Oxidation/Reduction problem from Chad's Quiz #7

[wired202808]

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Can someone please explain this to me? I just want to be sure as to why the products side is a +6. is it because it is originally +6 to begin with or did anything change and cause it to be +6.

Thanks everyone.

 

[MrFantastik]

The CHARGE of the products is +6 because there are two Cr+3 atoms. Just like the charge of the reactants is +12 because there are 14 H+ and one Cr2O7(2-).

The oxidation number of the Cr changes from +6 in Cr2O7(2-) to +3 in Cr3+

If this wasn't good enough of an explanation, let me know what you mean by

is it because it is originally +6 to begin with

and maybe I can help you out more.

 

[wired202808]

MR Fantastik, I understand the reactants and the 7H20 but how do you know the 2Cr3+ is equal to 6? is it merely 2 moles of Cr times the 3+ charge? Thats what I found to be confusing,

 

[MrFantastik]

Yup, thats what it is. Just like in the reactants the 14 moles of H+ give you a +14 charge that is then taken down to +12 when you include the Cr2O7(2-)

I completely understand your question now tho. How do you know whether or not to multiply the number of moles with the charge in order to get your total charge? And the way I remember it is by looking at those H+s. Since you understood everything that was going on in the reactants then you automatically made the assumption that 14 moles of H+ have a +14 total charge rather than a +1 total charge. Just remember that and you'll be fine

 

[wired202808]

Thats not what I meant lol I mean on the product side. how did would you know that the 2Cr3+ is now worth a 6+, since its the last one left I guess you assume that each Cr is worth +3 then times 2, therefore the product side is +6?

 

[MrFantastik]

Lol...yes i know that what youre asking about is if 2 moles of Cr3+ has a total charge of +6. This is true. I was just trying to show you that since you already understand that 14 moles of H+ has a total charge of 14+ that you already understand the concept when applying it to the reactants side. Now you just gotta apply the same concept to the products side to see that 2 moles of Cr3+ have a total charge of 6+. Its pretty simple and you seem to understand it so I wouldnt worry about it anymore.

 

[wired202808]

thanks for the help MrFantastik!