Na_2CO_3对乙酸乙酯皂化反应速率常数测定结果的影响

Effect of Na_2CO_3 on the determination of the rate constant of saponification of ethyl acetate

氢氧化钠易吸收空气中的二氧化碳生成碳酸钠.如果配置的氢氧化钠溶液中含有碳酸钠,就会影响其浓度的准确标定,进而影响乙酸乙酯皂化反应速率常数的测定结果.通过探究反应物中碳酸钠对乙酸乙酯皂化反应速率常数的影响,初步探讨了该实验的改进方案.结果表明:随着碳酸钠浓度的不断增加,测得的反应速率常数不断增大;但当碳酸钠的浓度增大到一定浓度后,反应速率常数测定值变化趋于平缓;当碳酸钠的浓度超过0.010mol/L时,测得的反应速率常数数值急剧变小,这可能是由于实际氢氧化钠溶液浓度远低于标定值的原因造成的.建议:利用稀释饱和氢氧化钠溶液的方法配置实验所需的氢氧化钠溶液.该方法既省时,又可以尽量避免碳酸钠对实验结果的影响,适合在本科生物理化学实验教学中进行推广.

Sodium hydroxide,when exposed to the air,will react with the carbon dioxide,to form sodium carbonate.It is very hard to determine the exact concentration of the sodium hydroxide solution,which contains a small amount of sodium carbonate.Sodium carbonate in sodium hydroxide is one of the factors that affect the accuracy of the experimental results on the determination of the rate constant of saponification ethyl acetate.By investigating the influence of sodium carbonate on the rate constant of ethyl acetate saponification in the reactants,the improved scheme of the experiment was discussed.The results show that the measured reaction rate constant increases with the increase of sodium carbonate concentration.However,when the concentration of sodium carbonate increases to a certain concentration,the change of reaction rate constant value tends to be flat;when the concentration of sodium carbonate Above 0.010 mol/L,the measured value of the reaction rate constant sharply decreases,which may be due to the fact that the actual sodium hydroxide solution concentration is far below the calibration value.We suggest that the scheme of preparing the sodium hydroxide solution by dilution with the saturated sodium hydroxide solution can not only save time,but also avoid the effect of sodium carbonate on the experimental results.And this change is suitable for promotion in the teaching of physical chemistry experiments for undergraduates.