[sn1sn2e1e2]

Here is a set of mechanisms..

SO2(aq) + H2O(l)     HSO3-(aq)  +   H3O+(aq)

2HSO3-(aq)  +    O2(aq)          S2O7(aq)  +  H2O(l)

S2O7(aq)   +   3H2O(l)        2SO4 2-(aq)   +   2H3O+(aq)
_______________________________________________________________

HSO3- is an intermediate in this mechanism but it shows up in the overall reaction when you add them up!  Aren't intermediates supposed to be created and destroyed in steps?

[Borek]

Show how you add them.

As written they are not all balanced.

[sn1sn2e1e2]

  You cannot multiply each equation by a coefficient because you do not know the "overall" reaction. They only give you steps. Also, each individual step is already balanced. Therefore, when you add up all the individual steps, the whole reaction is technically "balanced". Couldn't H2O(l) be the intermediate (it's not) since HSO3 - is the intermediate also?

[Borek]

you do not know the "overall" reaction.

You do. Overall reaction is oxidation of SO₂ to H₂SO₄ in the presence of water.

Also, each individual step is already balanced.

No.

What does it mean "balanced reaction"?