IGCSE Chemistry 0620: Cracking, Atom Conservation and the Bromine Water Test

A refinery has a surplus of a long-chain hydrocarbon fraction with molecular formula $C_{14}H_{30}$ , but there is much greater demand for shorter-chain alkanes (used as fuels) and for alkenes (used as a chemical feedstock). The $C_{14}H_{30}$ fraction is heated strongly with a catalyst to crack it.

(a) State two reasons why the refinery cracks the $C_{14}H_{30}$ fraction rather than selling it as it is. [2]

(b) In one cracking reaction, a single molecule of $C_{14}H_{30}$ splits into one molecule of a saturated product, $C_9H_{20}$ , and one molecule of another product, X.

(i) Using conservation of atoms, deduce the molecular formula of product X. [2]

(ii) State whether X belongs to the alkane or the alkene homologous series. Justify your answer using its general formula. [2]

(c) The $C_9H_{20}$ product and product X are collected separately in two unlabelled gas jars. Describe a simple chemical test, including the observation for each jar, that would let a student identify which jar contains product X. [2]

Part (a): Why crack the $C_{14}H_{30}$ fraction?

Two acceptable reasons:

Matching supply to demand. Long-chain fractions like $C_{14}H_{30}$ are less useful and less in demand than shorter-chain alkanes, which are needed in much larger quantities as fuels; cracking converts the surplus long-chain fraction into more useful, shorter molecules.
Supplying alkenes as feedstock. Cracking also produces alkenes, which fractional distillation of petroleum does not provide in large enough quantities, and which industry needs as a starting material (feedstock) for making other chemicals.

Part (b)(i): Deducing the formula of X by conservation of atoms

The atoms present in $C_{14}H_{30}$ must all still be present, split between the two products, $C_9H_{20}$ and X.

Carbon atoms: $14 = 9 + (\text{carbon atoms in X}) \implies \text{carbon atoms in X} = 14 - 9 = 5$

Hydrogen atoms: $30 = 20 + (\text{hydrogen atoms in X}) \implies \text{hydrogen atoms in X} = 30 - 20 = 10$

So product X has molecular formula $\boxed{C_5H_{10}}$ .

Part (b)(ii): Is X an alkane or an alkene?

Check X’s formula, $C_5H_{10}$ , against the two general formulas, using $n = 5$ :

Alkane general formula: $C_nH_{2n+2} = C_5H_{12}$ . does not match.
Alkene general formula: $C_nH_{2n} = C_5H_{10}$ , matches exactly.

So X is an alkene.

Part (c): Distinguishing the two gas jars

Add a small amount of aqueous bromine (bromine water, orange) to a sample of gas from each jar.

The jar containing X (the alkene): the bromine water decolourises, changing from orange to colourless, because the C=C double bond in X reacts with the bromine in an addition reaction.
The jar containing $C_9H_{20}$ (a saturated alkane): the bromine water stays orange, since a saturated alkane does not react with aqueous bromine in this way.

Final answers

(a) Cracking matches the supply of long-chain hydrocarbons to the greater demand for shorter alkanes, and produces alkenes needed as a chemical feedstock.
(b)(i) X = $C_5H_{10}$ .
(b)(ii) X is an alkene, since $C_5H_{10}$ fits $C_nH_{2n}$ (n = 5) rather than $C_nH_{2n+2}$ .
(c) Bromine water decolourises with the jar containing X (alkene); it stays orange with the jar containing $C_9H_{20}$ (alkane).