1990 AL Chemistry paper II marking scheme

Transcription

1 1990 AL Chemistry paper II marking scheme 1.(a)(i) According to the equation: A 2 (g) + 3B 2 (g) 2AB 3 (g) 4 mol of AB 3 will consume 2 mol of A 2 & 6 mol of B 2. Therefore at equilibrium: [B 2 ] = (12-6) / 20 = 3 / 10 mol dm -3 [A 2 ] = (4-2) / 20 = 2 / 20 = 1 / 10 mol dm -3 [AB 3 ] = 4 / 20 = 2 / 10 mol dm -3 (ii) K = [AB 3 ] 2 / ([A 2 ][B 2 ] 3 ) = (0.2) 2 / [(0.1)(0.3) 3 ] = mol -2 dm 6 (iii) At new equilibrium: 9 mol of B 2 is present (3 mol of B 2 are consumed). Let V be the new volume [B 2 ] = 9 / V [1/2m] [A 2 ] = [(Original no. of mol.) - (1/3)(no. of mol. of B2 used)] / V = [4-(1/3)(3)] / V = 3 / V [1/2m] [AB 3 ] = 2 / V [1/2m] K = = (2 / V) 2 / [(3 / V)(9 / V) 3 ] = x 10-3 V 2 [1/2m] Therefore V = 90 dm 3 (b)(i) Reduction occurs at cathode Cr 2 O 2-7 (aq) + 14H + (aq) + 6e - 2Cr 3+ (aq) + 7H 2 O(l) Oxidation occurs at the anode Fe 2+ (aq) Fe 3+ (aq) + e - (ii) From anode to cathode (iii) e.m.f. = (+0.77) = 0.56 V (iv) Pt Fe 2+ (aq),fe 3+ (aq) H + (aq),cr 2 O 2-7 (aq),cr 3+ (aq) Pt (c) [Any 2 of the following factors, max. 2m per factor] Temperature - an increase in temperature increases the no. of particles having sufficient energy to produce fruitful collison, and also that the average kinetic energy will be increased. This will result in higher velocities & higher velocities hence more collisions per unit time. Concentration - an increase in concentration will give more particles per unit volume & hence more collisions per unit time. Catalyst - a catalyst that increase a reaction rate does so by providing new mechanism. If the activation energy of the rate determining step of the new mechanism is lower than the activation energy of the rate determining step of the uncatalyzed reaction, then the rate is increased. 2.(a)(i) Since the two gaseous atoms are at the same temperature, they have the same average kinetic energy. (ii) Since the two gaseous have the same kinetic energy, the magnesium atom, having much lower mass, must have higher velocities [1m + 1m] (iii) The no. of atoms leaving per second is determined by the no. of "collision" per second of gaseous atoms with the hole. The vapour pressure indicates the no. of molecules per unit volume, and the higher the vapour pressure, the more molecules that strike the surface per second. In addition, the higher the (relative) molecular mass, the slower the particles are moving at a given temperature. (Hence both the vapour pressure and molecular mass are important factors governing the no. of atoms leaving the container per second.) (b)(i) Ag 2 C 2 O 4 2Ag + + C 2 O 2-4 [1/2m] Ksp = [Ag + ] 2 [C 2 O 2-4 ] [1/2m] [Ag + ] = 2.10 x 10-5 M [C 2 O 2-4 ] = 2.50 x 10-2 M (The original amount of C 2 O 2-4 >> that used to form ppt. when the ppt. Ag 2 C 2 O 4 "first appeared") Ksp = (2.10 x 10-5 ) 2 (2.50 x 10-2 ) = 1.10 x mol 3 dm -9 value, unit [1/2m] (ii) When (red) Ag 2 CrO 4 appeared which means that the concentration of Ag + and CrO 2-4 satisfied the relationship Ksp = [Ag + ] 2 [CrO 2-4 ] [1/2m] Since Ag 2 CrO 4 2Ag + + CrO 2-4 [1/2m] [CrO 2-4 ] = 1.44 x 10-5 M [Ag + ] = (Ksp / [CrO 2-4 ]) 1/2 = [1.2 x / (1.44 x 10-5 )] 1/2 = 2.89 x 10-4 M Since the concentration of Ag + and C 2 O 2-4 are related to the Ksp of AgC 2 O AL CHEM P II - 1

2 [C 2 O 4 2- ] = Ksp / [Ag + ] 2 = 1.10 x / (2.89 x 10-4 ) 2 = 1.32 x 10-4 M Initial [C 2 O 4 2- ] = 2.5 x 10-2 M [C 2 O 4 2- ] after appearance of Ag 2 CrO 4 = 1.32 x 10-4 M Therefore no. of moles of Ag 2 C 2 O 4 precipitated in 1 dm 3 solution = 2.5 x x 10-4 = mol [1.5m] (c) Balmer series is a series of lines resulting from photons emitted or absorbed by the hydrogen atom with wavelength or frequency in the visible region. [1m + 1m] These spectral lines are the result of transition of the electron (not electrons) from one energy level to another with the lower energy level with quantum no. n1 = 2 (if emission) The wavelengths of this series satisfy the formula: 1 / λ = R H (1 / / n 2 2 ) R H : Rydberg constant; n 2 : 3,4,5..., λ: wavelength 3.(a) The modern method of determining relative masses of particles uses the mass spectrometer [3m] [1/2m for each of the following points] - The sample (an element) is bombarded by electrons to form positively charged ions. - The ions are accelerated by the electric field between plates A and B. - The ions travel along until deflected by magnetic field. - The magnetic field is adjusted so that ions of a particular mass are focused onto the ion detector. - The ion detector responds to mass-to-charge ration (m/e) (NOT mass!) of the ionized species. *[Most ions detected are +1] - Knowing the magnetic field strength, accelerating voltage and the radius of the path, the mass of the particle can be calculated. (b) Outside syllabus (c) Melting involves the overcoming of forces between the units of which the solid is made. Two of the given solids: HF & F 2 would be expected to form molecular solids; the other, NaF would be expected to form ionic solid. The coulombic forces holding an ionic solid together are strong, hence NaF would be expected to have high melting point. The attraction between F 2 molecules is quite weak and due only to van der Waal's forces would be expected to have a low melting point. Even though HF forms molecular solid but because of the presence of hydrogen bonding between H and neighbouring F atom, which can hold the molecules together more strongly and therefore, expected to have much higher melting point than F 2. m.p.: F 2 < HF < NaF [If NaF > HF > F 2 only 1/2m] For reference: NaF(993 C), HF(-83.1 C), F 2 ( C) 4.(a) Increase in effective nuclear charge from Li to Ne, because electrons are added in the same principle shell. [2m] Stability: Be(1s 2 2s 2 ) > B(1s 2 2s 2 2p 1 ) extra stability for filled 2s orbital N(1s 2 2s 2 2p 3 ) > O(1s 2 2s 2 2p 4 ) extra stability for half-filled subshell (b) SiO 2 : - three-dimensional polymeric / network solid with very strong Si - O bond [Si - O must be stated!] [1m + 1m] CO 2 : - exists as discrete molecules with only weak intermolecular van der Waal's forces [1m + 1m] (c) N 2 O 4 2NO 2 H 2 O 2 2HO or homolytic fission / state the products 1990 AL CHEM P II - 2

3 weak N-N / and O-O bond strengths due to lone pair - lone pair repulsion on adjacent O (& repulsion between N δ+ - N δ+ as N is less electronegative than O ) (d) Sulphur has empty low lying energy 3d orbital - to expand its coordination sphere, thus capable of forming compounds with wide range of oxidation numbers. Oxygen has a much higher electronegativity than S and therefore it usually acts as an oxidant, oxygen cannot expand its octet / has no low lying energy orbital for bonding [2m] (e) Li + has very high charge density of the small ionic radius, therefore Li(H2O) n + has high hydration energy *Li salt - lattice energy is low because of the crystal packing is not efficient, hence they are very soluble in water. 5.(a) LiCl colourless ionic solid with Li +, Cl - AlCl 3 colourless covalent solid, dimeric structure,undergoes sublimation PCl 3 covalent colourless liquid can be oxidized to POCl 3 6.(a) K 2 NiCl 4 coloured ionic solid with complex anion. LiCl and K 2 NiCl 4 are ionic solids whereas AlCl 3 and PCl 3 are covalent molecules. LiCl and K 2 NiCl 4 do not react with water. Whereas AlCl 3 and PCl 3 are hydroscopic and readily hydrolysed by water. About the bonding: LiCl ionic chloride AlCl 3 covalent Cl- as ligand PCl 3 covalent bond between P and Cl 2- NiCl 4 dative covalent bond. [Structure: 1/2m x 4 = 2m; bonding: 1/2m x 4 = 2m physical properties: 1/2m x 4 = 2m; chemical properties: 1/2m x 4 = 2m Overall presentation: - highlight on their difference 2m, if in table form 1m only] (b) Ammonia N 2 + 3H 2 2NH 3 < 0 / exothermic Condition of reaction: Effect of temperature on the equilibrium constant Effect of pressure Effect of concentration of N 2 and H 2 Catalyst is required because of the inertness of N 2 or H 2 molecules. [1m or 0m] NH 3 is removed from the reaction mixture. Overall presentation [Alternative] Sulphuric acid S + O 2 SO 2 2SO 2 + O 2 2SO 3 H < 0 very slow in the absence of catalyst H 2 O SO 3 + H 2 SO 4 H 2 S 2 O 7 2H 2 SO 4 Steps involved production of SO 2 ; conversion to SO 3 ; removal of SO 3 [3m] Conditions of conversion (state the facts only) Effect of [O 2 ], removal of SO 3 Effect of pressure Effect of temperature on the equilibrium constant [Explanation of any one 1m] Catalyst is used to lower the activation energy for the SO 2 + O 2 reaction. O 2 is kinetically inert. [1m, without explanation, 0m] Overall presentation 1990 AL CHEM P II - 3

4 (i) NCl 4 (ii) SI 6 (iii) ArCl 2 (iii) [NaI 4 ] 3- Nitrogen cannot have 9e - in its valence shell / and cannot expand its valence shell because N has no low energy vacant oribtals for bonding S(VI) has a high redox potential and would oxidized I - to I 2. SI 6 is therefore very unstable with respect to decomposition. / Explain in term of size factor. (repulsion between I - ) Ar does not form compounds because of the stability of closed-shell electronic configuration. Because there is no low energy vacant orbitals for bonding. / Na + has too low charge density for complex ion formation (iv) [PO 4 ] 2- P cannot give oxidation state higher than +5 (vi) [Co(NH 3 ) 8 ] 3+ Co(III) prefers six-coordinated complexes. For eight-coordinated Co(III), the number of valence electrons around Co is greater than 18. [1.5m x 6 = 9m] (b) H 2 O FeSO 4 Fe(H 2 O) 2+ n / Fe 2+ (aq) Fe(H2O) n + O 2 Fe(H 2 O) n 3+ 2Fe(H 2 O) n Fe 2 O 3 / Fe(OH) 3 + 6H + [3m] Fe(CN) 4-6 is stable because CN - is a better ligand than H 2 O / CN - ligands stabilize Fe 2+ The redox potential of Fe(CN) 4-6 is different from Fe(H 2 O) 2+ n. (No mark for "Fe(CN) 4-6 is a stable complex") [2m] (c) Cl is much more electronegative than I / the size of I - is larger than Cl - Cl - is more resistant to oxidation than I - / Cu 2+ is not oxidizing enough to oxidize Cl - to Cl 2 / I - is a stronger reducing agent / I - ion s polarized by Cu 2+ CuI 2 decomposes CuI 2 CuI + 1/2 I 2 7.(a) (b) X: Structural isomerism Y: Geometric isomerism / cis-trans isomerism Z: Enantiomerism / optical isomerism / stereoisomerism (c) 1990 AL CHEM P II - 4

5 8.(a)(i) titration of known concentration against standard base [2m] (ii) decolourization of bromine water / KMnO 4 solution [2m] (iii) hydrolysis with NaOH/KOH(aq), acidify with HNO 3 followed by additon of AgNO 3 (aq) formation of white ppt. of AgCl Ł B (iv) (v) It is NOT optically active, because there is no chiral centre (b) It is not optically active because it is a racemic mixture. [2m] 9.(a)(i) The intermediate carboncation is stabilized by the electron donating alkyl substituents. [2m[ (ii) Rate: (fastest) Q > P > R (slowest) Taking P as a reference. Q is faster than P because the F substituent is electronegative Ł carbonyl carbon is more electrophilic Ł more susceptible to nucleophilic attack by -NH 2 [3m] R is slower than P because the -CH(CH 3 ) 2 group is more bulky and hinders the approach of the - NH 2 to the carbonyl group [2m] (iii) 2,4-dinitrophenylhydrazones of carbonyl compounds are high melting solid / crystals with bright colour Ł useful for characterization of the carbonyl compound / identification of the carbonyl group. [2m] (b) Chemically reasonable reaction sequence is acceptable Possible reaction sequence: (i) (ii) 1990 AL CHEM P II - 5

6 [2 x 3m = 6m] 1990 AL CHEM P II - 6