Is petrol a reducing species?

2 Answers
Aug 4, 2015

No, petrol is reducing.


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."

Aug 4, 2015

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


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.