HCI Prelim C2_P2_MS

1 HWA CHONG INSTITUTION  2017 C2 CHEMISTRY PRELIMINARY EXAMINATIONS  PAPER 2 MARK SCHEME    1 (a) (i) H 2(g) +I 2(g) ⇌ 2HI(g) Initial conc/ mol dm¯3 0.05 0.05 0 Change / mol dm¯3 - 0.03 - 0.03 + 0.06 Eqm conc / mol dm¯3 0.02 0.02 0.06 2 22 [HI] [H ][I ] cK  [1] Kc =   2 2 0.02 0.06 = 9 [1] (ii) H 2(g) +I 2(g) ⇌ 2HI(g) Initial conc/ mol dm¯3 0.02 0.02 0.06 Change / mol dm¯3 +x +x -2x Eqm conc / mol dm¯3 0.02+x 0.02+x 0.06-2x Kc =   2 2 0.06 2 0.02 x x   = 0.36    0.06 2x 0.02 x   = 0.6 x = 0.01846 [1] [H2] = [I2] = 0.02+x = 0.0385 mol dm¯3 [HI] = 0.06-2x = 0.0231 mol dm¯3 shown [1]

Hwa Chong Institution 2017 9729 / 02 / C2 Prelim 2017 2 (iii) Correct axis labels [0.5] Appropriate scale [0.5] Correct shape of curves [0.5] Horizontal lines from 30 – 40 mins and 60 – 70 mins [0.5] *Correct concentration and time values for HI at t = 0, 30 and 60 min [1] *Correct concentration and time values for I2 at t = 0, 30 and 60 min [1] *-[0.5] for every wrong plot (iv) To prevent the position of equilibrium from shifting during the cooling process or when H I is removed when dissolved in water. [1] (b) IIIx xx xxxx xxxx xx xx or   [1] (c) Both I2 and HI have simple molecular structure / consist of simple discrete molecules [0.5] held together by dispersion forces. However, I2 has a larger number of electrons, therefore a larger electron cloud than HI, leading to stronger dispersion forces. [1] So, a larger amount of heat energy [0.5] is needed to separate the molecules, leading to a higher boiling point. 0 0.01 0.02 0.03 0.04 0.05 0.06 0 1 02 03 04 05 06 07 08 0 concentration / mol dm−3 t/ min I2 HI

Hwa Chong Institution 2017 9729 / 02 / C2 Prelim 2017 3 2    (a) (i) A ligand is an ion or molecule with one or more lone pairs of electrons available to be donated into the vacant orbitals of transition metal atom or ion.         (ii)  From the graph, VFe2+:Vphen = 2.5:7.5 therefore you can deduce the following reacting ratio - Fe2+:phen is 1:3 Formula of the complex: [Fe(phen)3]2+             (iii)   Fe 2+ has an incomplete/ partially filled 3d subshell (insufficient to just give electronic configuration)  In the presence of ligands (phen), the degenerate 3d orbitals of Fe 2+ split into two different energy levels with an energy gap ΔE  ΔE falls within the visible region of the electromagnetic spectrum  An electron in a lower energy d-orbital can absorb energy from the visible spectrum and be promoted to a higher energy d orbital that is vacant  The orange-red colour seen is the complement of the blue light absorbed.         (iv)  The energy gap, ∆E, is of a different magnitude in both complexes. Hence wavelength of light absorbed by the ferrozine complex is different from that absorbed by the phen complex and different colours are