Chemistry Question

The reaction progress is monitored by titrimetric analysis. At each 15-minute interval, 5.0 cm3 of the reaction mixture is transferred into a beaker and an excess fixed amount of standard hydrochloric acid is added to it.



a) What is the purpose of adding excess HCl to the reaction mixture? (2 marks)


Ensure all the NaOH is reacted by excess HCl and then the reaction stop immediately.

解釋:如果唔加HCl, CH3COOCH2CH3同 NaOH會繼續反應，甘[NaOH]或[CH3COOCH2CH3]就會下降，影響個結果。


b) Describe how to find the amount of NaOH present in the reaction mixture. (3 marks)


1. Find the volume of HCl added
2. Find the no. of mole of HCl added
3. Find the HCl remained ( HCl is in excess) by titration of standard NaOH solution. (逆滴定法)
4. Find the no. of mole of HCl used ( no. of mole of HCl added- no. of mole of HCl remained)
5. Find the no. of mole of NaOH ( =the no. of mole HCl used )



c) i) State one alternative way instead of adding excess HCl to serve the same purpose. (1 mark)


Adding larger amount of ice water (the name of mothod is quenching)

解釋: 低温使反應停止，CH3COOCH2CH3同 NaOH就唔會繼續反應





最後個條比我諗諗=]

2014-03-07 16:00:51 補充：
ii) State one disadvantage of this alternative way.

Adding cold water just slow down the reaction, (maybe few of reactant have enough energy to overcome the energy barrier and react),while adding excess HCl can react the NaOH completely, so that it can prevent the further hydrolysis of the ester.