Background

Esterification and the reverse process, hydrolysis of an ester are the forward and back reactions of the ester equilibrium.

Esterification

CH₃COOH(l) + C₂H₅OH(l) ⇋ CH₃COOC₂H₅(l) + H₂O(l)

Hydrolysis of an ester

CH₃COOC₂H₅(l) + H₂O(l) ⇋ CH₃COOH(l) + C₂H₅OH(l)

The equilibrium constant is given by dividing the multiple of the concentrations of the products raised to the power ot their stoichiometric coefficients, by the multiple of the concentration of the reactants raised to the power of their stoichiometric coefficients.

For the equilibrium:

aA + bB ⇋ cC + dD

kc = [C]^c[D]^d/[A]^a[B]^b

Clearly, the actual value of kc depends on which way round the equilibrium is written. The value of kc for the forward reaction is the reciprocal of the value of kc for the back reaction.

kc_(forward) = 1/kc_(backward)

For the esterification and the hydrolysis of the ester a catalyst speeds up the attainment of the equilibrium condition, but does not affect the value(s) of the equilbrium constant. Dilute hydrochloric acid is used as the catalyst.

At room temperature the equilibrium is slow to establish, so the reaction mixtures should be prepared a couple of weeks before the analysis.

Analysis is a simple titration to find the total acid in the equilibrium mixture. The catalyst acid remains unchanged, while the hydrolysis reaction increases the total acid in the mixture due to the formation of ethanoic acid.

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Requirements

Chemicals

• Ethylethanoate
• Hydrochloric acid, 2.0 mol dm^-3
• Sodium hydroxide, 0.1 mol dm^-3
• Phenolphthalein indicator

Apparatus

• Test-tubes, bungs and rack
• Pipette, 10ml (x2)
• Volumetric flask, 250ml
• Burette
• Clamp, stand and boss
• White tile

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Procedure

• Three test tubes are sealed each containing 10cm³ of ethylethanoate and 10cm³ of hydrochloric acid.
• After standing for 2 weeks, the contents of the test tube are made up to 250 cm³ in a volumetric flask.
• 10cm³ aliquots of this solution are transferred to a conical flask and titrated against the sodium hydroxide solution using 5 drops of phenolphthalein indicator to the first permanent tinge of pink.
• Repeat the titration to concordant results.

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Safety

• Hydrochloric acid is corrosive

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