SolutionsPreparation
1. Fractional distillation
Arrange these petroleum fractions in the order in which they leave the distillation tower, from the most to the least volatile: gas oil, gas, kerosene, gasoline and residue.
Gas (C1–C4) → gasoline → kerosene → gas oil → residue.
The more carbons the molecules have, the higher their boiling point and the lower down the tower they leave.
2. Cracking and reforming
What is the difference between cracking and reforming? Which petroleum fraction is each one applied to?
Cracking breaks the large molecules of the heavy residues into smaller ones (gas and gasoline) using heat and catalysts such as zeolites. Reforming turns light molecules into more complex and valuable ones, for example linear alkanes into aromatic hydrocarbons.
3. Balance the reforming reaction
Heptane is converted into toluene, C7H8, during reforming. Balance the reaction.
C7H16 → C7H8 + 4 H2
Reforming releases hydrogen: it is a dehydrogenation.
4. Octane number
A gasoline behaves in the engine like a mixture of 92% isooctane and 8% heptane. What is its octane number? Why is a high number desirable?
Its octane number is 92. A high number means that the mixture resists premature self-ignition under pressure, which prevents engine knocking.
5. Additives
Why was tetraethyllead abandoned as a gasoline additive?
Because of its high toxicity: the lead released with the exhaust gases accumulates in living organisms and in the environment. It also poisons the catalytic converters of cars. Ethers are now used as anti-knock additives.