DHS_H2_CHEM_P3_ANSWERS Prelim

© DHS 2011 9647/03/Answers 1 Dunman High School 2011 Year 6 H2 Chemistry Preliminary Exam – Paper 3 Answers 1 Iron is the cheapest and one of the most abundant of all metals, comprising nearly 5.6% of the earth's crust and nearly the earth’s entire core. It exists in a wide range of oxidation states, from −2 to +6, although ferrous (Fe 2+) and ferric (Fe 3+) compounds are more common. (a) Acidic solutions containing ferrous ions are oxidised to ferric ions in air, with no precipitation seen. On the other hand, ferrous ions give a precipitate in alkaline solutions and the precipitate turns reddish–brown in air. With reference to the Data Booklet , explain the two reactions using relevant E values, writing equations where appropriate. [4] In acidic solution O2 + 4H+ + 4e 2H2O Eo = +1.23 V Fe3+ + e Fe2+ Eo = +0.77 V 4Fe2+ + O2 + 4H+  4Fe3+ + 2H2O Eo cell = +1.23 – (+0.77) = + 0.46 V In alkaline solution Fe2+ (aq) + OH– (aq)  Fe(OH)2 (s) OR Ferrous ion react with hydroxide ions to form the ppt, Fe(OH)2. O2 + 2H2O + 4e 4OH– Eo = +0.40 V Fe(OH)3 + e Fe(OH)2 + OH– Eo = –0.56 V Eo cell = +0.40 – (–0.56) = + 0.96 V 4Fe(OH)2 + O2 + 2H2O  4Fe(OH)3 OR The ppt will be oxidised in air to reddish–brown Fe(OH)3. (b) Ferric ions can catalyse the reaction between I–(aq) and S2O8 2–(aq). By considering relevant E values, describe and explain the role of the ferric ions in this reaction, writing equations where appropriate. Fe3+ acts as a catalyst and reacts with I–. Fe 3+ + e Fe2+ Eo = +0.77 V I2 + 2e 2I– Eo = +0.54 V 2I– (aq) + 2 Fe3+ (aq)  I2 (aq) + 2Fe2+ (aq) E cell = +0.77 – (+ 0.54) = + 0.23 V >0. Reaction is feasible. Fe2+ intermediate reacts with S2O8 2–. Fe3+ catalyst is regenerated.

© DHS 2011 9647/03/Answers 2 Fe3+ + e Fe2+ Eo = +0.77 V S2O8 2– + 2e 2SO4 – Eo = +2.01 V S2O8 2– (aq) + 2Fe2+(aq)  2SO4 2– (aq) + 2Fe3+(aq) E cell = +2.01 – (+ 0.77) = + 1.24 V >0. Reaction is feasible. [3] (c) Ferric chloride is an industrial scale commodity inorganic compound which is often used as catalyst in organic synthesis. One example is its use as a Lewis acid for catalysing the alkylation reaction of benzene by chloroethane to form phenylethane. This reaction is similar to the reaction between benzene and chlorine. (i) Write a balanced equation for the overall reaction of chloroethane and benzene. (ii) State and outline the mechanism, with equations only, for the above reaction using ferric chloride as a catalyst. Electrophilic substitution. unstable cyclic carbocation (iii) Hence, or otherwise, suggest how you w ould synt