(40) Chemistry question, plz help (HKCEE's question)

2007-10-02 10:16 am

The student placed a drop of potassium dichromate solution at A and adrop of a deep blue solution at C.It is known that the two solutions do not react and the deep blue colour of the solution at C is due to the cation present.
(a) write the formula of the ion responsible for the orange colour of potassium dichromate.
(b) Why way the filter paper moistened with sodium sulphate solution?

更新1:

(c) An electric current was passed through the circuit for some time. (i) What would be the colour change at A? (ii) What would be the colour change at B? Explain your answer.

更新2:

(d) Using the same apparatus and materials, suggest how you could show that the colour changes in (c) are NOT due to diffusion.

回答 (1)

Gabriella Montez

2007-10-03 1:14 am

✔ 最佳答案

a) Cr2O7- [ Dichromate ions are orange in colour.]
b) It provides ions for the conduction of electricity. [ Sodium sulphate is an electrolyte which contains mobile ions to conduct electricity. ]
ci) The orange colour at A becomes paler.
ii) Green colour would be observed. Under the influence of electric field, the cations and anions are respectively attracted towards the negative and positive poles. The orange negative ions and the blue positive ions mix at B to give the green colour.
d) Reverse the polarity of the d.c. supply. Then the orange colour will appear at the left of A and the blue colour will appear at the right of C. It shows that the colour changes in ( c ) are not due to diffusion.

2007-10-02 17:19:58 補充：
Sorry for a typing error, in part a), the ion should be Cr2O7 2- but NOT Cr2O7-

參考: My Chem Knowledge

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