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Redox Reaction: Ferrous Ion and Permanganate Ion

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The ferrous ion, Fe2+(aq), reacts with the permanganate ion, MnO4- (aq), in an acidic solution to produce the ferric ion, Fe3+(aq). A 6.893 g sample of ore was mechanically crushed and tehn treated with concentrated hydrochloric acid, which oxidized all of the iron in the ore to the ferrous ion, Fe2+(aq). Next the acid solution containing all of the ferrous ions was titrated with .100 M KMnO4 solution. The end point was reached when 13.899mL of the KMnO4 solution was used.

a. Write the oxidation half reaction
b. Write the reduction half reaction
c. Write the balanced final redox reaction
d. Identify the oxidizing agent, the reducing agent, the species being oxidized, and the species being reduced
e. Calculate the number of moles of iron in the sample or ore
f. Calculate the mass percent of the iron in the ore

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Solution Summary

To balance the overall equation the number of electrons oxidized and reduced must be the same. By definition, the oxidizing agent is reduced and the reducing agent is what is oxidized (the reason for this is that, for example, the oxidizing agent allows oxidation to occur). When you are asked to find the mass percent of the ore, you want to know how many grams of the sample were iron. Mass iron / mass ore x 100. 475 words.

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This is a standard titration problem. The first thing we need to do is determine the balanced chemical equation for the reaction. Let us first write the half-reactions.

1) oxidation. Fe2+ is converted to Fe3+: Fe2+ --> Fe3+ + e-
2) MnO4- is reduced to Mn2+ in acidic solution: MnO4- + H+ --> Mn2+ + H2O

Note that the H+ is needed in the second reaction to combine with the O's to create water. This was not needed in the oxidation reaction since iron is the only atom in the oxidation. Note also that the oxidation reaction is balanced in both atoms and charge. In contrast, we have some work to do on the bottom reaction. In fact you know you need the water and the H+ in the second reaction to balance the O's on both side. Adding in coefficients for the reduction reaction, we are able to balance all the atoms:

MnO4- + 8H+ --> Mn2+ + ...

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