IGCSE · Practice questions · Cracking

Crack it, balance it.

Six original Cambridge-style questions on cracking long-chain alkanes.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Heat, catalyst, split.

Cracking: thermal decomposition of large alkanes.
Conditions: high temperature and a catalyst (silica or alumina).
Products: smaller alkanes + alkenes (+ sometimes hydrogen).
Why: match supply to demand; make alkenes for polymers.

Recall

[2 marks]

State the conditions used to crack long-chain alkanes.

A high temperature (about 500–600 °C). ✓
A catalyst such as silica or alumina (zeolite). ✓

Calculation

[1 marks]

Complete the equation: C₁₄H₃₀ → C₁₀H₂₂ + ......

Carbon left: 14 − 10 = 4. Hydrogen left: 30 − 22 = 8. ✓
Missing product: C₄H₈ (or 2C₂H₄). ✓

Calculation

[2 marks]

Hexadecane, C₁₆H₃₄, is cracked to give octane, C₈H₁₈, and ethene only. Write the balanced equation.

Carbon left after octane: 16 − 8 = 8, so 4 C₂H₄. ✓
Hydrogen check: 18 + 4 × 4 = 34. ✓
C₁₆H₃₄ → C₈H₁₈ + 4C₂H₄ ✓

Explain

[3 marks]

Explain why oil companies crack some of the gas oil fraction.

Crude oil contains more long-chain fractions such as gas oil than are needed. ✓
There is a higher demand for short-chain fractions such as petrol than crude oil supplies. ✓
Cracking converts the surplus into petrol and also makes alkenes for polymers. ✓

Analysis

[2 marks]

A student bubbles the gaseous product of cracking through bromine water. Describe what is seen and explain what it shows.

The bromine water changes from orange to colourless. ✓
The product contains an alkene: it is unsaturated (contains a C=C double bond). ✓

Explain

[2 marks]

Explain why cracking is described as thermal decomposition.

Heat (thermal) is used to break the reactant down. ✓
One compound is broken into two or more simpler substances (decomposition). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.