volatile (they evaporate easily), which is why they’re used in perfumes and flavourings — volatility gets the molecules to your nose. A question may also hand you the reaction the other way round: given that compounds A and B react to make an ester plus one other product, that other product is water — a fact that becomes the key to condensation polymerisation in section 10.
The acid’s –OH and the alcohol’s –H (blue) leave together as the water; the carbonyl and the alcohol’s O (red) join into the –COO– ester link.
Both are colourless liquids, so “describe a test to distinguish them” is a natural 2-marker. Add a carbonate (e.g. sodium carbonate) to each: the carboxylic acid fizzes (CO2), the alcohol does nothing. Universal indicator works too — red-orange in the acid, green (neutral) in the alcohol. Quoting the result for both liquids is what completes the answer.
Now that you’ve met them both, never call ethanol or ethanoic acid a hydrocarbon. If the formula contains an O, the molecule is not a hydrocarbon — hydrocarbons contain hydrogen and carbon only (section 1). Ethanol (CH3CH2OH) and ethanoic acid (CH3COOH) each carry oxygen, which is exactly what makes them an alcohol and a carboxylic acid rather than members of the alkane or alkene families.
Why carboxylic acids are weak acids Higher
A strong acid (HCl, sulfuric, nitric) ionises completely in water — effectively every molecule releases its H+. A weak acid only partially ionises: most molecules stay whole, and the ionisation is a reversible reaction sitting at an equilibrium that lies well to the left:
CH3COOH(aq) ⇌ CH3COO−(aq) + H+(aq)
- Ethanoic acid only partially ionises in water / the equilibrium lies to the left. 1 mark
- So a solution of ethanoic acid has a lower concentration of H+ ions than hydrochloric acid of the same concentration… 1 mark
- …giving it a higher pH (closer to 7) and slower reactions — e.g. with the same carbonate, the weak acid fizzes less vigorously.
The reasoning chain is always partial ionisation → fewer H+ → higher pH / slower rate. Note the fair-test caveat: this comparison only makes sense at the same concentration — concentration (mol/dm³) and strength (degree of ionisation) are different ideas.
🧪 Strong & weak acids — ionisation, concentration & pH
Both acids start at the same concentration. Notice the strong acid is far more acidic. Then drag its slider to dilute it — and see how far you have to go before it only matches the weak acid’s pH.
● H⁺ hydrogen ion · ● A⁻ acid anion (Cl⁻ or CH3COO⁻) · H–A un-ionised acid molecule
≈100% ionised · picture schematic
· picture schematic
pH scale — compare the two acids
🧪 Exam-style questions
What is the name of the ester produced when ethanol reacts with ethanoic acid? Tick (✓) one box.
Dilute ethanoic acid is added to sodium carbonate in an open flask on a balance. What happens to the mass, and why? Tick (✓) one box.
Vinegar contains ethanoic acid. 250 cm³ of a vinegar contains 12 g of ethanoic acid. Calculate the mass of ethanoic acid in 1.0 dm³ of this vinegar.
Show answer
- 1.0 dm³ = 1000 cm³, which is 1000 ÷ 250 = 4 portions of 250 cm³. 1 mark
- Mass = 4 × 12 1 mark
- = 48 g (i.e. the concentration is 48 g/dm³ — the C3 concentration idea in organic clothing). 1 mark
Solutions of ethanoic acid and hydrochloric acid have the same concentration. Why does the ethanoic acid solution have a higher pH? Tick (✓) one box.
Esters are used in perfumes because they smell pleasant and are volatile. What does volatile mean? Tick (✓) one box.
Ethyl ethanoate (an ester) is made by warming ethanoic acid with which reagent? Tick (✓) one box.
9Addition Polymerisation T
Alkenes’ biggest job is making polymers. In addition polymerisation, thousands of small molecules (monomers) join together into one very large molecule (the polymer) — and because it’s an addition reaction, nothing else is produced. The monomers must be alkenes: it’s the C=C double bond that opens up to link each monomer to its neighbours. That’s also the whole answer to “why do alkenes form polymers but alkanes don’t” — alkanes have no double bond to open.
n ethene molecules → poly(ethene). The repeating unit contains exactly the same atoms as the monomer — nothing else is formed.
Going monomer → repeating unit:
- Replace the C=C with a single C–C bond.
- Keep every other atom and group exactly where it was — in poly(propene), the CH3 stays as a side group on one carbon.
- Draw square brackets with the two end bonds extending through them, and write n at the bottom right. Balance the equation with n on the left: n C2H4 → (C2H4)n.
Going polymer → monomer (the reverse question, often asked with PVC): find the repeating two-carbon unit in the chain, redraw those two carbons with a double bond, and keep their side groups — for poly(chloroethene), that’s chloroethene, a C=C with one Cl. The repeating unit and the monomer always contain the same atoms.
- Leaving a C=C inside the brackets. The polymer backbone is all single bonds — the double bond was spent joining the chain.
- Stopping the end bonds at the brackets. They must poke through — they show the unit repeats on both sides.
- Forgetting the n — or losing side groups. Atom-count the repeating unit against the monomer: they must match.
🧪 Exam-style questions
Which compound could be a monomer for addition polymerisation? Tick (✓) one box.
Explain why alkenes form polymers but alkanes do not. Tick (✓) one box.
A polymer chain has the repeating unit –CH2–CHCl– (one chlorine side group). What is its monomer? Tick (✓) one box.
In addition polymerisation, why does the repeating unit contain the same atoms as the monomer? Tick (✓) one box.
10Condensation Polymerisation & Amino Acids T H
Addition polymerisation needs a C=C. Condensation polymerisation works on a completely different principle: the monomers carry two functional groups each, one at each end of the molecule. When the functional groups react, the monomers join end-to-end — and each new link kicks out a small molecule, usually water. That lost water is what the name “condensation” records, and it’s the headline difference from addition polymerisation, where nothing is lost.
The simplest case uses two different monomers, each carrying two of the same group: a diol (an –OH at both ends, e.g. ethanediol) and a dicarboxylic acid (a –COOH at both ends, e.g. hexanedioic acid). An –OH and a –COOH react to form an ester link (section 8’s chemistry, working overtime), so the product is a polyester:
n is the number of repeating units; two ester links form per unit, so 2n water molecules are lost in total.
| Addition | Condensation | |
|---|---|---|
| Monomer(s) | One type of monomer | Usually two different monomers |
| Functional group(s) | A C=C double bond | Two functional groups per monomer |
| Products | The polymer only | Polymer + small molecule (usually water) |
| Repeating unit | Same atoms as the monomer | Fewer atoms than the monomers (water has left) |
“Compare the two types of polymerisation” is a textbook 4-marker: one mark per row, as long as each point is a genuine comparison (“addition has X whereas condensation has Y”).
Amino acids — one monomer, two different groups
The diol/diacid pair needs two different monomers because each carries two of the same group. An amino acid does the job single-handedly: it carries two different functional groups in one molecule — a basic amine group (–NH2) at one end and an acidic carboxylic acid group (–COOH) at the other. The –NH2 of one molecule reacts with the –COOH of the next, water is lost, and the chain grows by condensation polymerisation into a polypeptide.
The named example to learn is glycine, H2NCH2COOH:
The bond joining each unit is an amide, or peptide, link (–CO–NH–) — the amino acid brings both groups itself, so it polymerises single‑handedly.
Check the bookkeeping against the figure: each repeating unit has lost one H from the –NH2 and an OH from the –COOH — exactly one H2O per link, which is why n water molecules appear on the right. And when different amino acids are combined in the same chain, the product is a protein — the bridge into section 11.
🧪 Exam-style questions
Which pair of monomers could undergo condensation polymerisation to form a polyester? Tick (✓) one box.
In condensation polymerisation, what is produced as well as the polymer? Tick (✓) one box.
Amino acids can polymerise on their own, without a second type of monomer. Why? Tick (✓) one box.
Glycine polymerises: n H2NCH2COOH → (–NHCH2CO–)n + ? What completes the equation? Tick (✓) one box.
11DNA & Natural Polymers T
Polymer chemistry didn’t start in a refinery — nature got there first. This short final section (back to all tiers of Triple, not just Higher) asks for one idea: the big molecules of biology are polymers, and you should be able to name the monomers each one is made from.
DNA (deoxyribonucleic acid) is a large molecule essential for life: it encodes genetic instructions for the development and functioning of living organisms and viruses. Structurally, most DNA molecules are two polymer chains wound into the famous double helix, and each chain is built from just four different monomers, called nucleotides.
DNA: two polymer chains (sugar–phosphate backbones) wound into a double helix and built from four different monomers (nucleotides). The strands run off each end — the chain continues well beyond this short section.
| Natural polymer | Monomer(s) |
|---|---|
| DNA | Nucleotides (four different ones) |
| Proteins | Amino acids |
| Starch | Sugars (glucose) |
| Cellulose | Sugars (glucose) |
“Name the type of monomer that makes [polymer]” is the whole question — one row of this table per mark. Starch and cellulose share an answer: both are polymers of sugars.
🧪 Exam-style questions
What are the monomers that make up DNA called? Tick (✓) one box.
Which row correctly matches a naturally occurring polymer with its monomer? Tick (✓) one box.
Which two statements about DNA are correct? Tick (✓) two boxes, then press Check.
★Capstone: Name that molecule Triple
The whole of the second half of C7 is built on functional groups: the part of a molecule that decides how it reacts, and which family it belongs to. Given a name or a condensed formula, you should be able to place any small organic molecule into the right homologous series — alkane (C–C and C–H only, CnH2n+2), alkene (a C=C double bond, CnH2n), alcohol (the –OH group) or carboxylic acid (the –COOH group). Sort each molecule below into its family.
Drag each molecule into a box — or tap it to step through the boxes. Then press Check.
- Crude oil, hydrocarbons & alkanes: crude oil is a finite resource — the remains of an ancient biomass (mainly plankton) buried in mud. Hydrocarbons contain hydrogen and carbon only; most are alkanes — saturated (single bonds only), general formula CnH2n+2, first four methane, ethane, propane, butane. A homologous series shares a general formula and each member differs by CH2.
- Fractional distillation: separates crude oil into fractions of similar chain length by evaporation then condensation. Large molecules (high boiling point) condense low in the hot column; small molecules (low boiling point) rise higher before condensing. It is a physical separation — no bonds inside molecules break.
- Properties & combustion: as molecules get larger, boiling point and viscosity rise and flammability falls (stronger intermolecular forces — a C2 idea). Complete combustion oxidises both elements → carbon dioxide + water; incomplete combustion (limited air) also gives toxic carbon monoxide and carbon (soot).
- Cracking: breaks long-chain alkanes into smaller, more useful alkanes + alkenes (catalytic or steam cracking) — meeting demand for short-chain fuels and supplying alkenes for polymers. Bromine water tests for the C=C: alkenes turn it orange → colourless; alkanes leave it orange.
- Alkenes T: unsaturated hydrocarbons with the C=C functional group, general formula CnH2n (ethene, propene, butene, pentene). Their reactions are all addition across the C=C — with hydrogen (nickel catalyst → alkane), steam (catalyst → alcohol) and the halogens (room temperature → di-halo compound).
- Alcohols T: functional group –OH (methanol, ethanol, propanol, butanol). They react with sodium (→ hydrogen), burn, dissolve to near-neutral solutions and are oxidised to carboxylic acids; uses are fuels, solvents and drinks. Ethanol is made by fermentation of sugars (yeast, ≈30 °C, no air → ethanol + CO2) or by hydration of ethene (steam + catalyst).
- Carboxylic acids T: functional group –COOH (methanoic, ethanoic, propanoic, butanoic; ethanoic acid = vinegar). They react with carbonates (→ CO2), form acidic solutions, and react with alcohols to make esters. H They are weak acids — only partially ionised, so a higher pH than a strong acid of the same concentration.
- Polymers T: in addition polymerisation, alkene monomers join at the opened C=C — the repeating unit has the same atoms as the monomer and nothing else forms. H In condensation polymerisation, monomers carry two functional groups (e.g. a diol + a dicarboxylic acid → a polyester) and a small molecule (usually water) is lost at every link.
- Natural polymers T: amino acids (an –NH2 and a –COOH) condense into polypeptides and proteins. DNA is two polymer chains wound into a double helix, each built from four different monomers called nucleotides; starch and cellulose are both polymers of sugars (glucose).
And that’s the carbon trail complete: from plankton buried in mud, through the fractionating column and the cracker, to fuels, alcohols, acids, esters and polymers — ending with the natural polymers that make the chemistry of life. For Paper 2, pair these notes with C6 — Rates & Equilibrium: equilibrium conditions and catalysts questions love to use the ethene-to-ethanol reaction as their context, and cracking borrows the same rate ideas. Next, C8 — Chemical Analysis turns to identifying these compounds — testing for pure substances, separating mixtures by chromatography, and the lab tests for gases and ions.