IGCSE Chemistry 0620: Cracking Decane to Form an Alkene and Hydrogen

A chemical plant cracks decane, $\text{C}_{10}\text{H}_{22}$ , by heating it strongly in the presence of a catalyst. In one type of cracking reaction taking place in the plant, a single molecule of decane splits into one molecule of an alkene with the same number of carbon atoms as decane, plus one molecule of hydrogen gas, $\text{H}_2$ .

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

(b) Write a balanced symbol equation for this cracking reaction, including state symbols. (All species are gaseous under the high-temperature conditions used for cracking, even though decane itself is a liquid at room temperature.) [2]

(d) Give one use for the hydrogen gas produced by cracking reactions like this one. [1]

Part (a): Deducing the alkene’s formula by conservation of atoms

All the atoms present in one molecule of decane, $C_{10}H_{22}$ , must still be present afterwards, shared between the alkene product and the molecule of hydrogen gas, $H_2$ , that also forms.

Carbon atoms: the alkene has “the same number of carbon atoms as decane”, so it has 10 carbon atoms.

Hydrogen atoms: $22 = (\text{hydrogen atoms in the alkene}) + 2 \implies \text{hydrogen atoms in the alkene} = 22 - 2 = 20$

So the alkene product has molecular formula $\boxed{C_{10}H_{20}}$ .

Check against the general formula: for an alkene, $C_nH_{2n}$ , with $n=10$ : $2n = 20$ , which matches. Confirming $C_{10}H_{20}$ is indeed a valid alkene formula.

Part (b): Balanced symbol equation

$C_{10}H_{22}(g) \rightarrow C_{10}H_{20}(g) + H_2(g)$

Check: carbon $10=10$ ; hydrogen $22=20+2=22$ . The equation is already balanced with a coefficient of 1 throughout, since the reaction simply removes one $H_2$ molecule from decane to form a carbon–carbon double bond in its place.

All three species are given the state symbol $(g)$ : although decane is a liquid at room temperature, the cracking reaction is carried out at a high enough temperature that decane (and its products) exist as vapours/gases under those conditions.

Part (c): Conditions needed for cracking

Cracking of large alkane molecules like decane requires:

A high temperature.
A catalyst.

Part (d): A use for the hydrogen produced

The hydrogen gas produced by cracking reactions can be used, for example, as a fuel, or to hydrogenate other alkenes into alkanes in the presence of a nickel catalyst, a reaction used, for instance, in the manufacture of margarine from unsaturated vegetable oils.

Final answers

(a) The alkene product is $C_{10}H_{20}$ .
(b) $C_{10}H_{22}(g) \rightarrow C_{10}H_{20}(g) + H_2(g)$
(c) A high temperature and a catalyst.
(d) The hydrogen can be used as a fuel, or to hydrogenate alkenes into alkanes (e.g. in margarine manufacture).