| MadSci Network: Chemistry |
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This is a good question. You should ask yourself what the difference in meaning
is, between the subscript and the coefficient.
A molecular or empirical formula defines a particular compound; changing the subscripts would change the compound--CO is not the same as CO2, and NaNO2 is not the same as NaNO3. This is not admissible when balancing an equation because it corresponds to doing a different reaction. Instead, you have to change the coefficients: 2 H2O is half as much as 4 H2O, but in both cases you're dealing with water. Here's an example: Ammonium nitrate will decompose to nitrogen and water: NH4NO3 ® N2 + H2O We can balance by adding appropriate coeffients, and noting that there is a little oxygen left over:NH4NO3 ® N2 + 2 H2O + ½O2 But if we change subscripts, we are talking about a different reaction:NH2NO ® N2 + H2O is the decomposition of nitrosylamine rather than ammonium nitrate.
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