Smallest whole number coefficient?

What is the smallest whole number coefficent of $\ce {Cr^{2+}}$ when the equation is balanced?  

$$\ce {Cr^{2+}\_{(aq)} + Al\_{(s)} -> Cr\_{(s)} + Al^{3+}\_{(aq)}}$$


I don't understand, how can this be balanced? And why would 3 be the smallest coefficient? 




You need to factor in the electrons that are being moved around


Chromium is doing this:  

$\ce {Cr^{2+}\_{(aq)} + 2 e- -> Cr\_{(s)}}$


Aluminum is doing the reverse of this:


$\ce {Al^{3+}\_{(aq)} + 3 e- -> Al\_{(s)}}$


Now, like a math equation flip around the aluminum reaction.  

And balance out the electrons


$\ce {[Cr^{2+} + 2 e- -> Cr\_{(s)}] \times 3}$


$\ce {[Al\_{(s)} -> Al^{3+} + 3 e- ] \times 2}$




---


$\ce {3Cr^{2+} + 6 e- + 2 Al\_{(s)} -> 3Cr\_{(s)} + 2 Al^{3+} + 6 e-}$


Hope this helps 


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