Why can't I change the subscripts in a molecule when I balance a chemical reaction?

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Why can't I change the subscripts in a molecule when I balance a chemical reaction?

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Answer 1 · 2016-12-05T01:43:08

Because you change the integrity of the compound you're looking at.

For example:

$6 HCl + 2 Al (s) \to 2 {AlCl}_{3} + 3 H_{2} (g)$ $6"HCl" + 2"Al"(s) -> 2"AlCl"_3 + 3"H"_2(g)$

If you suddenly write $6 HCl$ $6"HCl"$ as $3 H_{2} {Cl}_{2}$ $3"H"_2"Cl"_2$ , then what you've done is replace $HCl$ $"HCl"$ with a new molecule whose formula is $H_{2} {Cl}_{2}$ $"H"_2"Cl"_2$ , and this molecule may or may not exist. This one doesn't.

The new chemical formula of $H_{2} {Cl}_{2}$ $"H"_2"Cl"_2$ literally tells you that there are two hydrogen atoms and two chlorine atoms in this hypothetical molecule, but $HCl$ $"HCl"$ only had one of each.

Likewise, $3 H_{2} {Cl}_{2}$ $3"H"_2"Cl"_2$ is also not the same as $3 H_{2} + 3 {Cl}_{2}$ $3"H"_2 + 3"Cl"_2$ .

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Why can't I change the subscripts in a molecule when I balance a chemical reaction?

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