The blank space in the chemical equation for burning fossil fuels represents the products of the reaction. When fossil fuels burn, they react with oxygen gas (O₂) to produce:

* Carbon dioxide (CO₂)

* Water (H₂O)

* Energy (heat and light)

So, the completed chemical equation would look something like this:

Fuel + O₂ → CO₂ + H₂O + Energy

The specific type of fuel will determine the exact chemical formula of the fuel and the specific products produced.