Cracking sim
Cracking breaks long alkane molecules into shorter alkanes and alkenes.
Cracking · covers the GCSE, IGCSE, O-Level topic
What the sim shows
Cracking breaks long alkane molecules into shorter alkanes and alkenes. Mineral wool holds liquid paraffin, the source of long-chain alkanes; broken porcelain chips are the catalyst. Heat the chips strongly first, then the heat spreading along the tube warms the wool gently, so the paraffin vapour passes over hot chips and cracks. Here the paraffin stands in as the lesson's decane: C₁₀H₂₂ → C₂H₄ + C₃H₆ + C₅H₁₂ (10 carbons and 22 hydrogens on each side). At room temperature ethene and propene are gases; pentane boils at 36 °C, but there is so little of it that it stays as vapour in the gas. So each mole of decane cracked gives 3 moles of gas, 72 dm³ at r.t.p., 2 of them alkene. The gas is collected over water; the first tubeful is mostly the air that was in the apparatus. The gas decolourises bromine water, orange-brown to colourless, because each C=C takes up one Br₂: so alkenes were made. Heat the paraffin end instead and the vapour meets cool chips: little cracks, it condenses as an oily liquid and almost no gas comes over. To finish, slide the clamp up the rod to lift the delivery tube out of the water, then turn off the Bunsen. Turn the Bunsen off first and the cooling gas contracts: cold water is sucked back up the delivery tube into the hot tube, and the glass cracks. 1.0 cm³ of paraffin; 2 cm³ of 0.02 mol/dm³ bromine water per test. The temperatures, how fast the paraffin boils off and how much cracks on one pass (about 45% at its best) are this desk's estimates for a school boiling tube; the equation and the method are the lesson's. The clock runs 10 times faster than real time.
- Blue flame: the Bunsen is on
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