The phrase diesel and petrol refers to two common fuels obtained from crude oil. From a general chemistry perspective, both are mixtures of hydrocarbons, but they occupy different boiling ranges and therefore have different volatility and vapour pressure.
1) What “diesel” and “petrol” are in chemical terms
Both diesel and petrol are not single compounds; they are complex mixtures of many hydrocarbons (primarily alkanes, cycloalkanes, and aromatics). The key difference is the typical molecular-size distribution:
- Petrol (gasoline) is richer in lighter hydrocarbons (roughly \(C_4\) to \(C_{12}\)), making it more volatile.
- Diesel is richer in heavier hydrocarbons (roughly \(C_{10}\) to \(C_{20}\)), making it less volatile and more viscous.
2) Why vapour pressure (volatility) differs
Vapour pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. A higher vapour pressure means the liquid evaporates more readily (higher volatility).
Diesel molecules are, on average, larger and have more surface area than petrol molecules. This increases intermolecular attraction (dispersion forces), making it harder for molecules to escape the liquid phase. Therefore, at the same temperature:
- Petrol has higher vapour pressure and evaporates more easily.
- Diesel has lower vapour pressure and evaporates less easily.
The temperature dependence can be summarized by the Clausius–Clapeyron form: \[ \ln P \;=\; -\frac{\Delta H_{\mathrm{vap}}}{R\,T} + C \] where \(P\) is vapour pressure, \(\Delta H_{\mathrm{vap}}\) is enthalpy of vaporization, \(R\) is the gas constant, and \(T\) is absolute temperature.
3) Practical property comparison (diesel and petrol)
| Property | Petrol (gasoline) | Diesel | Chemistry reason (high level) |
|---|---|---|---|
| Typical carbon range | Roughly \(C_4\)–\(C_{12}\) | Roughly \(C_{10}\)–\(C_{20}\) | Different fractions from crude oil |
| Boiling range | Lower | Higher | Stronger dispersion forces for larger molecules |
| Vapour pressure (at same \(T\)) | Higher (more volatile) | Lower (less volatile) | Evaporation is easier for smaller molecules |
| Viscosity | Lower | Higher | Larger molecules increase internal friction |
| Ignition quality metric | Octane rating (resists auto-ignition) | Cetane number (ignites readily under compression) | Different engine requirements and fuel blend targets |
4) Combustion chemistry connection
Both fuels undergo oxidation to carbon dioxide and water when combustion is complete. A representative petrol-like hydrocarbon is octane, and a representative diesel-like hydrocarbon can be modeled by a longer alkane.
Petrol model (octane): \[ 2\,C_{8}H_{18} + 25\,O_{2} \rightarrow 16\,CO_{2} + 18\,H_{2}O \]
Diesel model (dodecane): \[ 2\,C_{12}H_{26} + 37\,O_{2} \rightarrow 24\,CO_{2} + 26\,H_{2}O \]
Real diesel and petrol contain many hydrocarbons, so actual combustion involves a distribution of similar reactions. Incomplete combustion (limited oxygen or poor mixing) can yield \(CO\) and soot (carbon particles), especially for heavier fuel droplets that do not vaporize quickly.
5) Visualization: crude oil fractionation and where petrol and diesel come from
6) Summary of the chemistry behind “diesel and petrol”
Diesel and petrol differ mainly because diesel contains, on average, heavier hydrocarbons than petrol. Heavier molecules experience stronger dispersion forces, which increases boiling points and decreases vapour pressure, explaining why petrol evaporates more readily while diesel is less volatile.