F.5 -- Chemical Cells

2009-09-10 7:23 am

Given a diagram:
There is a U-shaped tube.
On the right hand side is carbon electrode B and left hand side is carbon electrode A.
B is soaked in iron (III)sulphate solution. A is soaked in sodium sulphite soultion.
In between the soultion in the U-shaped tube, a cotton wool seperate the two. An external wire connected electrode A and B. An ammeter is also connected to them.
(1)What would be observed in the iron (III)sulphate solution after some time. Write a half-equation for the reaction involved.
(2)State and explain the pH change if a universal indicator is added to the sodium sulphite solution. Write the equation involved, if any.

回答 (1)

老爺子

2009-09-15 7:15 am

✔ 最佳答案

(1)
The brownish-yellow solution changes to pale green.
Fe3+(aq) + e- → Fe2+(aq)

(2)
The pH of sodium sulphite decreases. This is because the oxidation of sulphite ion to sulphate ion gives also hydrogen ion, and the increase in the concentration of hydrogen ion makes the pH value decrease.
SO32-(aq) + H2O(l) → SO42-(aq) + 2H+(aq)

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