Hi, I would like to know the pseudo order with respect to ester in the hydrolysis of ester in basic medium. I did three experiments with different concentration of ester (0.1;0.15 and 0.2 M) , [OH-]= 0.001 M. The initial rate increases with the increase in [ester], but the ratio of the initial rate is not proportional to the [ester] ratio. I tried also to draw k vs [ester] but no linear relation, so is there any explanation for this and how I can determine the order with respect to ester? ( this exp for 3d year) May be because the reaction is not complete
If you have the initial rate of hydrolysis for each run, find the ratio of the rates between two runs. Divide the logarithm of this ratio by the logarithm of the ratio of the ester concentrations. if it's 1, it's first order; if it's 2, it's second order, etc.
Question, though- is the solution buffered? In a basic solution, you will not get base-catalyzed hydrolysis, but the reaction RCOOR' + OH- -> RCO2- + R'OH. If your hydroxide ion concentration isn't constant, that will completely ruin your results.
The ester is in large excess with respect to OH-, The pH of the solution varies with time I measured pH vs time to determine the order of the reaction. Ln Vi vs ln [ester] gives a linear relation with a slope 0.28, R2:0.95. For the order with respect to OH- no problem it is 1, but for ester , the predictive order is one but my results did not give me 1!
Hi Jamal,
you are following change in OH, it is okay you will be able to fined order of reaction with respect to OH.Do again your experiment and measure change in ester concentration and plot as you did for OH or then go to what Dr. George said.
Hi Salim,
Dr George asked me to do the find the relation b/w the ratio of the rate and the ration of [ester] . this is equivalent to plot ln V vs ln [ester]o. the slope which corresponds to the order is 0.28 ! so it is not 1 and not 2. but in the literature the order is 1 for ester. I can not measure the variation of [ester] vs time since it is in excess. my question how I will measure the change in ester concentration as you suggested
You can use UV spectrometer to measure absorbance of your solution .It is directly proportion with concentration.
Hi,
The exp is for chemistry students, so I am afraid that the students break the UV cell (which is expensive) during exp, ,another thing, perhaps, ester CH3COOC2H5 do not absorb UV above 200 nm. What I did I made exp with 0.1 M ester and 0.1 M NaOH, the order becomes 2 instead one so I can deduce the order with respect to ester (1 for ester and 1 for NaOH)
Thx in any way
Hi,
Just been doing the same reaction with students and the apparent order in ester was 0.5 (1 in hydroxide). An reproducibility was great.
Cheers,
Laurent.
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