GChem Normality?

[IdahoDoc]

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Cr2O7-2 ------> Cr3+ the dichromate is 2.5 molar

What is the normality if the reaction goes to completion?

a. 7.5
b 2.5
c. 0.83
d. 5.0
e. 1.13











The answer is 7.5 normal but i dont understand why it is not 5 normal. i always thought normality was dependent how many parts the reactant divided into. THe solution talks about how there are 3 electors passed so we need to times by 3 not 2. any help here would be great.

 

[UCB05]

My answer wouldn't be any of those. Cr+6 > Cr+3 needs 3 equivalents of electrons. Dichromate has two of these so it requires 6 equivalents total to reduce to completion. My answer would be 2.5x6 = 15, not 7.5. It's not 5N because in dealing with redox reactions the basic unit of normality is one charge so you have to take that into account beyond the molar stoich coefficients.

 

[aahitunaa]

1

 

[IdahoDoc]

I think beause its 3 equiv of electrons you take 2.5 x 3 = 7.5. that is how I would do it.

I see that there are three electrons but I don't know why i need to care about electrons in this problem i haven't on any other normality problems. So was i just getting lucky on the other problems or is this one different?

any other ideas?

 

[JBarr29]

Hey man, I understand your confusion. This is from Wikipedia and helped clear it up for me a couple months ago.

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.

 

[UCB05]

I see that there are three electrons but I don't know why i need to care about electrons in this problem i haven't on any other normality problems. So was i just getting lucky on the other problems or is this one different?

any other ideas?

because normality as a concept is based on the specific reaction you're referring to. In a redox reaction, normality scales with the amount of electrons donated or received. In a titration reaction it scales with equivalents of acid or base contributed. In a displacement reaction, on the effective acceptability of said other reagent. If you've never had to bother with electrons on any normality questions, you either haven't done one or have only dealt with 1:1 redox reactions.

 

[UCB05]

Hey man, I understand your confusion. This is from Wikipedia and helped clear it up for me a couple months ago.

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.

Right and a dichromate requires six equivalents of electrons to be reduced. My guess, if 7.5N is the right answer, is that the reactant normality is 15N but since the reaction creates twice as much product, the final normality is 15/2=7.5N? Someone help us out here~

 

[luv8724]

you nailed it UCB