Alkanes: saturated hydrocarbons, CnH2n+2
A hydrocarbon contains carbon and hydrogen only; saturated means all carbon–carbon bonds are single (C–C), so alkanes undergo no addition reactions. General formula . The first four — methane, ethane, propane, butane (Mr 16, 30, 44, 58) — are gases at RTP, and boiling point rises down the series. Alkanes are unreactive because the C–C and C–H bonds are strong; their main reactions are combustion and, at Extended, chlorine substitution.
Combustion of alkanes
Complete combustion (excess oxygen) gives carbon dioxide and water: . Incomplete combustion (limited oxygen) gives toxic carbon monoxide (CO) and/or carbon (soot) plus water. To balance combustion, the coefficient is — double through if it is a fraction. Water is liquid (l) at RTP, not steam.
(Extended) Chlorine substitution in UV
(Extended) In UV light (sunlight) an alkane reacts with chlorine by substitution — one H is replaced by one Cl, giving a chloroalkane plus HCl, e.g. . No reaction occurs in the dark; UV breaks the Cl–Cl bond. It continues stepwise to a mixture (up to tetrachloromethane) because each product can be chlorinated again. Substitution replaces an atom; alkanes cannot add.
Drawn from real examiner reports.
Alkanes are saturated, not unsaturated
Writing "alkanes are unsaturated" reverses the definition. Alkanes are saturated — every carbon–carbon bond is a single bond (C–C), with no C=C. The mark-scheme form of "saturated" refers to the C–C bond type (all single bonds), not being "full of hydrogens"; that vague phrasing scores zero. Only alkenes can add extra atoms across a double bond.
Alkanes do not decolourise bromine water
The bromine-water test detects unsaturation: alkenes decolourise orange-brown bromine water, alkanes do not. Stating that an alkane decolourises bromine water — or naming it as the positive result — is a zero-mark error, confusing alkanes with alkenes. Alkanes have no C=C for bromine to add across, so the colour is retained.
W22 P32 Q1(b); S22 P31 Q6(a)(i) — test for unsaturated hydrocarbons poorly recalled.
Carbon has 4 bonds, H has 1
In displayed formulae, drawing carbon with five bonds (pentavalent) or hydrogen with two bonds (divalent) is impossible and loses the structure mark. Carbon always forms exactly four bonds, each hydrogen exactly one, and each Cl in a chloroalkane exactly one. Count the bonds at every atom before finishing: methane has 4 on C and 1 on each H.
W22 P31 Q7(a)(i); W22 P41 Q6(c); W22 P42 Q6(d) — displayed formulae show pentavalent carbon or divalent hydrogen.
(Extended) Substitution needs UV, not heat
(Extended) The condition for alkane + chlorine is ultraviolet (UV) light (accept sunlight). Giving "heat", "a catalyst" or "high pressure" scores no condition mark — high pressure belongs to the Haber process, not here. The equation alone is not enough: the UV condition must be stated (write "UV" or "ultraviolet", not just "light"). No UV, no reaction.
Substitution vs addition
Alkanes react by substitution (one atom replaced by another, producing a by-product such as HCl), never by addition. Addition needs a C=C double bond, which alkanes do not have, so "chlorine adds across the alkane" is wrong. In substitution of methane, one product molecule plus HCl form; nothing adds across a bond. Reserve "addition" for alkenes.
Incomplete combustion ≠ CO2 only
With limited oxygen, alkanes undergo incomplete combustion, giving toxic carbon monoxide (CO) and/or soot (carbon) plus water — not carbon dioxide. Writing when the question specifies a limited air supply loses marks. Only complete combustion (excess oxygen) gives and water, with a clean blue flame.
Balance a chlorination equation
(Extended) Write the alkane + , then apply one-in one-out: one Cl replaces one H, the displaced H joins the other Cl as HCl. Products are chloroalkane + HCl. Put "UV light" above the arrow. Check: C unchanged; H alkane = H chloroalkane + 1; Cl 2 = 1+1.
Balance combustion: C, H, then O
Balance combustion in the order C → H → O. Set CO2 from the carbons, water from the hydrogens, then count oxygen on the right and halve for the coefficient (double through if fractional). Water is liquid (l) at RTP, not steam (g).
Define saturated by bond type
When asked what "saturated" means, name the bond type: all carbon–carbon bonds are single bonds (no C=C). Vague answers like "full of hydrogen" or "has no spare bonds" are not credited. Give the general formula too if defining an alkane.
State the UV condition explicitly
For substitution equations, state the condition as "ultraviolet (UV) light" (or sunlight) — "light" alone is not enough for the mark, and "heat"/"catalyst"/"high pressure" are wrong. Write it above the arrow; give the balanced equation separately if asked.
Hydrocarbon: A compound containing carbon and hydrogen only — no other elements.
Saturated: A hydrocarbon in which all carbon-carbon bonds are single bonds (C–C). No C=C double or C≡C triple bonds are present.
Alkane: A saturated hydrocarbon with the general formula where is the number of carbon atoms.
Definition-precision trap: "Saturated" does NOT mean "has the maximum possible number of hydrogen atoms relative to carbon" as a vague statement — the mark-scheme form is "all C–C bonds are single bonds". Vague answers ("full of hydrogens", "no double bonds to oxygen") do not score.
Define the term "saturated" as applied to a hydrocarbon.
Butane has the molecular formula .
Calculate the relative formula mass () of butane.
(: C = 12, H = 1)
