Question

What is the chemical equation of burning fossil fuels?

Answer 1

Most fossil fuels are hydrocarbons.
Hydrocarbon + Oxygen (`O_2`)= Carbondioxide (`CO_2`) + Water (`H_2O`)

Answer 2

A chemical equation of burning the alkenes within petroleum could be
`C_"n"H_"2n+2"+ (1+3n)*O_2 -> n*CO_2 +(n+1)*H_"2"O`

Explanation:

There are different types of crude oil present on earth. Each of these crude oils consists out of a composition of molecules. These compositions are never the same and therefore we cannot simply say that crude oil consists out of some molecules.

If we take for example petroleum which is found in the earth and assume we have an oil that has mostly alkenes in it, we can, for example, burn:
`C_"n"H_"2n+2"`
The `n` is a placeholder for an unknown number.

This is a general form of an alkene. A burning reaction means that the compound reacts with oxygen.
We know experimentally that `CO_2` and `H_2O` are the mean products of a burning reaction.

Now we have enough information to write this:
`C_"n"H_"2n+2"+ O_2 -> CO_2 +H_"2"O`
Of course, this reaction is not yet balanced, but since we do not know the `n`, we cannot fill in all the number yet. Therefore we must balance the reaction by multiplying the reactants and the products with a form of `n`. For `CO_2` and `H_2O`, this is easy.
Then we use math to find out that we need to place `1+3n` in front of `O_2`.

We obtain:
`C_"n"H_"2n+2"+ (1+3n)*O_2 -> n*CO_2 +(n+1)*H_"2"O`
You can test this yourself by filling in a random number for `n` and see if the reaction is balanced.

In practice, the composition of an amount of fuel is determined and a chemical reaction can be written accordingly. The above method is therefore highly theoretical and shows only how to write an overall reaction.