Alkanes

Alkanes earn their marks through equations: a balanced complete-combustion equation appears on most Paper 3 and Paper 4 organic questions, and the Extended-only chlorine substitution is a reliable 2-3 marks for candidates who remember two details: UV light and the HCl by-product. The chemistry itself is short, because alkanes barely react.

What alkanes are

Alkanes are saturated hydrocarbons with the general formula CnH2n+2. Two definitions earn marks here:

• Hydrocarbon: a compound of hydrogen and carbon only. (Writing “contains hydrogen and carbon” without “only” drops the mark: ethanol contains both, and is not a hydrocarbon.)
• Saturated: all carbon-carbon bonds are single bonds.

The first four members are methane CH4, ethane C2H6, propane C3H8 and butane C4H10. Naming stems and displayed-formula rules are on the homologous series page. On the Extended paper, butane has a branched structural isomer, methylpropane.

Because they have no functional group, alkanes are unreactive: they do not react with acids, alkalis, or bromine water (which is exactly why bromine water distinguishes them from alkenes). They undergo two reactions only.

Reaction 1: combustion

Complete combustion (in a plentiful supply of oxygen) produces carbon dioxide and water:

CH4 + 2O2 → CO2 + 2H2O

C3H8 + 5O2 → 3CO2 + 4H2O

Balance in this order: carbons first, hydrogens second, oxygens last. A fraction in front of O2 is allowed (C2H6 + 3½O2 → 2CO2 + 3H2O), or double everything through if you prefer whole numbers.

Incomplete combustion (in a limited supply of oxygen) produces carbon monoxide (or carbon, as soot) and water:

2CH4 + 3O2 → 2CO + 4H2O

Carbon monoxide matters because it is toxic: it binds to haemoglobin in red blood cells and prevents the blood carrying oxygen. That sentence is the standard 2-mark “explain why incomplete combustion is dangerous” answer, and it links forward to the air pollutants in chemistry of the environment.

Reaction 2: substitution with chlorine (Supplement)

In the presence of ultraviolet light, alkanes react with chlorine. A chlorine atom substitutes for (replaces) a hydrogen atom:

CH4 + Cl2 → CH3Cl + HCl

Three details carry the marks:

1. Condition: UV light (sunlight accepted). Without it, no reaction. This is what “alkanes are unreactive in the dark” questions probe.
2. Type: substitution, defined as a reaction in which one atom (or group) replaces another.
3. Products: chloromethane and hydrogen chloride. The HCl is the routinely forgotten product.

Substitution can continue step by step (CH3Cl to CH2Cl2 and onwards), but 0620 only requires the first step. Keep the contrast sharp: alkanes substitute (two products), alkenes add (one product).

Worked exam question

Butane is used as a fuel in portable stoves. (a) Write the balanced equation for the complete combustion of butane, C4H10. [2] (b) In a poorly ventilated room, the stove produces a toxic gas. Name the gas and explain why it is toxic. [2] (c) Explain why butane does not decolourise aqueous bromine. [1]

Model answer, mark by mark:

• (a) M1: correct products, CO2 and H2O. M2: correctly balanced: C4H10 + 6½O2 → 4CO2 + 5H2O, or 2C4H10 + 13O2 → 8CO2 + 10H2O.
• (b) M3: carbon monoxide. M4: it binds to haemoglobin / prevents the blood carrying oxygen. (“It’s poisonous” restates the question and scores nothing.)
• (c) M5: butane is saturated (it has no C=C double bond), so no addition reaction occurs and the bromine stays orange.

The mistakes that cost marks

1. Oxygen balanced first. Start with O2 and the equation collapses. Carbons, hydrogens, then oxygens: the only reliable order.
2. CO2 written for incomplete combustion. Limited oxygen gives CO or C. If the question says “limited supply of air” or “poorly ventilated”, carbon dioxide is the wrong product.
3. “Carbon monoxide is toxic because it’s poisonous.” Circular. The mark needs the mechanism: binds to haemoglobin, blood cannot carry oxygen.
4. Missing HCl in substitution. CH4 + Cl2 → CH3Cl is unbalanced and incomplete. Count the atoms: the displaced hydrogen and the second chlorine leave as HCl.
5. “Hydrocarbon” without “only”. Definitions are all-or-nothing in 0620. Hydrogen and carbon only.

How examiners want it phrased

Student wording	Mark-scheme wording
”Alkanes are saturated so they’re stable"	"Alkanes contain only single carbon-carbon bonds, so they do not undergo addition reactions"
"It burns to make smoke"	"Incomplete combustion in limited oxygen produces carbon monoxide and carbon (soot)"
"Chlorine reacts with methane in light"	"In ultraviolet light, chlorine substitutes for a hydrogen atom: CH4 + Cl2 → CH3Cl + HCl"
"CO stops you breathing"	"Carbon monoxide binds to haemoglobin, preventing the blood from carrying oxygen”

Alkanes are the easiest section of organic chemistry to secure in full: two reactions, three definitions, one balancing routine. If combustion equations still wobble under time pressure, bring one to a free trial lesson and we will fix the balancing order in a single session.

Test yourself

Answer all three before clicking. The answers stay hidden until you do.

Q1 (2 marks). Define the terms hydrocarbon and saturated.

Show answer

• hydrocarbon: a compound of hydrogen and carbon only [1] • saturated: all carbon-carbon bonds are single bonds [1]

Q2 (2 marks). Write the balanced symbol equation for the complete combustion of ethane, C2H6.

Show answer

• correct products CO2 and H2O [1] • balanced: 2C2H6 + 7O2 → 4CO2 + 6H2O (or C2H6 + 3½O2 → 2CO2 + 3H2O) [1]

Q3 (3 marks). (Supplement) Ethane reacts with chlorine. State the condition required, name the type of reaction, and give the two products.

Show answer

• ultraviolet light [1] • substitution [1] • chloroethane and hydrogen chloride: C2H6 + Cl2 → C2H5Cl + HCl [1]