AJC_Prelims H2 Chem Paper 3_Answers

AJC H2 Chemistry Prelim 2008 - Paper 3 Suggested Answers 1 (a) - 3d and 4s electrons are of similar energies - lose different numbers of these electrons to form ions of similar stability (b) (i) Empirical formula of Y = MnO2 (ii) Z = MnO4 - Step II is a disproportionation reaction 3 MnO4 2- + 2H2O → 2 MnO4 - + MnO2 + 4OH- (iii) - aq CO2 is acidic / CO2 dissolves in water to form carbonic acid, - removes OH- as HCO3 - - Position of equilibrium shifts to the right, hence aids disproportionation (c) (i) Anode: Na → Na+ + e Cathode: MnO2 + H2O + e → MnO(OH) + OH- (ii) E•(MnO2/MnO(OH)) = - 1.71 V (d) (i) Since t½ is constant at about 1.5 mins, order of reaction w.r.t. H2O2 is 1. (iii) k = 0.478 min-1 (iv) k increases, because from the rate equation, if rate increases even when H 2O2 concentration remain the same, then value of k must also increase. (can explain using Arrhenius equation k = A e(-Ea/RT)) AJC/H2 Chemistry/Prelims2008/P3Answers 1

2 (a) (i) ∆Hsol = – 157 kJ mol–1 (ii) - The temperature of the water will increase. (iii) - ∆Hhyd of Ca2+ will be less exothermic than Cu2+. - From Data Booklet, ionic radius of Ca2+ is 0.099nm while Cu2+ is 0.069nm. - This is because Ca2+ has a larger ionic radius and thus smaller charge density than Cu2+. (b) (i) - In reaction I and II, 2 similar Cu–O bonds are broken and - 2 similar Cu–N bonds are formed . Thus, their ∆Hr { values are similar (ii) - In reaction I, one ethylenediamine replaces 2 H2O ligands. - There is an increase in the number of particles in aqueous solution and - thus increase in disorderness or ways of rearrangement. (iii) 1o r 1o r kJmol 5 . 43 = GΔ : II Reaction kJmol 9 . 60 = GΔ : I Reaction - - - ∆Gr { for reaction I is more negative than II. - Ligand exchange is more spontaneous in reaction I than II. - Thus, complex product for reaction I is more stable than II. (iv) - Ligand exchange with polydentate ligands and its analogous monodentate ligands gives similar ∆Hr { values due to formation of similar Cu−N bonds. - Polydentate ligands increases the disorderness of the system more compared to monodentate ligands. - Thus, the positive entropy is the factor which determines a more negative ∆Gr { value and formation of more stable polydentate complex. * mathematically, ∆H r { (reaction I) ≈ ∆Hr { (reaction II); Increase in disorderness ⇒ ∆Sr { (reaction I) > 0 JK–1mol–1 ∴∆Gr { (reaction I) more negative than ∆Gr { (reaction II) (c) (i) CC C N C N S SHH CC S C N C N SH H ci