Question

What is stoichiometry, and how is it used in chemical reactivity?

Answer 1

You must (i) have a balanced chemical equation, and (ii) know some basic arithmetic.

Explanation:

Let's look at another scenario. You go into a shop, and you purchase an item whose price is `£23-30`. You give the clerk a `£50-00` note. The clerk gives you the item and `£17-70` in change. Would you know whether you had been short-changed? I think you would know, and you would have cause for complaint.

In the banking and finance caper, there is a saying, `"for every credit there must be a corresponding debit"`. What does this mean? It means that if you credit an account, some other account must be debited, or there must be cash on hand (cash, so I am told, is a debit item).

What applies in the banking and finance industries, applies even more so in chemistry, because there are more particles to balance. This illustrates the fundamental principle of `"stoichiometry"`. Mass is ALWAYS conserved. If you start with `10*g` of reactant, at most you are going to get `10*g` of product. In practice, you are not even going to get that, because losses invariably occur on handling. So bear this in mind the next time you balance a chemical equation: if the reactants were money, would you be ripped off? In honesty, I do not think you would be.

So (finally!) a problem. I combust `16*g` of methane, `CH_4`, according to the following rxn:

`CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O(l)`

What is the mass of oxygen gas that I need? What mass of carbon dioxide will I get? Note that mass (and atoms) are necessarily conserved in every chemical reaction.

Try the process with another example; the combustion of hexanes, `C_5H_12`. This combusts according the equation:

`"Pentane + oxygen "rarr" carbon dioxide + water"`

`C_5H_12 + xO_2(g)rarryCO_2+zH_2O`

How to balance this? Remember, if it were money you would do it automatically. Balance `C`, then `H`, and then `O`.