Question

Question #5411c

Answer 1

The amount of atoms of each species on both sides of the reaction have to be the same.

Explanation:

There isn't really much more than this. When you know what your reactants are, and what your products of reaction are, you write down the equation of the reaction. Then you change the amount of each compound until you have the same amount of atoms of each species on both sides of the reaction.

For example, say you have a hydrocarbon combustion. In a combustion, the products are always `CO_2` and water.
Let's assume we are burning propane (always with oxygen; heat takes no part in the stoichiometric equilibrium). So the unadjusted equation would be:
`C_3H_8 + O_2 -> CO_2 + H_2O`

On one side we have 3 carbon atoms, in the other side, only one. Also, on one side we have 8 hydrogens, and 2 on the other side. So let's start by getting 3 `CO_2` and 4 `H_2O` molecules.
`C_3H_8 + O_2 -> 3` `CO_2 +4` `H_2O`

Now, the only unadjusted element is the oxygen. We have 2 on one side, and 10 on the other side. That means we need 5 oxygen molecules in the reactant side, instead of one.
`C_3H_8 +5` `O_2 -> 3` `CO_2 +4` `H_2O`
And this is the stoichiometry of the combustion of propane.