Chem Quenching problem

2012-02-01 2:34 am

Let me write something from the book about quenching.

Consider the alkaline hydrolysis of ethyl ethanoate:
CH3COOCH2CH3(l) + NaOH → CH3COONa + CH3CH2OH

Ethyl ethanoate and dilute sodium hydroxide solution are first mixed. At regular time intervals, a small portion (let say 10cm^3) of the reaction mixture is withdrawn using pipette. The reaction in the small portion is quenched by pouring it into ice-cold distilled water.

The quenched portion is then titrated against standard HCl, using phenolphthalein as indicator. The amount of unreacted NaOH in the small portion can be determined.

My question concerning quenching:
How to determine the unreacted sodium hydroxide in the small portion?
係咪因為我地知道用左幾多standard HCl ，所以可以計到出黎？
但係那個reaction mixture 裡面除了 NaOH外，仲應該會有 CH3COOCH2CH3,CH3COONa ,CH3CH2OH 。咁我地個titration咪唔準囉？

回答 (1)

知識就是力量

2012-02-01 3:28 am

✔ 最佳答案

1. How to determine the unreacted sodium hydroxide in the small portion?
After titration, you know that the volume of HCl reacted (in average account and ignore the trial). Also, it is standardized. With known molarity, the no. of moles of NaOH can be easily calculated.

2. 有 CH3COOCH2CH3,CH3COONa ,CH3CH2OH 。咁我地個titration咪唔準囉？
In fact, HCl does not react with CH3COOCH2CH3, CH3COONa, CH3CH2OH. So, in titration, only NaOH reacts with HCl.

2012-01-31 19:44:08 補充：
2. Note that the reaction between NaOH and CH3COOCH2CH3 is very slow under low temperature (after quenching). They will not react with each other in titration.

參考: Knowledge is power.

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