Some chemistry questions

2014-11-06 5:56 am

1. Which of the following aqueous solutions, when electrolysed, would its pH value decrease ?
||||||Anode||||||||||||Cathode |||||||||||Aqueous solution
(1) Pt(s)||||||||||||||||Pt(s)|||||||||||||||||||||0.2M H2SO4(aq)
(2) C(graphite)||| C(graphite)|||||||| 2M NaBr(aq)
(3) Ag(s)||||||||||||C(graphite)||||||||||0.2M AgNO3(aq)
A. (1) only
B. (2) only
C. (1) and (3) only
D. (2) and (3) only

The ans is A, i dont know why 3 is not correct. In 3, Ag+ is preferentially discharged rather than H+ at cathode while OH- is preferentially discharged rather than NO3- at anode. So, the pH should decrease.

2. Phosphorus forms two chlorides, PCl3 and PCl5 where PCl3 has similar shape as
ammonia.

Why PCl5 has a higher melting point than PCl3?
the ans given is saying that PCl5 has a higher relative molecular mass, so PCl5 molecules has a greater intermolecular force.

However, we all know that PCl5 is non-polar, PCl3 is polar. So, PCl5 molecules and PCl5 molecules should be held strongly by dipole-dipole attraction while PCl3 molecules and molecules are just held together by weak VDF. Then why PCl5 has a higher melting point than PCl3?

Thanks for answering.

回答 (1)

pingshek

2014-11-07 1:38 am

✔ 最佳答案

The answer is as follows :

圖片參考：https://s.yimg.com/lo/api/res/1.2/sH9nT6uLh4M.lbAvyNJjMg--/YXBwaWQ9dHdhbnN3ZXJzO3E9ODU-/https://farm4.staticflickr.com/3956/15699586016_c7c15608ee_b.jpg

https://farm4.staticflickr.com/3956/15699586016_c7c15608ee_b.jpg

2014-11-09 18:40:56 補充：
Refer to standard reduction potential, OH⁻ is a stronger reducing agent than Ag when the concentration of OH⁻ ions is 1 M. However, in silver nitrate solution, the concentration of OH⁻ is very low, and thus Ag is a stronger reducing agent than OH⁻.

2014-11-09 18:41:03 補充：
Note that the process of electroplating of silver on the cathode. Silver oxidized to silver ions at the anode, and silver ions are reduced to silver at the cathode.

參考: pingshek, pingshek, pingshek

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