Saponification reaction of ethyl acetate

1. Purpose of experiment
1. To master the principle and method of determining the rate constant of the saponification reaction of ethyl acetate by conductance method.
2. To understand the characteristics of the second-order reaction and learn to obtain the rate constant of the second-order reaction by graphic method.
3. To be familiar with the use of conductivity meter.

Second, experimental principle
The saponification of ethyl acetate is a typical second-order reaction, and its reaction formula is:
\ (CH_ {3} COOC_ {2} H_ {5} + NaOH\ rightarrow CH_ {3} COONa + C_ {2} H_ {5} OH\)

During the reaction, the concentration of each substance changes with time. If the initial concentration of ethyl acetate is\ (a\) and the initial concentration of sodium hydroxide is\ (a\), and the reaction time is\ (t\), the concentration of\ (CH_ {3} COONa\) and\ (C_ {2} H_ {5} OH\) generated by the reaction is\ (x\), then the concentration of\ (CH_ {3} COOC_ {2} H_ {5}\) and\ (NaOH\) is\ (a - x\). The rate equation for this reaction is:
\ (\ frac {dx} {dt} = k (a - x) (a - x) \)

where\ (k\) is the reaction rate constant. Integrating the above equation yields:
\ (k =\ frac {1} {t}\ cdot\ frac {x} {a (a - x) }\)

In order to determine the value of\ (x\) at\ (t\) time, the conductivity method is used in this experiment. In this experiment, since\ (NaOH\) and\ (CH_ {3} COONa\) are strong electrolytes, while\ (CH_ {3} COOC_ {2} H_ {5}\) and\ (C_ {2} H_ {5} OH\) do not have obvious conductivity, the conductivity change of the reaction system is mainly determined by the concentration change of\ (OH ^ {-}\) and\ (CH_ {3} COO ^ {-}\).

Let\ (G_ {0}\) be the conductance of the solution at the beginning of the reaction,\ (G_ {t}\) be the conductance of the solution at the reaction time\ (t\), and\ (G_ {\ infty}\) be the conductance of the solution at the end of the reaction. Since the conductivity of the solution is proportional to the concentration of the electrolyte, and the conductivity of\ (OH ^ {-}\) is much greater than that of\ (CH_ {3} COO ^ {-}\), there are:
\ (G_ {0} = A_ {1} a\)
\ (G_ {t} = A_ {1} (a - x) + A_ {2} x\)
\ (G_ {\ infty} = A_ {2} a\)
where\ (A_ {1}\),\ (A_ {2}\) are proportionality constants related to temperature, solvent, and electrolyte properties. From the above three formulas:
\ (x =\ frac {G_ {0} - G_ {t}} {G_ {0} - G_ {\ infty}} a\)

Substitute\ (x\) into the rate equation integral formula to obtain:
\ (k =\ frac {1} {t\ cdot a}\ cdot\ frac {G_ {0} - G_ {t}} {G_ {t} - G_ {\ infty}}\)

It can be seen that as long as\ (G_ {0}\),\ (G_ {t}\) and\ (G_ {\ infty}\) are determined, and\ (\ frac {G_ {0} - G_ {t}} {G_ {t} - G_ {\ infty}}\) plotted against\ (t\), if the result is a straight line, it is proved that the reaction is a second-order reaction, the slope of the straight line\ (m =\ frac {1} {a\ cdot k}\), so that the reaction rate constant\ (k\) can be obtained.

III. Experimental instruments and reagents
1. ** Instrument **
- 1 conductivity meter
- 1 set of constant temperature water bath
- 1 double tube conductivity cell
- Pipette (25mL) 3
- Large test tube (50mL) 4
- 1 stopwatch
2. ** Reagent **
-\ (0.0200mol/L\) \ (NaOH\) solution
-\ (0.0200mol/L\) ethyl acetate solution (to be temporarily prepared)

Fourth, experimental steps
1. ** Adjust the thermostatic water bath temperature **
Adjust the thermostatic water bath temperature to\ ((25.0\ pm0.1) ^ {\ circ} C\).
2. Determination of **\ (G_ {0}\) **
Accurately absorb\ (25mL\) \ (0.0200mol/L\) \ (NaOH\) solution with a pipette and inject it into a large dry test tube, put it in a constant temperature water bath at constant temperature\ (10min\). Then rinse the conductivity electrode with distilled water, suck it dry with filter paper, insert it into the large test tube, and measure its conductivity\ (G_ {0}\).
3. **\ (G_ {t}\) Determination **
Accurately absorb\ (25mL\) \ (0.0200mol/L\) \ (NaOH\) solution and\ (25mL\) \ (0.0200mol/L\) ethyl acetate solution with a pipette, inject it into another large dry test tube, put the test tube into a constant temperature water bath\ (10min\). Quickly pour the ethyl acetate solution into the\ (NaOH\) solution, start the stopwatch at the same time, after mixing evenly, immediately insert the conductance electrode into the mixed solution and start measuring the conductivity of the solution\ (G_ {t}\). After the reaction starts, the conductance values of\ (2min\),\ (4min\),\ (6min\),\ (8min\),\ (10min\),\ (12min\),\ (15min\),\ (20min\),\ (25min\),\ (30min\) are measured once.
4. ** G_ {\ infty}\) Determination **
The solution after the reaction in step 3 is placed in a constant temperature water bath\ (24h\), or heated in a water bath of\ (50 - 60 ^ {\ circ} C\) for half an hour, and then cooled to the experimental temperature to determine its conductivity\ (G_ {\ infty}\).

V. Data recording and processing
1. ** Data Record **
Record the experimental temperature,\ (G_ {0}\),\ (G_ {t}\) and the corresponding time\ (t\), fill in the data in the following table:

|\ (t/min \) | \( G_ {t}/(\ mu S/cm )\) | \(\ frac {G_ {0} - G_ {t}} {G_ {t} - G_ {\ inf t y }}\) |
| ---- | ---- | ---- |
| 2 | | |
| 4 | | |
| 6 | | |
| 8 | | |
| 1 0 | | |
| 1 2 | | |
| 15 | | |
| 2 0 | | |
| 2 5 | | |
| 3 0 | | |

2. ** Data Processing **
- Plot with\ (\ frac {G_ {0} - G_ {t}} {G_ {t} - G_ {\ infty}}\) for\ (t\) and find the slope of the line\ (m\).
- According to\ (m =\ frac {1} {a\ cdot k}\), calculate the reaction rate constant\ (k\), where\ (a = 0.0100mol/L\) (because the total volume is doubled after mixing, the concentration is halved).

6. Precautions
1. The ethyl acetate solution needs to be prepared temporarily. When preparing, it should be prevented from volatilization loss and preheated at constant temperature.
2. The conductivity electrode should be rinsed with distilled water before and after use, and dried with filter paper to avoid affecting the measurement results.
3. When mixing the ethyl acetate solution and\ (NaOH\) solution, it should be done quickly, and the timing should be started immediately after mixing.
4. When measuring\ (G_ {\ infty}\), the heating temperature should not be too high, and the time should not be too long, so as not to cause side reactions and affect the measurement results.

7. Question for Thought
1. Why should this experiment be carried out under constant temperature conditions, and the ethyl acetate and\ (NaOH\) solution should be pre-heated before mixing?
2. If\ (NaOH\) and ethyl acetate solutions are concentrated solutions, can the\ (k\) value be calculated by this method?
3. In this experiment, if the initial concentrations of the reactants\ (NaOH\) and ethyl acetate are not equal, how should the\ (k\) value be calculated?