Question 38

[h99h20baby]

how do you know not to use this equation with the Cr3?
Cr3 e ↔ Cr2


-0.41 V

thanks

[admin]

Because the question asks for the production of Cr metal which is solid. The equation that you specified produces a cation which is in solution.

[asfi]

could this problem have been done looking at the standard reduction potential for both reactions?

[admin]

I do not see an easier way to solve this problem than the one described in the Explanation (which also uses the stochiometric relationship described by the Standard Reduction Potential Table).

[blechz2803]

Maybe i just don't understand this...but wouldn't the moles of chromium metal obtained be less than 2/3- because the reduction potential is less and therefore not reduced as well as sulfate ions?
Unless it doesn't make a difference what the reduction potentials are? And if so-why??

[jellywing_2058]

Correct, the reduction potential in this case is not used to calculate the moles, as it is the required potential needed per mole synthesis for each of the separate elements. Thus, +0.17 V is required for SO2 synthesis and -0.74 V is required for Cr metal synthesis.

These are basically the required potentials to reduce the reactants to give the desired products. As such, the potentials are different for each reaction and have no direct bearing on the calculations of moles synthesized.

As for the moles of products synthesized, one needs to look at the stoichiometry of each reaction and the moles of electrons involved.
The question simply asks how many moles of Cr metal would be formed. You need to look at the reaction and thus the electrons to calculate the answer as was correctly performed in the explanation on the exam.

The potential is there to show how much voltage is required to reduce each of the reactants to give the products made and to determine the moles of products to be formed. You need to consider the moles of electrons needed to reduce the reactants.