Consider the following intermediate chemical equations.
C(s) +202(g) → CO(g)
CO(g)+302(g) → CO2(g)
How will oxygen appear in the final chemical equation?
O O2(g) as a product
O O2(g) as a reactant
O O(g) as a product
O 20(g) as a reactant

O₂(g) as a reactant

In chemical reactions, there is a reaction between reagents and products

Reaction:

aA + bB ⇒ cC

Substances A and B are called a reactants

Substance C is called a product

In the above reaction:

CO functions as a product in the first reaction and functions as a reactant in the second reaction  

In the final chemical equation :

CO(g)+30₂(g) → CO₂(g)

O₂ is on the left so it is called the reactant

B. O₂(g) as a reactant

2fe + 3h2so4 -> fe2(so4)3 + 3h2

it is necessary to balance equations in order to get the proper amount of reactants as well as products in order not to make a miscalculation.

you cannot change the subscripts as that would change the substance itself. for instance, diatomic elements such as h2 only have the subscript two when a gas. another example would be so4, if you were to change the subscript, the element would no longer be sulfate.

we change the coefficients instead as that only changes the number of molecules rather than the substance itself.

hope this !