Question

Some molecules of ${{C}_{11}}{{H}_{24}}$ crack to produce ethene and propene. How would you write a balanced equation to show a cracking reaction which makes ethene and propene?

Answer 1

Hint: Cracking is the breaking of larger molecules into smaller useful bits. This given molecule is supposed to break in 3 products among which 2 are already mentioned in the question. So, we will apply the law of conservation of mass and find the last product as well, hence getting the balanced chemical equation.

Complete step by step answer:
- Cracking can be described as breaking up of large hydrocarbon molecules into small useful bits. This can be achieved by using high temperature and pressure conditions without a catalyst or lower temperatures and pressures in the presence of a catalyst. These hydrocarbon molecules are broken up in a fairly random way to yield mixtures of smaller hydrocarbons (some of which have carbon-carbon double bonds).

- The molecule ${{C}_{11}}{{H}_{24}}$is cracked to produce 3 products. The two of them are ethene and propene as mentioned in the question. The chemical formula for ethene is ${{C}_{2}}{{H}_{4}}$ and the chemical formula for propene is ${{C}_{3}}{{H}_{6}}$.

- We can find the other product by applying the law of conservation of mass as it will contain the rest of the carbon and hydrogen atoms.
The reaction can be presented as:-
${{C}_{11}}{{H}_{24}}\to {{C}_{2}}{{H}_{4}}+{{C}_{3}}{{H}_{6}}+{{C}_{a}}{{H}_{b}}$

- Calculation of ‘a’ and ‘b’:-
According to the law of conservation of mass, “during a chemical or physical change, the matter is neither created nor destroyed”.
Therefore the number of C and H atoms on the reactant side should be equal to the number of C and H atoms on the product side.

Value of ‘a’:
11=2+3+a
11=5+a
a=6

Value of ‘b’:
24=4+6+b
24=10+b
b=14

The third product of the reaction is ${{C}_{6}}{{H}_{14}}$i.e., Hexane.

- Hence the balanced chemical equation is: ${{C}_{11}}{{H}_{24}}\to {{C}_{2}}{{H}_{4}}+{{C}_{3}}{{H}_{6}}+{{C}_{6}}{{H}_{14}}$

Note:
- Cracking involves breaking of molecules in smaller ones but not too small so hexane would not break further.
- The source of the large hydrocarbon molecules is mostly the gas oil fraction from the fractional distillation of crude oil. These oil fractions are obtained as viscous liquids from the distillation process, but are kind of re-vaporized before cracking.