Tuesday, October 1, 2019
Chemistry Questionnaire Essay
What is meant by the term cis-trans isomerism? How many cis-trans isomers are present in Structure I? Draw any 2 possible cis-trans isomers. (iii) State the physical state of this lipid at room temperature. Explain your answer.  (b) Suggest a possible identity for each of the organic compounds A-E below. Explain your reasoning and write balanced equations for the reactions involved where possible. i) A, C11H14O2 is a neutral compound. Hydrolysis of A gives B, C7H6O2 which is a white precipitate and C, C4H10O which does not decolourise acidified potassium manganate(VII) on heating. (ii) D, C4H10O2 reacts with 2 moles of sodium metal, but not with solid sodium carbonate. On boiling under reflux with aqueous potassium manganate(VII), D gives a compound, E, C4H6O3. E reacts with alkaline aqueous iodine to give a yellow precipitate and evolves carbon dioxide when solid sodium carbonate is added. Draw structural formulae of the products of the reactions between glutamic acid and I aqueous NaOH II aqueous HCl (ii) Draw the structural formula of a dipeptide formed between glutamic acid and phenylalanine, showing the ionic form in which it would exist at pH 10.  [Total: 20] 5 3 (a) Iron(III) chloride is a dark brown solid which dissolves in water to give an acidic solution. This solution is used to etch, i. e. dissolve, unwanted copper from printed circuit boards. When a copper-coated printed circuit board is immersed in aqueous iron(III) chloride, the solution turns greenish blue. i) Explain, with the aid of an equation, why aqueous iron(III) chloride is acidic. (ii) Use the Data Booklet to explain why iron(III) chloride etches copper. Give a relevant equation to explain the greenish blue solution observed. (iii) Suggest what happens to the overall E0 value if the thickness of the copper coating is increased. Anhydrous aluminium chloride behaves similarly as anhydrous iron(III) chloride in that they sublime readily when heated. When 1. 56 g of aluminium chloride was placed in an evacuated vessel of volume 1. 00 dm3 at 1800C, the pressure produced was 32. kPa. (i) Calculate the apparent relative molecular mass of the aluminium chloride under the stated conditions. (ii) In the vapour phase, aluminium chloride partially exists as a dimer. Using your answer obtained for b(i), calculate the percentage of the dimer under the stated conditions. (iii) Suggest a reason why the dimerisation process of aluminium chloride is exothermic. 6 (iv) Predict, giving a reason in each case, the effect on the proportion of dimer in the equilibrium mixture if I the pressure is lowered, II the temperature is lowered. MgO(s) + CO2(g) ?H? = +117 kJ mol-1; ?S? = +175 J mol-1 K-1 8 (i) Why does the entropy increase when magnesium carbonate decomposes? (ii) Calculate the standard Gibbs free energy change, ? G? , for the decomposition of magnesium carbonate. Comment on the spontaneity of the decomposition of magnesium carbonate at room temperature. (iii) Assuming that ? H? and ? S? for the reaction are independent of temperature, calculate the temperature at which the decomposition reaction becomes feasible. (iv) Compare the ease of thermal decomposition of magnesium carbonate with barium carbonate.  Turn over 4 (d) Suggest a simple chemical test to distinguish between each pair of compounds. For each test, give reagents and conditions, and state what would be seen with each compound. The dissociation reactions and Ka values of carbonic acid are given below. H2CO3 HCO3- + H+ Ka1 = 4. 2 x 10-7 mol dm-3 HCO3- CO32 Ã¢â¬â + H+ Ka2 = 4. 8 x 10-11 mol dm-3 0. 10 mol dm-3 aqueous NaOH is added slowly to 20. 0 cm3 of carbonic acid and the change in pH is followed and shown below. pH x x B x A x x 10 20 30 40 10 Vol of aqueous NaOH added / cm3 (i) Calculate the initial pH of the carbonic acid, stating any assumptions you make. (ii) Calculate the pH at the first equivalence point, A. (iii) What is the pH at point B? Give a reason for your answer.  [Turn over 5 (b) Maintenance of pH is vital to the cells of all living organisms because enzyme activity is influenced by pH. For instance, protection from harmful pH change in the blood plasma is provided by the hydrogencarbonate/carbonic acid system (HCO3-/H2CO3). At 370C, the temperature of the human body, pH of blood plasma is 7. 4. i) Explain, with relevant equations, how the hydrogencarbonate/carbonic acid system regul ates the pH of blood plasma. (ii) Calculate the ratio of [HCO3-]/[H2CO3] for maintaining the optimum pH of blood plasma.  (c) Describe two differences in physical properties between transition metals and a s-block metal such as magnesium, suggesting reasons for these differences based on their atomic structures.  11 (d) The oxidation of manganese(IV) oxide in alkaline solution in air produces green manganate(VI) ion, MnO42-. When manganate(VI) is added to a dilute acid, a purple solution and dark brown solid manganese(IV) oxide reforms.