please help with redox balancing

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japdjj
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Posts: 1
Joined: Sun Oct 25, 2015 2:49 pm

please help with redox balancing

Post by japdjj »

How do I balance these equations? Please explain the half equations as well.

Fe2+ + MnO4- ----> Fe3+ + MnO2

H2O2 + Cr3+ ----> CrO42- + OH-
GrahamKemp
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Posts: 396
Joined: Mon Mar 19, 2012 10:12 pm

Re: please help with redox balancing

Post by GrahamKemp »

Split into half reactions, removing spectator ions and water and its ions.

Balance Oxygen using water. Balance Hydrogen using protons. Then balance the charges using electrons. Merge so that electrons cancel.

Note: To make alkaline, add sufficient

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OH{-}
on both sides to neutralise all

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H{+}
(make water) and then cancel any water common to both sides.

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 Fe{2+} + MnO4{-} = Fe{3+} + MnO2
Split in Half:

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 Fe{2+} = Fe{3+} || MnO4{-} = MnO2

Balance O:

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 Fe{2+} = Fe{3+} || MnO4{-} = MnO2 + 2 H2O

Balance H:

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 Fe{2+} = Fe{3+} || MnO4{-} + 4 H{+} = MnO2 + 2 H2O

Balance e:

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 3 Fe{2+} = 3 Fe{3+} + 3 e || MnO4{-} + 4 H{+} + 3 e = MnO2 + 2 H2O

Merge:

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3 Fe{2+} + MnO4{-} + 4 H{+} = 3 Fe{3+} + MnO2 + 2 H2O


Now you go:

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H2O2 + Cr{3+} = CrO4{2-} + OH{-}
Split in Half:

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H2O2 = ? || Cr{+} = CrO4{3-}
Balance O:
Balance H:
Alkalise:
Balance e:
Merge:
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