redox(急) - Ans.fyi 參考答案

redox(急)

[匿名]

2012-08-05 1:08 am

1. in an experiment,an acidified solution containing 1.5X10^-3 mole of Cr^n+ ions was allowed to react with 0.0250 M potassium permanganate solution. it was found that 49.0 cm3 of the permangante solution were required to oxidize the Cr^n+ ions.

a. in terms of n ,write the ionic half equation for the reaction that occurred for the Cr^n+ ions

b. deduce the no. of moles of Cr^n+ ions oxidized by one mole of permangante solution.

c. using your answer in b, deduce the value of n.

回答 (1)

己式庚辛

2012-08-05 2:45 am

✔ 最佳答案

This is an interesting question.
a.
permanganate is a stronger oxidant than dichromate, so chromium ions could be oxidized to give dichromate (in acidic medium!) :
2Cr(n+) + 7H2O ------> Cr2O7(2-) + 14H(+) + [6x2 - 2xn = 12-2n]e-

b.
in reaction, no. of mole of permanganate used = 0.0250 x 49/1000 = 1.225x10^-3 mole
no. of mole of Cr(n+) reacted = 1.5x10^-3 mole

so, PER no. of mole of permanganate, no. of mole of Cr(n+) reacted
= 1.5x10^-3 / 1.225x10^-3 = 1.2245 mole

c.
in the half equation
MnO4(-) + 8H(+) + 5e- ------> Mn(2+) + 4H2O
every 1 mole of permanganate reacted consumes 5 mole of electrons ;

for every mole of Cr(n+) ions reacted, 12-2n mole of electrons are given out,
so 1.2245 mole of Cr(n+) gives 1.2245 x (12-2n) mole
= 14.694 - 2.449n mole of electrons

in redox, no. of electrons must be balanced,
so 5 = 14.694 - 2.449n
n = 4

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