Carbon sits in Group 4, so it can form four covalent bonds and link to itself again and again. That single fact builds thousands of compounds — fuels, plastics, alcohols and acids.
We'll work through the whole CCEA Unit 2.5 story: crude oil and fractional distillation, the alkanes and alkenes, combustion and pollution, cracking, the bromine-water test, addition polymers, alcohols and carboxylic acids.
Read each idea, then answer to unlock the next one. Get them right to earn ⭐ and finish with full marks. Press Start!
Idea 1
Families called homologous series
A hydrocarbon is a molecule made of hydrogen and carbon only. To make sense of the thousands of carbon compounds, we sort them into families called homologous series — alkanes, alkenes, alcohols and carboxylic acids.
Every member of a homologous series:
has the same general formula;
shows similar chemical properties;
shows a gradation (steady trend) in physical properties such as boiling point;
differs from the next member by a CH2 group.
Why carbon? It is in Group 4, so each carbon forms four covalent bonds and can bond repeatedly to other carbon atoms — the backbone of every organic molecule.
Quick check
What makes a homologous series?
?Members of a homologous series differ from the next member by which group?
Idea 2
Crude oil & fractional distillation
Crude oil is a finite (non-renewable) resource and the main source of hydrocarbons and a feedstock for the petrochemical industry. It is a mixture of hydrocarbons, mostly alkanes, with chains of many different lengths.
Because different chain lengths have different boiling points, they are separated by fractional distillation in a tall tower that is hot at the bottom and cool at the top:
Vapour rises and condenses where the tower has cooled to its boiling point. Short chains (low b.p.) leave near the top; long chains (high b.p.) condense at the base.
The trend down the tower: longer chains → higher boiling point, more viscous, less flammable, darker.
Match it
Fraction → use
Tap a fraction on the left, then its correct use on the right.
Idea 3 · Alkanes
Alkanes — saturated hydrocarbons
The fractions are largely alkanes. Alkanes have only single C–C bonds, so they are saturated (the maximum number of hydrogen atoms). They have no functional group, which makes them relatively unreactive.
General formula: CnH2n+2methane CH₄ · ethane C₂H₆ · propane C₃H₈ · butane C₄H₁₀ — all gases at room temperature
An alkane: every bond is a single bond, so it holds the maximum hydrogen.
Alkanes are insoluble in water and their boiling points rise as the chains get longer.
Calculate
Use the general formula
1Propane is the alkane with 3 carbon atoms. Using CnH2n+2, how many hydrogen atoms does one propane molecule contain?
H atoms
Hint: 2n + 2 with n = 3.
Idea 4
Combustion of hydrocarbons
Complete combustion (plenty of oxygen) produces carbon dioxide and water only:
Incomplete combustion (too little oxygen) produces carbon monoxide and water, and sometimes carbon (soot).
Carbon monoxide is toxic. CO combines with haemoglobin in the blood, reducing its capacity to carry oxygen. It is colourless and odourless, so it cannot be detected without a monitor.
• Water turns anhydrous copper(II) sulfate from white to blue (or blue cobalt chloride paper to pink).
Quick check
Why is carbon monoxide dangerous?
?A gas heater burns with too little air. Which statement about the carbon monoxide it produces is correct?
Idea 5
Cracking
The light fractions (petrol, refinery gases) are in greater demand than heavy ones like bitumen. Cracking breaks down larger saturated hydrocarbons (alkanes) into smaller, more useful ones — and some of the products are unsaturated alkenes.
C10H22 → C8H18 + C2H4decane → octane (a useful alkane) + ethene (a reactive alkene)
Two useful outcomes: shorter alkanes for fuels, and alkenes for making polymers and ethanol. Check the equation balances: 10 C and 22 H on each side.
Quick check
What does cracking make?
?A long-chain alkane is cracked. Which best describes the products?
Idea 6 · Alkenes
Alkenes — unsaturated hydrocarbons
Alkenes contain a C=C double bond — that is their functional group. A C=C means the molecule is unsaturated (it does not have the maximum hydrogen) and reactive.
General formula: CnH2nethene C₂H₄ · propene C₃H₆ · but-1-ene & but-2-ene C₄H₈ — all gases at room temperature
Spot the difference from ethane: a double C=C bond, and each carbon carries one fewer hydrogen.
Read each formula, then tap whether it is a saturated alkane or an unsaturated alkene.
Idea 7
Addition reactions & the bromine test
Because the C=C bond can open up, alkenes undergo addition reactions — a small molecule adds across the double bond. Ethene reacts with bromine, hydrogen and steam:
ethene + hydrogen → ethane (C2H4 + H2 → C2H6)
ethene + steam → ethanol (C2H4 + H2O → C2H5OH)
ethene + bromine → a colourless product (name not required)
That last reaction is the famous bromine-water test for a C=C double bond:
Orange/brown bromine water stays orange with a saturated alkane, but is decolourised (turns colourless) by an unsaturated alkene.
Watch the wording: the alkene decolourises bromine water (orange → colourless). It does not turn it a new colour.
Predict it
Predict the bromine test
?A gas is bubbled through orange bromine water and the colour quickly fades to colourless. What does this tell you about the gas?
Idea 8
Addition polymerisation
Because of the C=C bond, many small alkene monomers can join into one very long chain — a polymer. This is addition polymerisation. Many ethene molecules make poly(ethene); chloroethene makes PVC.
The C=C of ethene opens up to form single bonds joining the units. The repeat unit is drawn with brackets and a subscript n.
Disposal problem: addition polymers are non-biodegradable — microbes cannot break them down. Landfill fills up; incineration releases toxic gases (but recovers heat). Recycling is costly but reduces both problems.
Quick check
From monomer to repeating unit
?Ethene (CH2=CH2) is polymerised to poly(ethene). What happens to the C=C double bond, and why are these polymers a disposal problem?
Idea 9 · Alcohols
Alcohols & making ethanol
Alcohols are a homologous series with the –OH (hydroxyl) functional group and general formula CnH2n+1OH. Members: methanol, ethanol, propan-1-ol, propan-2-ol. Ethanol is a good solvent and a fuel. Two ways to make ethanol:
1 · Hydration of ethene
Ethene + steam, passed over a catalyst → ethanol. A continuous, fast industrial process.
C2H4 + H2O → C2H5OH
2 · Fermentation
Add yeast to a sugar solution and keep it warm and without air. Enzymes in the yeast turn the sugar into ethanol and carbon dioxide. This makes the ethanol in alcoholic drinks.
Oxidation: alcohols are oxidised to carboxylic acids — in air, or with acidified potassium dichromate (orange → green). Ethanol → ethanoic acid (this is how wine turns to vinegar).
Quick check
Which manufacture method?
?A brewer makes ethanol by adding yeast to a warm sugar solution. Which process is this, and what else is produced?
Idea 10 · Carboxylic acids
Carboxylic acids
Carboxylic acids are a homologous series with the –COOH functional group: methanoic, ethanoic, propanoic and butanoic acid. Ethanoic acid (CH3COOH) is the acid in vinegar.
They are weak acids — only partially ionised in solution — but they still react like other acids: