Destroyer Gen Chem #70

[Versati1ity]

Could someone explain this problem to me. For whatever reason I can't seem to understand the solution. Since it is a redox reaction Cr 6+ -> Cr 3+ I realize that 3 e- will be required to accomplish this, but I don't understand why that would be used in finding the normality.

I guess I am still a little confused about normality. I thought that normality was only used for acids and bases N = M * # OH- or H+ of base or acid respectively. Since it is a redox reaction do we find normality based on equivalents of electrons? Help!

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[ongmeister]

For that problem I believe u don't find the Normality you are just balancing the electrons for the half reactions.

 

[Versati1ity]

For that problem I believe u don't find the Normality you are just balancing the electrons for the half reactions.

The solution say's that the balancing portion of the problem is just for added practice. The solution is 3 * Molatity = 7.5N

But I want someone to explain why the 3 e- are used as the multiplier.

 

[whoaaitzkyle]

Could someone explain this problem to me. For whatever reason I can't seem to understand the solution. Since it is a redox reaction Cr 6+ -> Cr 3+ I realize that 3 e- will be required to accomplish this, but I don't understand why that would be used in finding the normality.

I guess I am still a little confused about normality. I thought that normality was only used for acids and bases N = M * # OH- or H+ of base or acid respectively. Since it is a redox reaction do we find normality based on equivalents of electrons? Help!

Well, once you determine the oxidation number of Cr ( = 6) you subtract it from the Cr on the other side.

Cr2O72- ---> 2Cr3+
+6-----------> +3

Transferring 3 electrons.

We know that M (#e-) = Normality

3(2.5) = 7.5N

 

[Versati1ity]

Well, once you determine the oxidation number of Cr ( = 6) you subtract it from the Cr on the other side.

Cr2O72- ---> 2Cr3+
+6-----------> +3

Transferring 3 electrons.

We know that M (#e-) = Normality

3(2.5) = 7.5N

I guess what I'm confused about is that I thought Normality was specific to OH- and H+ equivalents, so only used in acid/base problems. So normality is broader than that? In this case e- were the quantity of interest.

 

[giant]

I just took this off from wikipedia! Normality is broader than just acid-bases.
Wikipedia:
As ions in solution can react through different pathways, there are three common definitions for normality as a measure of reactive species in solution:

* In acid-base chemistry, normality is used to express the concentration of protons or hydroxide ions in the solution
* In redox reactions, normality measures the quantity of oxidizing or reducing agent that can accept or furnish one mole of electrons. Here, the normality scales from the molarity, most commonly, by a fractional value. Calculating the normality of redox species in solution can be challenging.
* In precipitation reactions, normality measures the concentration of ions which will precipitate in a given reaction. Here, the normality scales from the molarity again by an integer value.

 

[whoaaitzkyle]

I guess what I'm confused about is that I thought Normality was specific to OH- and H+ equivalents, so only used in acid/base problems. So normality is broader than that? In this case e- were the quantity of interest.

Maybe someone else can help you out on this one (I can't really explain it...I probably have learned this the incorrect way). However, the way I interpret Normality is looking at equivalence and not in terms of just proton transfer. So in this scenario I was looking to just balance the electrons making it equal on both sides. It always has seemed to work for me, but if someone can give a more proper explanation if this is incorrect I'd be interested in learning!

 

[doc3232]

I just took this off from wikipedia! Normality is broader than just acid-bases.
Wikipedia:
As ions in solution can react through different pathways, there are three common definitions for normality as a measure of reactive species in solution:

* In acid-base chemistry, normality is used to express the concentration of protons or hydroxide ions in the solution
* In redox reactions, normality measures the quantity of oxidizing or reducing agent that can accept or furnish one mole of electrons. Here, the normality scales from the molarity, most commonly, by a fractional value. Calculating the normality of redox species in solution can be challenging.
* In precipitation reactions, normality measures the concentration of ions which will precipitate in a given reaction. Here, the normality scales from the molarity again by an integer value.

Welcome giant

 

[whoaaitzkyle]

I just took this off from wikipedia! Normality is broader than just acid-bases.
Wikipedia:
As ions in solution can react through different pathways, there are three common definitions for normality as a measure of reactive species in solution:

* In acid-base chemistry, normality is used to express the concentration of protons or hydroxide ions in the solution
* In redox reactions, normality measures the quantity of oxidizing or reducing agent that can accept or furnish one mole of electrons. Here, the normality scales from the molarity, most commonly, by a fractional value. Calculating the normality of redox species in solution can be challenging.
* In precipitation reactions, normality measures the concentration of ions which will precipitate in a given reaction. Here, the normality scales from the molarity again by an integer value.

thanks for the explanation! welcome to the SDN community!

 

[Versati1ity]

I just took this off from wikipedia! Normality is broader than just acid-bases.
Wikipedia:
As ions in solution can react through different pathways, there are three common definitions for normality as a measure of reactive species in solution:

* In acid-base chemistry, normality is used to express the concentration of protons or hydroxide ions in the solution
* In redox reactions, normality measures the quantity of oxidizing or reducing agent that can accept or furnish one mole of electrons. Here, the normality scales from the molarity, most commonly, by a fractional value. Calculating the normality of redox species in solution can be challenging.
* In precipitation reactions, normality measures the concentration of ions which will precipitate in a given reaction. Here, the normality scales from the molarity again by an integer value.

Thanks giant, this is what I was looking for!

 

[yakuza]

I just took this off from wikipedia! Normality is broader than just acid-bases.
Wikipedia:
As ions in solution can react through different pathways, there are three common definitions for normality as a measure of reactive species in solution:

* In acid-base chemistry, normality is used to express the concentration of protons or hydroxide ions in the solution
* In redox reactions, normality measures the quantity of oxidizing or reducing agent that can accept or furnish one mole of electrons. Here, the normality scales from the molarity, most commonly, by a fractional value. Calculating the normality of redox species in solution can be challenging.
* In precipitation reactions, normality measures the concentration of ions which will precipitate in a given reaction. Here, the normality scales from the molarity again by an integer value.

Nice catch. I never knew this lol

 

[giant]

Hi everyone! Thanks for the warm welcomes

 

[klutzy1987]

You are fogetting about lewis acids and bases. The Cr6+ is acting as a lewis acid here. Normality applies to all acids not just bronstead lowry.