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.


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?

  • 1
    $\begingroup$ 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, 2015 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). $\endgroup$
    – math_lover
    Jan 12, 2015 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, 2015 at 21:26
  • 1
    $\begingroup$ 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, 2015 at 20:22

1 Answer 1


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

  • $\begingroup$ 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. $\endgroup$
    – math_lover
    Jan 12, 2015 at 18:51
  • 1
    $\begingroup$ 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, 2015 at 18:56

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