AP Chemistry Test FRQ: Balancing Equations

[cadillac]

I have no idea how to do the balanced equations portion of the FR. I know how to balance equations using stoichiometric coefficients, but not the particular type of "balanced equations" on the AP test.

For example: "a small piece of sodium is placed in distilled water"

my initial thinking:
Na + H2O --> NaOH + H+

but they have as the right answer:
2Na + 2H2O --> 2Na+ + 2OH- + H2

Why is my answer wrong? (I have the right species and my answer is balanced...)
Also, I have no idea when to combine certain species, or keep them separate as ions--do solubility rules account for this?


Any ideas?

 

[cadillac]

Nevermind, I asked a friend of mine, and got it resolved... But if anyone has any tips for these unique reaction questions, I would really appreciate it!

 

[Borek]

You didn't have a orrect species and your reaction was not balanced. H⁺ is not the same as H₂ and not only atoms must be balanced - charge must be balanced too.

And free hydrogen is diatomic. You were close.