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# How do I derive the half equation Watch

1. I wrote the answer in the markscheme but I don't know how they got it

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2. Basically: one half equation + other half equation = full equation

[NiO(OH) + H2O + e- ---> Ni(OH)2 + OH-]
+[half equation you need]
--------------------------------------------------------------
[MH + NiO(OH) ---> M + Ni(OH)2]

So anything that is on the left of the full equation but not in the given half equation => you need on your left (MH)

Anything on the left of the half equation that isn't on the left of the full equation => you need on your right (H2O + e-)
Other way around with your right hand side...

Anything that is on the right of the full equation but not in the given half equation => you need on your right (M)

Anything on the right of the half equation that isn't on the right of the full equation => you need on your left (OH-)
Put it all together: MH + OH- ---> M + H2O + e-

Just check that the species and charges balance It gets quicker with practice

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Updated: April 17, 2014
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