AL化學漂白水!!!!!

問題:
1.點解要用過量的碘化鉀和稀硫酸 ?
Ans: Excess KI is added to react with bleach completely and dissolve the iodine formed (iodine will not dissolve in water but KI solution). Dilute sulphuric acid is added to increase the rate of reaction.

2.令到漂白水失效的原因?(請列明方程式和詳細解釋)
Ans: Decompose the ClO- ion.

3.點解不能過早加入澱粉溶液?
Ans: The starch indicator is added when the solution has become a pale straw colour. If added too early, the indicator forms an insoluble complex with iodine, and the titre is too low.

4.點解要依此次序加入錐形瓶中?(漂白水-->碘化鉀-->稀硫酸)

Ans: Reaction of bleach and KI is slow, but when dilute sulphuric acid is added the reaction rate increase.
If KI sulphuric acid first, some I2 will formed from KI, then the mole of I2 formed by bleach will be affected.
If bleach sulphuric acid first, Chlorine gas will released and escaped to the atmosphere, iodine formed will be decreased. In addition, Chlorine gas is toxic and corrosive which will attack our eyes and lungs.

5.點解要馬上用硫化硫酸鈉溶液滴定?
Ans: Although, vapourization of iodine is slower than chlorine, but it is still vapourizing, so the solution must be titrated immediately.

6.為何要加入澱粉溶液?作用是什麼?
Ans: The end-point will be detected easily from blue to colourless, (better than yellow to colourless)