What does normality have to do with reduction? DESTROYER Q 70 GEN CHEM

craigory

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    I thought normality was similar to Van't Hoff factor, i.e. amount of particles something dissociates into. However, in this problem, Chromium is reduced from +6 to +3, so because 3 moles of electrons were required to reduce, the normality is 3[Cr(+3)] concentration. However, in the balanced equation, Cr207(-2) --> 2Cr(+3). Can someone explain to me why the normality is linked to the amount of e- required to reduce vs. the amount of particles of Chromium dissociated? I thought normality would be 2[Cr(+3)]...
     

    doc toothache

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      I thought normality was similar to Van't Hoff factor, i.e. amount of particles something dissociates into. However, in this problem, Chromium is reduced from +6 to +3, so because 3 moles of electrons were required to reduce, the normality is 3[Cr(+3)] concentration. However, in the balanced equation, Cr207(-2) --> 2Cr(+3). Can someone explain to me why the normality is linked to the amount of e- required to reduce vs. the amount of particles of Chromium dissociated? I thought normality would be 2[Cr(+3)]...

      Can you write the complete question?
       

      doc toothache

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        #70. Consider the reaction of 2.5M dichromate:

        Cr2O7 2- ----> Cr 3+ (acidic medium)

        What would be the solution Normality if the reaction goes to completion?

        Normality is defined as number of equivalents/L of solution. In acid-base reaction it is the amount of substance which reacts with or liberates 1 mol of H+. In a redox reaction it is the amount of substance which reacts with or liberates 1 mol of electrons. In the example given the equivalent weight of dichromate is equal to the mol wgt/oxidation state change. Since the total change is 6e- a 1M solution is equal to 6N; a 2.5M is 15N.
         
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        Zerconia2921

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          Normality is defined as number of equivalents/L of solution. In acid-base reaction it is the amount of substance which reacts with or liberates 1 mol of H+. In a redox reaction it is the amount of substance which reacts with or liberates 1 mol of electrons. In the example given the equivalent weight of dichromate is equal to the mol wgt/oxidation state change. Since the total change is 6e- a 1M solution is equal to 6N; a 2.5M is 15N.



          According to the solutions the answer is 3(2.5) = 7.5N

          Cr is reduced from plus six to plus three so 3 e- difference. The normality will be 3 times molarity. Normality greater than or equal to molarity. this is what is written for the solution.
           

          doc toothache

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            According to the solutions the answer is 3(2.5) = 7.5N

            Cr is reduced from plus six to plus three so 3 e- difference. The normality will be 3 times molarity. Normality greater than or equal to molarity. this is what is written for the solution.

            Hmm. As you say, Cr+ is reduced from 6+ to 3+. However, there are 2 Cr+ in Cr2O7(-2) for a total gain of 6e. Admittedly most times we are the ones making miscalculations. However, this is one of those occasions when the source answer is questionable. But if you like their answer better, stick with it.
             

            doc toothache

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              In redox chemistry, equivalent refers to the moles of electrons transferred. In this case, 3 electrons are lost going from Cr(+6) to Cr(+3). I don't have the Destroyer so I don't know what the question is asking, but you would multiply by factor of 3.

              Is it safe to assume that some of the above posts went unread?
               

              tommyngu

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                Ok. I know this post is old, but I came across it via Google search for the same problem. I think Destroyer's answer of 7.5N is correct, but they did a bad job at explaining why. If you look at the balanced equation 3 e- are being transferred PER Cr atom and there are 2 of them.
                Here's my math: (2.5mol dichromate/L)(2mol Cr/mol dichromate)(3 eq e-/mol Cr)(1mol Cr/ 2mol Cr3+) = 7.5N.
                Yes, there are 6 total electrons transferred BUT those 6 electrons are being transferred from from 2 Cr atoms to produce 2 reduced Cr atoms. I hope this helps those who had the same problem as I did.
                 
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