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How do you write chemical equations for combustion reactions?

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Hint: Combustion is an exothermic reaction in which a fuel is getting oxidized. Every chemical equation consists of all the atoms that are undergoing a change in the chemical reaction.

Complete answer:
So in the question we are asked how we will write the equations for combustion reactions.
To write a chemical equation of a chemical reaction, we should first know the species of atoms, ions, molecules involved in the reaction. And show what chemical change is happening to these species so that it will yield the product. Hence we can simply say that we should be aware of the reactant species that is involved and how they are converted to product.
A chemical equation consists of two parts- the left side of the arrow which is the reactant side and the right side is the product side.
Now let’s see what a combustion reaction is and which species are involved for the chemical combination to yield the final product.
Combustion process is a process in which a substance commonly called as fuel is getting oxidized with the help of oxidizing agent and the substance on oxidizing gives out the energy as in the form of heat. Hence combustion is an exothermic process, since heat is evolved. The combustion reaction could be said as a redox reaction between the fuel and oxidizing agent as in this reaction, the fuel will get oxidized whereas the oxidizing agent will get reduced.
Generally we study the combustion reactions of the hydrocarbons. The hydrocarbons are used as the fuel. The hydrocarbons are those molecules which only contain H and C atoms in them.
The fuel in the presence of oxygen gets oxidized and on complete combustion the products yielded are $C{{O}_{2}}$ and ${{H}_{2}}O$ molecules.
We can generally write the equation as,
$Fuel+{{O}_{2}}\to C{{O}_{2}}+{{H}_{2}}O$
In the case of hydrocarbons, we write the general equation for complete combustion as${{C}_{x}}{{H}_{y}}+z{{O}_{2}}\to xC{{O}_{2}}+\frac{y}{2}{{H}_{2}}O$
Where the value of z is,$z=x+\frac{y}{4}$
For example: The chemical equation for the combustion of methane is,
$C{{H}_{4}}+2{{O}_{2}}\to C{{O}_{2}}+2{{H}_{2}}O$

Note:
Combustion reaction is generally called as burning but it is not necessary that in every combustion reaction flames are involved in it the reaction is a burning process.
If incomplete combustion takes place i.e. the oxygen supply was not enough to complete the combustion and incomplete combustion takes place and yields carbon monoxide as the final product.