SAJC H2 CHEM P1 WORKED SOLUTIONS Prelim

1 [Turn Over 1. A solid hydrocarbon was completely combusted in a closed vessel at 120 oC. The residual gas had a volume of 64 cm 3, which decreased by 24 cm 3 after bubbling through a dehydrating agent. After this, 40% of the final gas volume consisted of oxygen. What is the empirical formula of the hydrocarbon? A CH B CH2 C CH3 D C2H3 Vol of H2O = 24 cm3 Vol of CO2 = 0.6 x (64 – 24) = 24 cm3 Mole ratio of CO2 : H2O = 24 : 24 = 1 : 1 Mole ratio of C : H in CxHy = 1 : 2 2. When cobalt metal is reacted with a solution containing cobalt( III) ions, cobalt( II) ions are formed. How many moles of Co and Co 3+(aq) would result in a mixture containing both cobalt(II) and cobalt(III) ions in the mole ratio of 3:1 after the reaction had taken place? Moles of Co Moles of Co3+ A 1 2 B 1 3 C 1 5 D 2 3 Co + 2Co3+  3Co2+ I 1 3 0 (Co3+ was in excess) C -1 -2 +3 F 0 1 3

2 [Turn Over 3. Which graph does not share the same general shape with the other three graphs according to the ideal gas law for a fixed mass of gas? A pV against p (at constant T) pV = nRT At constant T, pV = constant B V/T against T (at constant p) pV = nRT At constant p, = constant C p against V (at constant T) pV = nRT At constant T, D pV against V (at constant T) pV = nRT At constant T, V/T T V

3 [Turn Over 4. Which element will possess an empty s orbital after forming a singly-charged cation? A Barium Ba+: [Xe]6s1 B Vanadium V+: [Ar]3d34s1 C Copper Cu+: [Ar]3d10 D Gallium Ga+: [Ar]4s2 5. Which of the following reactions will form a product with the same shape as the reactant with respect to the underlined element? A 2AlCl3  Al2Cl6 Trigonal planar to Tetrahedral B 2H2O2  2H2O + O2 Bent to Linear C PCl3 + Cl2  PCl5 Trigonal pyramidal to Trigonal bipyramidal D CH3CH2CHO + [O]  CH3CH2COOH Trigonal planar to Trigonal planar 6. Which of the following is arranged correctly in the order of decreasing melting points? A Sulfur > Chlorine > Argon S8 has the largest electron cloud, followed by C l2 and Ar, hence it has the most extentive id-id interactions. B Carbon dioxide > Silicon dioxide > Germanium dioxide Silicon dioxide (giant covalent structure) should have a higher m.p than carbon dioxide (simple covalent structure). C Beryllium chloride > Magnesium chloride > Calcium chloride Beryllium chloride (simple c ovalent structure due to high charge density of Be 2+) should have a lower m.p than magnesium chloride. D Sodium > Magnesium > Aluminium Al has the largest no. of delocalised electrons, followed by Mg and Na, hence it has the strongest metallic bonding.

4 [Turn Over 7. A student dissolved 0.238 mol of sodium fluoride in 300 cm3 of water. Given the following data: Lattice energy of NaF −918 kJ mol −1 Enthalpy change of hydration of F − −457 kJ mol −1 Enthalpy change of hydration of Na + −390 kJ mol −1 What would be the initial temperature of the water if the final temperature of the solution is 20.00 o C? Assume that the specific heat capacity of sodium fluoride solution is 4.18 J g −1 K −1 . A 6.52 o C B 13.48 o C C 20.00 o C D 33.48 o C = – 457 – 390 – (–918) = +71 kJ mol-1 (endothermic) Heat absorbed, q = 71 x 0.238 = 16.898 kJ T = - = - = - 13.48oC Initial T = 20 + 13.48 = 33.48oC

5 [Turn Over 8. Ammonia gas and hydrogen chloride gas react to form ammonium chloride as shown in the equation below: NH3 (g) + HCl (g) → NH4Cl (s) ΔHo = −176 kJ mol−1 The standard entropy change of this reaction is −284 J K−1 mol−1. Which of the following statements is not correct? A At room temperature, ΔGo = +84.4 kJ mol−1. ΔGo = ΔHo - TΔSo = - 176 – 298(-0.284) = -91.4 kJ mol−1 B The reactants are less stable than the product. C The reaction is spontaneous at low temperatures and non -spontaneous at high temperatures. At low T, | | | |, since both and are negative, G < 0. D There is a decrease in the degree of disorderliness. There is a decrease in no. of gaseous moles from 2 to 0.

6 [Turn Over 9. The following experimental results are obtained for a reaction with the general rate equation of: rate = k [M]x [N]y [L] Experiment [M] / mol dm-3 [N] / mol dm-3 [L] / mol dm-3 Relative rate 1 0.1 0.2 0.3 1 2 0.2 0.6 1.2 72 Which of the following are possible values of x and y? x y A 1 1 B 2 1 C 1 2 D 2 2 From expt 1 to 2, [M] increases by 2 times, [N] increases by 3 times and [L] increases by 4 times, rate increases by 72 times. Order of reaction wrt [L] is 1. A: 21 x 31 x 41 = 24 B: 22 x 31 x 41 = 48 C: 21 x 32 x 41 = 72 D: 22 x 32 x 41 = 144

7 [Turn Over 10. Which of the following will decrease the rate constant for a reaction? A Decrease in concentration B Decrease in temperature C Decrease in pressure D Addition of catalyst From Kinetics lecture notes, Change Effect on rate constant, k Effect on EA Increase in concentration No effect No effect Increase in pressure (gaseous system) No effect No effect Increase in temperature Increased No effect Use of catalyst Increased Decreased

8 [Turn Over 11. Phosphorus trichloride and chlorine react according to the following equation: PCl3(g) + Cl2(g) PCl5(g) A mixture of PCl3 and Cl2 was placed in a syringe and brought to equilibrium at time t and the following graph was obtained. Which of the following accounts for the drop in [PCl3] at time t? A PCl5 was added to the mixture B Argon gas was added to the mixture C The syringe was heated D The plunger of the syringe was withdrawn until the volu me was twice the initial volume Volume increases, resulting in a n immediate drop in pressure. Equilibrium then shifts left to increase the pressure by favouring the backward reaction which produces less gaseous molecules. 12. Which of the following statements about phosphoric(V) acid is not correct? A HPO4 2- has a higher Kb value than H2PO4 -. Ka of H3PO4 > Ka of H2PO4 - > Ka of HPO4 2- Kb of H2PO4 - < Kb of HPO4 2- since Kb = Kw / Ka B PO4 3- can react as a base. In presence of acid, PO4 3- + H+  HPO4 2- C H2PO4 - can react both as an acid and as a base. In presence of acid, H2PO4 - + H+  H3PO4 In presence of base, H2PO4 - + OH-  HPO4 2- + H2O D H3PO4 has a higher pKa than HPO4 2-. Ka of H3PO4 > Ka of H2PO4 - > Ka of HPO4 2- pKa of H3PO4 < pKa of H2PO4 - < pKa of HPO4 2-

9 [Turn Over 13. 95 cm3 of M2+ ions is mixed with an an equal volume of X - ions at 25oC. Which of the following initial concentrations of M2+ and X- will result in the precipitation of MX2? (Ksp of MX2 = 7.67 x 10-13 mol3 dm-9) [M2+] / 10-4 mol dm-3 [X-] / 10-4 mol dm-3 A 1 1 I.P = [M2+][X-] = (0.5 x 10-4)(0.5 x 10-4)2 = 1.25 x 10-13 mol3 dm-9 < Ksp B 1 2 I.P = [M2+][X-] = (0.5 x 10-4)(1 x 10-4)2 = 5 x 10-13 mol3 dm-9 < Ksp C 2 1 I.P = [M2+][X-] = (1 x 10-4)(0.5 x 10-4)2 = 2.5 x 10-12 mol3 dm-9 < Ksp D 2 2 I.P = [M2+][X-] = (1 x 10-4)(1 x 10-4)2 = 1 x 10-12 mol3 dm-9 > Ksp Precipitation occurs 14. Use of the Data Booklet is relevant