[Xenonman]

So these things people call "redox reaction" are annoying. I used to dislike them, to later find about the algebraic method, to later find the algebraic doesn't always work. WHY?
A book I found claimed the algebraic was the most powerful method, so one should become skillful with it ASAP.
But there are redox reactions that annoy the usual redox reactions. Those ones can't be solved by algebraic, only by acid or base conditions in a redox method.
Question is: would you discourage me from using the algebraic, because it is not fully reliable, and instead study the redox fully?

[Borek]

Algebraic method fails when the stoichiometry depends on the mechanism, not just atom counting.

[Big-Daddy]

At least the algebraic method will work for all non-redox reactions (AFAIK) so you definitely should study it fully, but at the same time do not ever try and use it for redox reactions - just apply the general methods for balancing redox reactions instead.

[Borek]

At least the algebraic method will work for all non-redox reactions (AFAIK)

C₇H₆O₃ + C₄H₆O₃  C₉H₈O₄ + C₂H₄O₂

And no, it is not something made up, it is a perfectly correct reaction of salicylic acid with acetic anhydride, yielding aspirin and acetic acid. Actually it is balanced as it is.

[Big-Daddy]

C₇H₆O₃ + C₄H₆O₃  C₉H₈O₄ + C₂H₄O₂

Hmm. Good example. It's easy to see why the method fails (H and O atom balance equations are identical) but at least we can say with certainty that there isn't a unique solution - in other words we cannot "balance this equation", we just have to observe the ratio of products to reactants experimentally. So perhaps, it will do to say that the algebraic method will work for all non-redox reactions where it is possible to attain a unique balancing of the reaction equation theoretically?

[Xenonman]

K₂CrO₄ + H₂O₂ + HCl -> CrCl₃ + O₂ + KCl + H₂O
I tried that one by algebraic a while ago. It was impossible. Webqc's good old balancer can't balance it, as it "can be balanced in an infinite number of ways: this is a combination of two different reactions"http://www.webqc.org/balance.php/balance.php. So I suppose it uses algebraic as well.

Guess I'll try to learn the redox one.

[Borek]

K₂CrO₄ + H₂O₂ + HCl -> CrCl₃ + O₂ + KCl + H₂O
I tried that one by algebraic a while ago. It was impossible

2H₂O₂ 2H₂O + O₂

4K₂CrO₄ + 20HCl 4CrCl₃ + 3O₂ + 8KCl + 10H₂O

Generally speaking every reaction with hydrogen peroxide being oxidized to oxygen will have this problem, as it is impossible to rule out simple decomposition.

Not that the latter makes any chemical sense, if anything, Cl₂ should be produced, something like

2K₂CrO₄ + 16HCl 2CrCl₃ + 3Cl₂ + 4KCl + 8H₂O

But balancers never care about chemical logic, they just do the accounting.