Determining an Equilibrium Constant

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Determining an Equilibrium Constant

Objective

To find out the equilibrium constant, Kc, for the reaction below, using acid hydrolysis:

Principle

Although the hydrolysis of ethyl ethanoate is very slow, by using dilute hydrochloric acid as catalyst, the above equilibrium can be attained in 48 hours. After 48 hours, the reaction mixture can then be titrated with standard sodium hydroxide solution. Finally, the equilibrium concentrations of four components below and hence, Kc of hydrolysis of CH3COOCH2CH3 can then be calculated:

Chemicals

2M HCl, 1.0335M NaOH, ethyl ethanoate, phenolphthalein indicator

Apparatus

5 small reagent bottles, 5ml pipette & filler, burette, measuring cylinder, conical flask, electronic balance, stand, white tile

Procedure -- Preparation, allow reaching equilibrium position

1.> Label 5 reagent bottles & their stoppers as 1A, 1B, 2, 3 and 4.

2.> Weigh each reagent bottles with their stoppers and record their corresponding masses in Table 3.

3.> Pipette 2M HCl into each bottles, using measuring cylinders to transfer ethyl ethanoate into bottles 2,3,4 and water into bottles 3,4 according to the

amount shown in Table 1.

Table 1 shows the amount of chemicals added:

Reagent bottle's number

A

B

2

3

4

Volume of HCl(aq) added /cm3

5.0

5.0

5.0

5.0

5.0

Volume of CH3COOCH2CH3(l) added /cm3

-

-

5.0

4.0

2.0

Volume of H2O(l) added /cm3

-

-

-

.0

3.0

Total volume of mixture /cm3

5.0

0.0

4.> Record the total mass before and after each addition of chemical into Table 3.
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5.> Set them aside for at least 48 hours and shake the bottles occasionally.

Procedure -- Titration (48 hours later)

1.> Rinse and fill a burette with standardized NaOH solution.

2.> Pour the contents of bottle 1A into a conical flask and rinse the bottle with deionized water.

3.> Add 2 - 3 drops of phenolphthalein indicator and titrate the mixture with standardized NaOH solution.

4.> Repeat steps 2 - 3 for other regent bottles. The results are recorded in Table 2.

Table 2 shows the results of titration:

Concentration of ...

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