IJC_2008_H1 Chemistry_Prelim II_Paper2 answers

1 © INNOVA 2008 8872/02 Preliminary 2 H1 Chemistry Paper 2 Mark Scheme 1. (a) Trigonal pyramidal (b) ………3N 2H4 Æ 4NH3 + N2 (c) ……… Formation of more stable molecules (d) No of moles of hydrazine = 25/1000 x 1.40 = 0.035 No. of moles of electrons = 4 x 0.035 = 0.140 No. of moles of KMnO4 = 35/1000 x 1 = 0.035 Mole ratio of KMnO4 : electrons = 1:4 Thus, final oxidation number = + 7 – 4 = + 3 Section A (e) (i) H-bonding H2NN H NN H 2 H H δ +δ − H (ii) There are stronger hydrogen bonding exis t between the hydrazine molecules compared to weak van der waalsforces of attraction between the molecules of ethane. Since more energy is required to break stronger H-bonding, the boiling point of hydrazine is higher. 2 (a) (i) Number of protons : ……31…… Number of neutrons: ……37…… Marker’s Comments: • Easy questions but many students made mistakes. • Many students use the Ar of Ga as 69.7 (average value of the different isotopes) given in Data Booklet. • Some students even mistaken Ga for Ge. (ii) –––––––– + + + + + + + + Source α Neutrons II. Deflection is proportional to argch e mass . Since deuterium ion is half the mass of alpha particle and half of it’s charge, the deflection remains the same at 3o.

2 © INNOVA 2008 8872/02 (b) (i) Al2O3 (s) + 2NaOH(aq) + 3H2O Æ 2NaAl[OH]4 (aq) AlCl3 (s) + 6H2O Æ [Al(H2O)6]3+ (aq) + 3Cl– (aq) [Al(H2O)6]3+ (aq) Æ [Al(H2O)5(OH)]2+ (aq) + H+ (aq) (ii) (c) (i) Isoelectronic refers to ions having the same number of electrons. (ii) 1s22s22p6 (iii) s-orbital: Spherical p-orbital: Dumb-bell (iv) Al: 1s22s22p63s23p1 Mg: 1s22s22p63s2 Since less energy is required to remove the valence electron from the higher energy p-orbital of the Al atom compared to the s-orbital of Mg atom. 3 (a) [ ] [] [] 2 3 c 3 22 NHK NH = Units: mol-2dm6 (b) (i) The system will reduce the pressure by shifting equilibrium to the left. (ii) The finely-divided iron acts as catalyst. The catalyst lowers the activation energy of the reaction. Equilibrium is attained faster as it speeds up the rates of both the forward and reverse reactions equally. Hence it has no effect on the position of the equilibrium. (c) (i) step I Reagent(s) and Conditions: … Cl 2 (g),…… FeCl 3 or AlCl3 catalyst ………… step II Reagent(s) and Conditions: … Br 2 in CCl4 ……… uv light……………… (ii) CHOHCH2COO- COO- Cl (iii) Free radical substitution This reaction is not specific

3 © INNOVA 2008 8872/02