GChem Destroyer 86

[NumbaOneStunna]

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Consider the rxn of 2.5 M dichromate:
Cr2O7 (-2) -> Cr +3


Whats the solution normality if it goes to completion?

I'm confused by this the answer is 7 and I thought it should be 5 (since 2Cr is produced for every reactant), instead the answer looks at the change in oxidation state to find the multiplying factor for normality. Can someone explain why?

 

[cjw7475]

Consider the rxn of 2.5 M dichromate:
Cr2O7 (-2) -> Cr +3


Whats the solution normality if it goes to completion?

I'm confused by this the answer is 7 and I thought it should be 5 (since 2Cr is produced for every reactant), instead the answer looks at the change in oxidation state to find the multiplying factor for normality. Can someone explain why?

The answer is actually 7.5, because if you notice the oxidation state of Cr in Cr2O7 is +6 and the product Cr is +3. So there are 3 electrons transferred from the reactant. So 3(2.5) = 7.5

 

[NumbaOneStunna]

Yes sorry about that 7.5 is correct. But you didn't explain anything, I know where the 3 came and that it times 2.5 is 7.5. I want to know what normality means in this case and how it relates to the change in ox state. I've only seen in used in acid/base type situations where some acid with say two readily ionizable protons has Normality of Acid = 2*Molarity of Acid.

 

[SupahShyGuy]

Think of normality as a similar way of looking at molarity except it deals with effective molecules in a solution or electron transfer (which means only the H+ or the OH- that will be dissociated into solution).. therefore, a 1 M H2SO4 will be 2 Normal since is forms 2H+ and 1SO42- (2 moles of e- transferred!) and 1 M HCl will be 1 Normal which forms H+ and Cl- (1 mole of e- transferred).

Therefore, 2.5 M of fully dissociated dichromate ion which gets reduced from 6+ to 3+, it transfers 3 moles of e- in this redox reaction.

3 mol e- x 2.5M = 7.5 N

=]

Happy Studying!

 

[Philippines03j]

Think of normality as a similar way of looking at molarity except it deals with effective molecules in a solution or electron transfer (which means only the H+ or the OH- that will be dissociated into solution).. therefore, a 1 M H2SO4 will be 2 Normal since is forms 2H+ and 1SO42- (2 moles of e- transferred!) and 1 M HCl will be 1 Normal which forms H+ and Cl- (1 mole of e- transferred).

Therefore, 2.5 M of fully dissociated dichromate ion which gets reduced from 6+ to 3+, it transfers 3 moles of e- in this redox reaction.

3 mol e- x 2.5M = 7.5 N

=]

Happy Studying!

Thanks for the question numbaone and for the explanation. I knew how to do both types of normality problems just was not connecting the dots!

 

[NumbaOneStunna]

So would it be correct to say solution normality when looking at a half reaction is the Molarity of electrons used or produced?

 

[SupahShyGuy]

So would it be correct to say solution normality when looking at a half reaction is the Molarity of electrons used or produced?

its more specific and correct to say electrons transferred.

 

[NumbaOneStunna]

its more specific and correct to say electrons transferred.

Yeah my b. Thanks.