ACJC 2022 Prelim P3 Answers

© ACJC2022 9729/03/Prelim/2022 ACJC solutions for H2 Chemistry Prelim Paper 3 2022 Section A 1 (a) Explain what is meant by the term Lewis acid. [1] Lewis acid is a substance that accepts an electron pair from a base via a dative covalent bond (i.e. an electron-pair acceptor) Examiner’s comments:  Do not be confused with the Bronsted-Lowry or Arrhenius’ definition of acids. (b) The water molecule can react in various ways: as an acid, as a base, as a nucleophile, as an oxidising agent and as a reducing agent. Study the following reactions and decide in which way water is reacting in each case. Explain your answers fully. [5] (i) 2F2 + 2H2O  4HF + O2 Reducing agent as itself is being oxidised. The oxidation number of O increases from −2 in H2O to 0 in O2. Or water reduces F2 to HF as the oxidation number of F decreases from 0 in F2 to –1 in HF. Examiner’s comments:  Change in oxidation number and relevant substance must be clearly stated as question stated to explain your answers fully. (ii) CH3COCl + H2O  CH3COOH + HCl Nucleophile. The carbon of the acyl chloride functional group is electron deficient and is susceptible to nucleophilic attack by water, which has a lone pair of electrons on its oxygen atom. Examiner’s comments:  Your answer should clearly reflect the definition of a nucleophile.  Base is not acceptable as a base involve donation of elec tron pair but without the expulsion of any groups. In this case, Cl is expelled and replaced with OH hence the reaction is a nucleophilic substitution rather than acid base. (iii) H2PO4− + H2O HPO42− + H3O+ Base. H2O accepts a proton from H2PO4− to form H3O+ and HPO42−. Examiner’s comments:  Your answer should clearly reflect the definition of a base, i.e. mention of the accepting of proton.

© ACJC2022 9729/03/Prelim/2022 (iv) Li + H2O  LiOH + ½ H2 Oxidising agent. Water oxidises Li to Li + as the oxidation number of Li increases from 0 in Li to +1 in LiOH Examiner’s comments:  Change in oxidation number and relevant substance must be clearly stated as question stated to explain your answers fully. (v) NO2− + H2O HNO2 + OH− Acid. H2O donates a proton to NO2− to form OH− and HNO2. Examiner’s comments:  Your answer should reflect the definition of an acid, i.e. proton donor. (c) The pKa values of three acids are listed in the Table 1.1 below: Table 1.1 acid formula pKa 1 CH3CH2