VJC Prelim P1

2  VJC 2015 9647/01/PRELIM/15 [Turn over Section A For each question there are four possible answers, A, B, C, and D. Choose the one you consider to be correct. 1 Which one of the following statements about 20.3 g of Co2(SO4)3 is incorrect? [Molar mass = 406 g mol-1, Avogadro’s constant = 6.0 x 1023 mol-1] A It contains 0.10 mol of Co3+ ions. B It contains 0.15 mol of SO4 2- ions. C It contains 47.3% of oxygen by mass. D It contains 1.5 x 1023 atoms. 2 In theory, 3 mol of NH3OH+(aq) is required to convert 1 mol of BrO3 –(aq) to Br–(aq). Which one of the following statements about this reaction is correct? A NH3OH+ is acting as an oxidising agent. B The change in oxidation number of nitrogen is +4. C N2O is the nitrogen–containing product. D The reaction is likely to have a high activation energy. 3 Which one of the following is a possible configuration of a stable M3+ ion in the ground state? A 1s2 2s2 2p3 B 1s2 2s2 2p6 3s2 3p1 C 1s2 2s2 2p6 3s2 3p6 3d2 D 1s2 2s2 2p6 3s2 3p6 3d2 4s2 4 The pressure rating of a car tire is typically given to be 210 kPa. This is based on an internal air temperature of 25C. A driver decides to inflate the tires of his car to the rated pressure after driving at a high speed for some time, during which the internal air temperature has risen to 40C. What will be the drop in pressure in each tire when the temperature has returned to 25C? A 9.3 kPa B 10.1 kPa C 78.8 kPa D 200 kPa

3  VJC 2015 9647/01/PRELIM/15 [Turn over 5 LiAlH4 and NaBH 4 are used as a source of hydride ion s, H –, in many organic reactions. Both chemical formulae consist of a metal cation and an anion made up of a group of atoms. Solid LiA lH4 decomposes upon heating to 150 C while solid NaBH 4 melts at a temperature of 400 C. Which one of the following statements about LiAlH4 and NaBH4 is correct? A The H – from LiA lH4 is more available for donation due to the higher polarity of the Al–H bond compared to B–H bond in NaBH4. B The release of H – from LiA lH4 and NaBH 4 involves homo lytic fi ssion of a covalent bond. C The lower stability of LiAlH4 to heat can be explained by the higher charge density of the cation and anion present compared to NaBH4. D The presence of a distinct melting point for NaBH 4 means that the covalent bonds in the anions are weaker than the ionic bonds between the cations and anions. 6 The enthalpy change of the following half equation (Δ Hr o) is determined using Hess’s Law: ½ I2(s) + e– I–(aq) I(g) + e– I–(g) Which one of the following shows the correct signs for the different enthalpy changes? ΔH1 o ΔH2 o ΔH3 o A + + – B + – – C + – + D – + + 7 Hydrogen peroxide is thermodynamically unstable and decomposes over time to form water and oxygen gas. H2O2(l)  H2O(l) + ½O2(g) The following data are provided:  O2(g) + 2H2(g)  2H2O(l) ΔH1 o = –572 kJ mol–1; ΔS1 o = –325 J mol–1 K–1  O2(g) + H2(g)  H2O2(l) ΔH2 o = –188 kJ mol–1; ΔS2 o = –225 J mol–1 K–1 What is the value of ΔGo, in kJ mol–1, for the decomposition of hydrogen peroxide? A –117 B –354 C –359 D –596 ΔHr o ΔH1 o ΔH2 o ΔH3 o

4  VJC 2015 9647/01/PRELIM/15 [Turn over 8 Artificial pacemakers are used to regulate the heartbeat of cardiac patients. The device is powered by a water–free lithium iodide battery as shown below. Adapted from UC Davis ChemWiki Which one of the following statements about the lithium iodide battery is correct? A The I2 complex cathode carries a negative charge. B The cell potential of the lithium iodide battery has a value of +3.5 8 V at the conditions prevailing in the battery. C The Ni mesh provides a medium for electrons to flow from Li(s) to I2 complex. D The pacemaker will be able to last for more than 5 years if it contains 6 g of reactive Li(s) and operates at an average of 0.8 mA. 9 A gaseous dimer, X, is introduced into an empty gas syringe which has a movable, tightly– fitting plunger. The gas is allowed to expand until equilibrium is reached at a controlled temperature at which 20 % of X dissociates into its monomer W. X(g) 2W(g) Which one of the following statements is correct? A The pressure inside the syringe at equilibrium will be higher than the atmospheric pressure. B The forward reaction is exothermic. C The value of the equilibrium constant, Kp is 0.167 atm D The dissociation of dimer X will be favoured when the plunger is pushed back into the equilibrium mixture. plunger gas self–sealing cap for introducing sample syringe oven for controlling equilibrium temperature (–) electrode (+) electrode LiI crystal Li(s) anode Ni(s) mesh I2 complex cathode

5  VJC 2015 9647/01/PRELIM/15 [Turn over 10 The table below gives the concentration and pH values of the aqueous solutions of two compounds S and T. Either compound could be a monoprotic acid or base. S T concentration 2 mol dm–3 2 mol dm–3 pH 2 11 Student P concluded that S(aq) is a strong acid. Student Q concluded that the extent of dissociation is lower in S(aq) than in T(aq). Student R concluded that no suitable indicator exists for the titration of S(aq) and T(aq). Which of the students are correct? A P and Q only C R only B Q and R only D none of the students 11 The reaction between Pb2+(aq) and different halides is investigated. When solid NaCl or Na I is added in increasing amounts to a solution of Pb 2+(aq) in separate experiments, the following graphs are obtained. Which one of the following deductions is correct? A The increase in both graphs after V cm3 is due to common ion effect. B The concentration of Pb2+(aq) for both graphs is at its minimum at V cm3. C The ionic product is higher than the solubility product for both salts between 0 and V cm3. D The precipitate that will be formed first if Pb 2+(aq) is added to a solution containing equal amounts of Cl–(aq) and I–(aq) is lead(II) chloride. amount of halide / mol total concentration of lead(II)–containing species in solution / mol dm–3 0 V Cl – I–

6  VJC 2015 9647/01/PRELIM/15 [Turn over 12 The energy profile of the reaction between M and N is shown below. Which one of the following is a correct statement? A The rate equation for the forward reaction is rate = k[M][N] B The activation energy for the reverse reaction is calculated by E1. C The equilibrium concentrations of M and N decrease when temperature decreases. D Only the rate of the forward reaction increases when temperature increases. MN2 progress of reaction energy M + 2N E1 MN + N E2

7  VJC 2015 9647/01/PRELIM/15 [Turn over 13 The graph below shows the ionic radius of five consecutive elements in the third period of the Periodic Table. Three corresponding graphs of the same elements are provided below, each with a point that is wrongly represented. Which of the following correctly identifies the error point in each of the graphs? melting point electrical conductivity 1st ionisation energy A X X W B X Y Y C Z X Y D Z Y W 14 Which one of the following statements of Group II elements (magnesium to bari