Balancing Chemical Equations Using Oxidation Numbers

[carlodelmundo]

Homework Statement

http://carlodm.com/pictures/problem.png

The Attempt at a Solution

I'm stuck. Usually I balance the equation by multiplying the Fe(OH)₂ and Fe₂O₃ by 3. However, I can no longer do this because Fe₂O₃ has two iron's per one molecule.

Do I multiply one by 6 and the other by 3?

 

[Borek]

Assume 2Fe(OH)₂ is your reagent (why? because Fe₂O₃ contains two Fe atoms, just keep iron atoms forming one molecule of product togehter) - then you will have to multiply everything by 3. Basically that's the same as multiplitcation by 3 & 6, but it is easier to follow the reasoning.

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[carlodelmundo]

I see. So I should balance the reactant iron first, to 2... then do the multiplications of irons by 3.

I've arrived at that step but the products side still looks like a mess. Is this where I stop all "techniques" and rely on intution and trial and error to figure out the balanced equation?

 

[Borek]

Once you have Fe and Cr balanced rest you have to "balance by inspection" - which, as you have correctly pointed out, is a fancy name for "intuition and trial and error"

Do you have to use ON method?

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[carlodelmundo]

No we're not required to use the ON method... but it is a tool that we are able to utilize for ugly equations such as this. Thanks for your help! This'll come handy for reviewing my Chem exam.

 

[Borek]

Half reactions method is more effective - it splits the problem into two, usually simpler ones.

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