Chemistry 2 Nov/Dec 2011

Chemistry Paper 2,Nov/Dec 2011

Questions:		1	2	3	4	7	8		Main

General Comments
Weakness/Remedies
Strength

Question 4
(i) What is meant by saponification? (ii) List the raw materials needed for the manufacture of soap. (iii) Name the main by-product obtained from the manufacture of soap. What type of reaction is represented by each of the following equations? (i) C3H8 + Cl2 C3H7Cl + HCl (ii) n(CH2 ‗ CH2) (CH2 - CH2)n (iii) C16H34 3C2H4 + C10H22 (iv) C2H4 + H2 C2H6 (v) C2H5OH + H2SO4 C2H5HSO4 + H2O An organic compound U produces effervescence when heated with sodium metal liberating gas V. U produces a sweet fruity smelling liquid when warmed with colourless liquid W in the presence of a catalyst. W reacts with sodium hydrogen trioxocarbonate (IV) solution to produce gas G. Write the functional group present in U and W. Identify gases V and G. Name the type of reaction between: U and sodium metal; U and W. Name the catalyst used in the reaction between U and W. If the molar mass of W is 74g mol-l, deduce the: Molecular formula; Structural formula of the compound. [H = 1.00; C = 12.0, O = 16.0] Explain briefly why the reaction between magnesium and 1.0 mol dm-3 ethanoic acid would be slower than the reaction between magnesium and 1.0 mol dm_3 hydrochloric acid. Majority of the candidates could not give the complete explanation of saponification but are familiar with the raw materials and main by-product in the manufacture of soap. Part (b) was fairly handled by majority of the responding candidates. Most of the candidates were able to substitute into the formula Cn H2n+1 COOH and were able to arrive at the molecular formula which is quite commendable.
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OBSERVATION
Majority of the candidates gave correct shape of s-orbital and p-orbital but many of the candidates could not identify the correct ion with the composition of the particles stated in the question. A fair attempt was made at identifying ionic and covalent bonds in the given oxides based on their properties but quite a large number of the candidates could not explain the formation of the covalent bonding in Z2O. A fair understanding of equilibrium reaction was displayed by majority of the responding candidates except for the features of an equilibrium reaction that was poorly tackled. Majority of the candidates were able to calculate the empirical and molecular formulae. However, few candidates lost marks because they cannot resolve the fractions in the mole ratio of the atoms into whole number e.g. 1 : 1.5 : 1 should have been resolved to 2 : 3 : 2 however, majority of the candidates approximated it to 1 : 2 : 1 which was wrong. The expected answers are: 3(a) (i) I. s-orbital = spherical shaped accept drawing O II. p-orbital – dumb-bell shaped accept drawing shape of figure 8 (ii) Helium nucleus/ion/He2+/alpha particle. (b) (i) X ionic/electrostatic forces/electrovalent/coulombic forces Y covalent Z covalent (ii) Two Z atoms donate one electron each while an oxygen atom donates two electrons to form two shared pair of electrons. I. Y2O is hydrogen bonding/dipole-dipole. II. Z2O is vander waal forces (c) (i) All the species in the equilibrium are gaseous/in same physical state/phase (ii) I. Equilibrium position is unaffected. There is equal number of gaseous molecules/moles on either side of the equilibrium. Rate increases. Increasing the pressure decreases volume/increase the concentration (iii) - The forward reaction proceeds at the same rate as reverse reaction The concentration of reactants and products are constant Equilibrium can only be achieved at a given temperature Equilibrium can only be achieved in a closed vessel if there is a gaseous component. Na S O (i) 29.1 40.5 30.4 23.0 32.0 16.0 1.26 1.26 1.90 Divide by the smallest 1.26 1.26 1.90 1.26 1.26 1.90 1 1 1.5 x 2 2 2 3 Empirical formula = Na2S2O3 (ii) (Na2S2O3)n = 158 (23 x 2 + 32 x 2 + 16 x 3)n = 158 n = 158 = 1 158 Molecular formula = Na2S2O3