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I truly don't understand the method for finding the net-ionic equation for a reaction. It is not clear to me how you should decide what the products are, especially when there are multiple possibilities.

Example:

Excess potassium cyanide solution is added to aluminum bromide solution.

The first thing that came to mind for the molecular equation is:

$\ce{3KCN\+AlBr3 \to 3KBr\+Al(CN)3}$

But apparently the correct molecular equation is:

$\ce{6KCN\+AlBr3 \to 3KBr\+Al(CN)6}$

I see no good reason why the one I thought of is incorrect. And to get the net ionic equation, the spectator ions are $\ce{K+}$ and $\ce{Br-}$. But why should I assume the solution is aqueous?

Another example:

Chlorine gas is bubbled through dilute sodium hydroxide.

Well its easy enough to get the reactants:

$\ce{Cl2\+NaOH \to ?}$

What first comes to mind is $\ce{Cl2\+2NaOH \to 2NaCl\+Cl2O\+H2}$

But the correct molecular equation is $\ce{Cl2\+2NaOH \to 2NaOCl\+NaCl\+H2O}$

What should be the thought process for arriving at the correct answer here. I am truly lost.

Another example:

Lithium oxide reacts with sulfur dioxide.

I recognize this as a synthesis reaction. However how can I predict if the product is lithium sulfite or lithium sulfate?

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    $\begingroup$ The 'correct' first equation does not balance. Could you provide us with the source of the first problem. I am not familiar with it. $\endgroup$ – bon Jan 12 '15 at 18:35
  • $\begingroup$ You are right it does not balance. But the molecular equation can't be correct because the ionic equation is $6CN^-+Al^{+3} \to Al(CN)_6^{-3}$. This is from a worksheet on net ionic equations (but its not homework). $\endgroup$ – Joshua Benabou Jan 12 '15 at 18:41
  • $\begingroup$ I looked it up but can find no references for Al(CN)6 - it could be that your worksheet has a mistake on it. $\endgroup$ – thomij Jan 12 '15 at 21:26
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    $\begingroup$ I have the impression is "how do I know which products to expect?". If I am right here, there is unfortunately no universal answer. Effectively, you'll have to do the experiment and analyse the products. There are a few guidelines and concepts, but this would get far to broad here. Alternatively, you'll have to look it up in a book or chemical encyclopedia. $\endgroup$ – Gerhard Feb 11 '15 at 20:22
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But apparently the correct molecular equation is $\ce{6KCN\+AlBr3 \to 3KBr\+Al(CN)6}$

That's not correct, what happenend to the other 3 potassium atoms?

It could form $\ce{Al2(CN)6}$ but you can't exclude $\ce{Al(CN)3}$ without further information beyond balancing of equations.

There are two main steps to balancing equation:

  1. Balancing of each type of atom

  2. Balancing number of electrons

If you've done both of those correctly, you've done all you can with balancing alone.

$\ce{Cl2\+2NaOH \to 2NaCl\+Cl2O\+H2}$

Above is clearly wrong because number of oxygen atoms doesn't balance

$\ce{Cl2\+2NaOH \to 2NaOCl\+NaCl\+H2O}$

Above is clearly wrong because number of oxygen atoms doesn't balance

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  • $\begingroup$ This does not answer my question. I am asking how we know what are the correct products when there are multiple possibilities, as in the examples I gave. $\endgroup$ – Joshua Benabou Jan 12 '15 at 18:51
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    $\begingroup$ You haven't given us an example where there are two equations that are balanced that you need to choose from. When there is such an example, you need experimental data. $\endgroup$ – DavePhD Jan 12 '15 at 18:56

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