Doc Brown's Advanced A Level Chemistry

Advanced A Level Chemistry

Kinetics-Rates Part 7

Selected Case Studies of a variety of chemical reactions and their rate expressions

7.5 Ester hydrolysis

RCOOR'(aq) + H2O/OH(aq) ==> RCOOH/RCOO(aq) + R'OH(aq)

Advanced A Level Kinetics Index

Case study 4.5 Ester hydrolysis

Explanation and derivation of orders of reactants and how to write the rate expression

  • Esters can be hydrolysed by (i) water alone (but very slow), but is (II) catalysed by acids (H+(aq) specifically) and (iii) alkalis (OH(aq) specifically). The latter hydrolysis (iii), is sometimes called a saponification reaction.

    • equation for (i)/(ii): RCOOR'(aq) + H2O(l) doc b RCOOH(aq) + R'OH(aq)

    • equation for (iii): RCOOR'(aq) + OH(aq) ====> RCOO(aq) + R'OH(aq)

  • Hydrolysis with water alone is usually too slow to obtain meaningful rate data.

  • However, with a fixed amount of acid catalyst e.g. HCl(aq), it is possible to follow the reaction by alkali titration and show that ...

    • rate = k1[RCOOR'(aq)], that is 1st order with respect to ester, however,

    • if the concentration of acid catalyst is varied, things are more complicated,

    • and the 2nd order expression, rate = k2[RCOOR'(aq)][H+(aq)] then applies.

  • With alkali, 2nd order kinetics are observed overall, and the reaction can be followed by titrating the remaining alkali with acid. Phenolphthalein (Pk(ind) = 9.3), would be a suitable indicator because one of the reaction products is the salt of a weak carboxylic acid (approx. pH 9). The product of the titration is neutral sodium chloride.

    • i.e. rate = k2[RCOOR'(aq)][OH(aq)], 1st order with respect to each reactant, 2nd order overall.

Advanced A Level Kinetics Index

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre–university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses) case studies in kinetics: orders of reaction and rate expressions for ester hydrolysis

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