balancing redox equation in topscore

[iamsyc]

I just took the last TopScore practice and made so many incredibly stupid mistakes and my DAT is on Friday .... but ANYWAYS ..

They asked for balancing redox equation. For the sum of the coefficients after it was balanced.

MnO4- + I- --> I2 + Mn2+

I did the whole splitting them up into half-reactions and added the H2O to the left, and H+ on the right for the Mn half but I only got 29 as the sum.

My balanced equation was

8H+ + 10 I- + MnO4 --> Mn 2+ + 4 H2O + 5 I2

And the answer was not that...

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[yumi43]

did u forget to add electrons in?? that could change the coefficients for everything

 

[suhshi]

10I- + 16H+ +2MnO4- 5I2 + 2Mn2+ +8h20

 

[iamsyc]

I did! I just don't understand why you need 2 MnO4- .. it balances out fine with it just being MnO4- ..maybe I didn't check close enough with all the electrons balancing out for that half-reaction?? blehh

 

[appulipzzz]

I also got 10I- + 2MnO4- + 16H+ ---> 5I2 + 2Mn2+ + 8H2O

Here's how I got it....

1) 2I- ----> I2 + 2e-
MnO4- + 8H+ + 5e- ----> Mn2+ + 4H2O

2) You multiply the top by 5 and the bottom by 2 in order to cancel out the electrons.
So,
10I- ----> 5I2 + 10e-
2MnO4- + 16H+ + 10e- ----> 2Mn2+ + 8H2O

3) You add everything together,
and you get 10I- + 2MnO4- + 16H+ ---> 5I2 + 2Mn2+ + 8H2O

I'm not really good at explaining things but I hope that this helps

 

[iamsyc]

Yeah, I just realized my stupid mistake ...totally forgot to multiply the MnO4- part in order to have 10 e- for both half-rxns to cancel out e-. Wow...haha but glad that finally got cleared up. Stupid mistakes will get the best of me when I take it on Friday