4.7.1.3,4.7.1.4 - Hydrocarbon properties, combustion and cracking

Molecular size and fuel properties

As hydrocarbon molecules become larger, their boiling points and viscosity generally increase, while their flammability decreases.

Viscosity describes resistance to flow: a viscous liquid flows slowly. Flammability describes how readily a substance ignites. Small hydrocarbon molecules have relatively weak intermolecular forces, so less energy is needed to separate molecules during boiling. Larger molecules have stronger intermolecular attractions overall and need more energy to separate, giving higher boiling points. Boiling separates molecules; it does not break the covalent bonds inside them.

Larger hydrocarbon molecules also give more viscous liquids. Small hydrocarbons flow more readily and evaporate more readily, producing vapour that ignites easily. Large hydrocarbons are generally less easy to ignite. “Less flammable” does not mean “cannot burn”: heavy fuel oil can still be used as fuel with suitable handling and heating.

These trends help explain why small-molecule fractions are useful as readily ignited fuels and why more viscous fractions can serve as lubricants. A comparison should identify the relevant property rather than say merely that a fraction is “better”.

To investigate viscosity, a teacher can compare the time taken for equal volumes of different liquid hydrocarbons to flow through the same apparatus. Temperature must be controlled because heating itself changes viscosity. A fair comparison uses repeated timings and the same volume and outlet. Flammability investigations need tiny quantities and appropriate safety controls because the liquids and their vapours are fuels; they are not suitable for unsupervised experimentation.

Complete combustion equations

Complete combustion oxidises the carbon and hydrogen in a hydrocarbon to carbon dioxide and water, releasing energy.

Combustion needs oxygen. When enough oxygen is available for complete combustion, all the carbon forms $\ce{CO2}$ and all the hydrogen forms $\ce{H2O}$. The overall reaction is exothermic: energy transfers to the surroundings, for example as heat. The fuel is chemically changed into new substances rather than simply evaporated.

Worked example 1 — propane combustion

Start a balance by putting the correct product formulae on the right. For propane:

Three carbons require $3\ce{CO2}$. Eight hydrogens require $4\ce{H2O}$. Find the oxygen count last:

This gives:

Check each element: carbon 3 on both sides, hydrogen 8 on both sides and oxygen 10 on both sides. Change coefficients in front of formulae, never the subscripts that define a substance.

Worked example 2 — butane combustion

For one $\ce{C4H10}$, carbon balance needs $4\ce{CO2}$ and hydrogen balance needs $5\ce{H2O}$. Count the required oxygen:

Multiply every coefficient by two to obtain whole numbers:

The method works for any supplied hydrocarbon formula, whether it is an alkane or an alkene. If oxygen is insufficient, combustion may be incomplete and carbon monoxide or carbon particles can form. They are not the products to use when the question specifies complete combustion.