Normality in Redox reactions

[dl9006]

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Could someone please explain how we use Normality in redox reactions?

I'm referring to the dat destroyer-gen chem #70

Question:
Reaction of 2.5M dichromate:
Cr2O72- goes to Cr3+

What would be solution Normality if reaction goes to completion?
Answer: 7.5 N


Do we always multiply the original concentration by the # of e- transferred to calculate normality in redox rxns?

thanks

 

[bennijai]

In redox reactions you will need to balance the equation out first and then take note on the number of e- transfered. Since there is a transfer of 3 electrons you will need to multiply the M by 3 giving you 7.5N.

In dissociation (acids/base) I believe normality depends on the particles dissociated.

 

[dl9006]

in acid/base chemistry, normality is used as a shortcut for calculating equivalent concentrations but what is normality used for in redox chemistry?

 

[bennijai]

For redox reactions, N measures the amount of oxidizing or reducting agent that can accept or donate one mole of electrons.

 

[sixkiller]

That is incorrect....

Dichromate is a single compound and the total oxidation is +12 to +6. Therefore, if the reaction goes to completion, the molarity must be multiplied by 6 not 3 as the solution says.

You can verify this with any textbook that discusses normality or any chemistry problem that deals with redox titration.

I have seen redox titration problems using dichromate....if you multiplied the concentration by 3, your answer will be incorrect.


Here is a textbook question that illustrates the application........

Acidified solutions of dichromate ion oxidize Iron(II) to Iron(III) forming chromium(III) in the process. What volume of 0.175M dichromate solution in mL is required to oxidize 60mL of 0.250M ferrous sulfate?

If you used the destroy logic, you get an incorrect answer.....

The application of normality is the same as an acid base titration...

0.25M(60mL)(1eq/mol) = 0.175M(6eq/mol)(V)

Solving for volume, we get 14.3ml.

*you can solve this problem just by using plain old stoichiometry, but if you recognize that this is a simple redox titration, you can solve it much much quicker*

 

[bennijai]

So destroyer is wrong? Hm...

So would the answer be 5.0N?

 

[sixkiller]

So destroyer is wrong? Hm...

So would the answer be 5.0N?

Well, you need to mutiply by a factor of 6; the balanced half reaction shows that one mole of dichromate reacts with 6 equivalents of electrons.

 

[bennijai]

Ah ok I see it now. Thanks.