Determining an Equilibrium Constant

A-Level: Chemistry

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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 1A 1B 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 ─ ─ ─ 1.0 3.0
Total volume of mixture /cm3 5.0 10.0
4.> Record the total mass before and after each addition of chemical into Table 3.
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 NaOH(aq) 1.0335M
Reagent bottle's number 1A 1B 2 3 4
Final burette reading /cm3 16.10 26.25 43.00 36.10 32.15
Initial burette reading /cm3 6.10 16.15 5.00 1.30 6.10
Volume of NaOH(aq) added /cm3 10.00 10.10 38.00 34.80 26.05
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Calculation & Results
Below, it shows how the values in individual items in Table 3 can be obtained:
1.> Since HCl(aq) and NaOH(aq) react in equimolar amounts, so,

2.> Total amount of acid at equilibrium in bottles 2,3 & 4
= no. of moles of HCl(aq) added + CH3COOH(l) formed
= concentration of NaOH(aq) x volume of NaOH(aq) added
3.> Equilibrium amount of CH3COOH(l)
= total amount of acid at equilibrium - amount of HCl(aq) added
= the value obtained in (2) - that in (1)
4.> Equilibrium amount of CH3CH2OH(l)
= equilibrium amount of CH3COOH(l)
= the value obtained in (3).
5.> Mass of pure HCl in the mixture
= amount of HCl(l) x molar mass of HCl (36.5 gmol-1)
= the value obtained in (1) x 36.5
6.> Mass of water in HCl(aq)
= mass of HCl(aq) added - mass of pure HCl
= mass of HCl(aq) added - value obtained in (5)
7.> Initial amount of CH3COOCH2CH3(l)


8.> Equilibrium amount of CH3COOCH2CH3(l)
= initial amount of CH3COOCH2CH3(l) - equilibrium amount of CH3COOH(l)
= value obtained in (7) - value obtained in (3)
9.> Initial amount of H2O(l)


10.> Equilibrium amount of H2O(l)
= initial amount of H2O(l) - equilibrium amount of CH3COOH(l)
= value obtained in (9) - value obtained in (3)
11.> Equilibrium constant, Kc


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