6B.8-6B.14 - Alkanes, Fuels And Pollution

Fuel Molecules And Saturation

A fuel molecule is useful because its bonds can be rearranged with oxygen to make more stable products, releasing energy. Before discussing engines and pollution, the molecule itself must be pinned down.

Alkanes are saturated hydrocarbons: compounds of carbon and hydrogen only.

For example, if $n = 5$, the alkane formula is $\ce{C5H12}$. This formula applies to straight-chain and branched-chain alkanes: pentane and 2-methylbutane both have $\ce{C5H12}$, but their atoms are connected differently.

Cycloalkanes are also saturated hydrocarbons, but a ring changes the hydrogen count. A cycloalkane with one ring has the general formula $\mathrm{C}_{n}\mathrm{H}_{2n}$, not because it contains a double bond, but because closing the chain into a ring removes two hydrogens compared with the equivalent open-chain alkane.

Worked example: formula and saturation

A hydrocarbon has seven carbon atoms. Give the formula of:

1. the open-chain alkane
2. the cycloalkane with one ring
3. state whether each is saturated if all carbon-carbon bonds are single bonds

For the open-chain alkane, use $\mathrm{C}_{n}\mathrm{H}_{2n+2}$:

$n = 7$, so H $= 2(7) + 2 = 16$ and the formula is $\ce{C7H16}$.

For the one-ring cycloalkane:

$n = 7$, so H $= 2(7) = 14$ and the formula is $\ce{C7H14}$.

Both are saturated if the displayed or structural formula shows only $\ce{C-C}$ single bonds. The formula $\ce{C7H14}$ alone should not make you shout "alkene"; it could be a cycloalkane.

From Crude Oil To Useful Fuel Fractions

Crude oil is a mixture of many hydrocarbons. A refinery does not simply "make petrol" in one step: it separates, breaks and rearranges hydrocarbons so that the fuel mixture has useful volatility and combustion behaviour.

[DIAGRAM: asset_name: Crude Oil Processing; asset_slug: edexcel_a_level_chemistry_l030_crude_oil_processing; recommended_method: deterministic_chart; description: Flow diagram showing crude oil entering fractional distillation, heavy fractions going to cracking, straight-chain hydrocarbons going to reforming, and fuel fractions leaving.]

The crude oil is vaporised and separated into fractions because different chain lengths have different boiling ranges. Longer-chain hydrocarbons have larger electron clouds and stronger London forces, so they usually have higher boiling points and condense lower in the fractionating column.

The products are usually a mixture containing shorter alkanes and alkenes. This helps supply lighter fuels and also makes alkenes for later organic chemistry, although alkene reactions are taught in later lessons.

Branched and cyclic hydrocarbons resist premature ignition (engine knocking), allowing smoother combustion and efficient engine operation. Reforming can also remove hydrogen when a ring forms; it does not mean breaking a long chain into smaller fuels.

Worked example: atom balance in cracking

Decane, $\ce{C10H22}$, is cracked to form ethene, $\ce{C2H4}$, and one alkane. Deduce the formula of the alkane and write a balanced equation.

Start with carbon:

10 carbons in decane − 2 carbons in ethene = 8 remaining carbons.

The remaining molecule is an alkane with $n = 8$, so:

formula $= \ce{C8H18}$

Check hydrogen: right-hand side H $= 4 + 18 = 22$, matching decane.

The equation is:

The chemistry sense check is that one long molecule has become two shorter molecules, and the atom count is conserved.

Common-error contrast

Fractional distillation sorts molecules by boiling range; cracking changes molecule size; reforming changes the carbon skeleton. An "explain" answer that says fractional distillation "breaks long chains" has swapped a physical separation for a chemical reaction.

Combustion Conditions Decide The Products

Complete combustion is the clean symbolic model for an alkane fuel burning in excess oxygen. For an alkane with formula $\mathrm{C}_{n}\mathrm{H}_{2n+2}$, complete combustion forms carbon dioxide and water:

For octane, a petrol-range alkane, the balanced equation is:

Real engines are not a perfect textbook flame. Oxygen supply, mixing, temperature and impurities affect the products.

Useful symbolic examples are:

The first incomplete-combustion equation forms carbon monoxide. The second forms solid carbon particulates, seen as soot or smoke. In real exhaust there may be a mixture: $\ce{CO}$, particulates and unburned hydrocarbons can all increase when there is not enough oxygen or mixing for full oxidation.