Question

Is petrol a reducing species?

Answer 1

No, petrol is reducing.

Explanation:

Petrol or gasoline is mostly a mixture of more than 200 different hydrocarbons.

It is used as a fuel in internal combustion engines.

The hydrocarbons range in length from 4 to 12 carbon atoms.

A typical hydrocarbon in petrol is octane, `"C"_8"H"_18`.

The equation for its combustion is

`2stackrel(color(blue)(-9/4))("C")_8"H"_18 + "25O"_2 → 16stackrel(color(blue)(+4))("C")"O"_2 + 18"H"_2"O"`

The average oxidation number of carbon in octane increases from `-9/4` to `+4`.

Since the oxidation number of `"C"` increases, the octane is oxidized.

The substance that is oxidized is a reducing agent.

A different way of saying it is,

"Oxygen is an oxidizing agent, so the substance it oxidizes must be a reducing agent."

Answer 2

While this is very much a formalism, petrol is in fact a reducing species inasmuch as it reacts with oxygen to form carbon dioxide.

Explanation:

To expand, in the standard combustion reaction hydrocarbons react with oxygen to form carbon dioxide and water. Zerovalent oxygen gas is reduced to water, `H_2O`, in which the oxygen has a formal `-II` oxidation state. Oxygen, of course, oxidizes the carbons of a carbon chain up to the `+IV` of `CO_2`.

In other words, since something is oxidized (the carbon), something else must be reduced, the oxygen gas: oxygen oxidizes but of course gets reduced. An alternative way to look at this is to assign formal oxidation numbers to the carbon atoms of methane, ethane, etc. The `C` nuclei of each species have formal oxidation states of `-IV`, and `-III`, respectively. Upon complete combustion, they give `CO_2`, a `+IV` oxidation state. Since they formally lose electrons, (and by definition must be oxidized), they have caused reduction in the oxidizing agent, here `O_2` gas.

To be a bit more succinct, while oxygen oxidizes the carbon, the carbon reduces the oxygen.